FAT-SOLUBLE VITAMINS.

XVII. THE INDUCTION OF GROWTH-PROMOTING AND CALCIFY- ING PROPERTIES IN A RATION BY EXPOSURE TO

ULTRA-VIOLET LIGHT.*

BY H. STEENBOCK AND A. BLACK.

(From the Department of Agricultural Chemistry, University of Wisconsin, Madison.)

(Received for publication, April 18, 1924.)

Last year Steenbock and Nelson (1) laid the foundations for an experimental method in which, by the use of ultra-violet light, growth can be used as a measure of the comparative amounts of vitamin A occurring in any food. Their method also allows demonstration, in case of inhibited growth and in the absence of a vitamin A deficiency, as to whether this inhibition is due to lack of the antirachitic vitamin or some other factor. Preliminary data on the use of this method have already been presented (2).

In attempting to use this method for determinations of vitamin A in butter a large series of experiments was run in which eight groups of rats, six animals to the group, were used (3). In each group two animals were exposed to ultra-violet light from a quartz mercury vapor arc, with the expectations that if there were a deficiency of the antirachitic vitamin in the ration, the two irra- diated animals would, by their increased growth, reveal such to be the case. It merely remained, then, to find out at what level of butter fat irradiation no longer had any effect. This would be the level at which the butter furnished a sufficiency of the antirachitic vitamin.

Contrary to what was expected, all the animals in each group,

*Published with the permission of the Director of the Wisconsin Agri- cultural Experiment Station, Madison.

To protect the interest of the public in the possible commercial use of theee and other finding8 soon to be published, applications for Letters Patent, both as to processes and products, have been filed with the United States Patent Office and will be handled through the University of Wisconsin.

405

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irradiated or not irradiated, irrespective of whether the butter was incorporated in the ration at an 8, 4, 2, 1, 0.5, 0.2, 0.1, or 0.0 per cent level, grew, on the average, equally well. This puzzled us because in dozens of other experiments, and in controls, irradia- tion had always been found to increase the amount of growth unless the animals gave signs of vitamin A deficiency as indicated by ophthalmia or infections of the respiratory tract. Then, of course, lack of vitamin A was the factor inhibiting growth.

Shortly after these results were obtained, there reached our hands an article by Hume and Smith (4) which appeared to have a bearing on this point. They irradiated glass jars with ultra- violet light and found that rats kept in such jars grew to a larger size than controls. Ozone, well known to be produced by the mercury lamp, was shown not to produce this effect nor did irra- diated jars from which the air had been blown out after irradiation. They concluded that “it is the air and not some property impressed on the glass jars which is active.” They speak of ionization as being a probable factor.

A little later Goldblatt and Soames (5), studying what they erroneously speak of as fat-soluble A, found that, when the livers of rats in which growth had been induced by irradiation were fed to suitably prepared and rationed animals, growth previously inhibited was restored; with the livers of non-irradiated rats taken under the same circumstances and fed to similar animals this did not take place. They found the results difficult of interpretation. They considered it unjustifiable to postulate synthesis of “fat- soluble A.” They suggest that it may be a mobilization from other organs, but conclude that “the correct explanation must await the results of further studies on the nature of the fat solu- ble growth promoting factor as well as on the site and mode of its action,”

It appeared to us that our results might be explained and some idea obtained as to the difficulty experienced by the aforementioned investigators if it were determined, in the first place, whether or not an irradiated animal when put in the cage with a non-irradi- ated animal is able to influence its growth in any way whatsoever. This suggested itself because in our butter fat experiments irradi- ated and non-irradiated animals had been kept together in the same cage for the first time-and it was in this instance and in no

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H. Steenbock and A. Black 407

other that approximate equality of growth was obtained. In the second place, it appeared justifiable to carry out a test to deter- mine if growth-promoting properties could be conferred upon an ordinary ration by direct irradiation just as Goldblatt and Soames had found that liver irradiated in the living rat was activated. Obviously, positive or negative data on these two points would clarify the situation greatly. Experiments dealing with the first will be published later; experiments dealing with the second form the subject of the present communication.

EXPERIMENTAL.

As data upon one point after another were accumulated the experimental procedure was modified from time to time, but essentially the same technique was followed as in previously pub-

lished work. White rats and black hooded white rats were used as the experimental animals, They were kept in our standard sized cages provided with false screen bottoms (6) in our rat room in which all t.he window curtains were drawn except those of north- ern exposure. This latter precaution was taken even though our experiments were started in early November and completed early in March, when solar radiation, especially such as finds its way through window glass, is virtually impotent. Especial emphasis was placed upon the preparation of our animals, in fact, it was the condition of our animals that first gave us the clue to this method of experimentation and particular care was taken to main- tain their condition as uniform as possible. The critical point in the preparation of our experimental animals to allow us to demon- strate what we desired, lay in having them suitably depleted in the antirachitic vitamin and yet stocked with the liberal reserves of vitamin A necessary to prevent early failure from ophthalmia and infections of the respiratory tract.

Our basal stock ration (7) supplemented with milk from stall fed COWS kept continually on the same ration, allowed us on the whole to meet the requirements. At times, however, when we were obliged to use poor alfalfa hay in this ration we were forced to supplement the ration temporarily with a small amount of butter fat to keep up the rate of reproduction in our stock neces- saryto supply US with a sufficiency of experimental animals.

When our young rats weighing 40 to 50 gm. at 21 to 25 days of

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Fat-Soluble Vitamins. XVII

age were put upon a ration, free from vitamin A and the antira- chitic vitamin, growth usually ceased within 2 weeks as the stores of the antirachitic vitamin became exhausted. With the intro- duction of the antirachitic factor growth was always restored; without it the animals maintained themselves practically at constant body weight; in either case, with the exhaustion of the reserves of vitamin A, death ultimately resulted from ophthalmia or infections of the respiratory tract.

It is obvious that the decisiveness of our demonstration of the occurrence or non-occurrence of the antirachitic factor lay in the length of time that failure due to lack of vitamin A could be delayed. To prolong this period we initially used a ration of hog millet which contains vitamin A and but little of the antirachitic vitamin; later, we used a synthetic ration supplemented with a small amount of alfalfa, 0.3 per cent cured with special precautions. This we had previously found to carry considerable amounts of vitamin A and but little of the antirachitic vitamin (8). Larger amounts we did not see fit to use because of the danger of the introduction of the antirachitic factor of which rats, on our rations, need but little for normal growth and bone calcification. Our synthetic ration con- sisted of alcohol-extracted and heated casein 18, salt 40, 4, agar 2, yeast 8, and dextrin 68. The preparation of these has been previously described (1).

For irradiation we used a Cooper-Hewitt quartz mercury vapor lamp with an arc 2 inch long at a distance of 23 inches, run for the most part at a voltage of 120.

Our first experiments were carried out with a ration of hog millet 84, purified casein 12, and salts 40, 4. The ration was made up in quantity and fed to the rats ad libitum in tin cups. The irra,diated ration was exposed for 10 minutes to the radiations of the quartz mercury vapor lamp in 50 gm. quantities, spread out in a thin layer in a 10 inch porcelain dish. After irradiation it was fed immediately. Unconsumed residues were discarded daily.

The results of this trial are shown graphically in Chart I. In each case a very pronounced stimulation of growth was observed. The experiment was terminated at the end of 7 weeks because by this time all the rats were afflicted either with ophthalmia or in- fections of the respiratory tract due to insufficiency of vitamin A.

Chart II shows the results obtained in a similar experiment

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H. Steenbock and A. Black

CHART I. Irradiation of a supplemented hog millet ration with a quartz mercury vapor lamp has a very pronounced effect upon growth. The rats were taken at 21 to 23 days of age. The position of the arrow indicates the incidence of ophthalmia and the asterisk the incidence of infections of the respiratory tract.

CHART II. Irradiation of a casein, salt, yeast, agar, dextrin ration was also effective in providing a growth stimulus, terminated only by exhaustion of reserves of vitamin A. As this ration carried no intact living plant cells. the synthetic action of cells in the production of the vitamin is excluded, These rats were started at an age of 21 to 24 days.

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410 Fat-Soluble Vitamins. XVII

where no vitamin A was furnished in the ration. The early fail- ure of our controls indicates that this group of animals was but very poorly provided with reserves of vitamin A. Yet evenehere in

CHART III. When more vitamin A was p?ovided in the ration, irradia- tion could be continued over a longer time and more decisive results were obtained. The rats were started at 21 to 24 days of age. Unfortunately, Litter 1 was an inferior litter as shown by their growth. Rat 9813 failed

rapidly due to a bad pneumonic condition revealed upon post-mortem. Analysis of the femurs for ash gave values of 48.8 and 45.6 per cent with Rats 9817 and 9819; of 55.2, 55.3, 58.6, and 57.5 per cent with Rats 9808, 9809, 9810, and 9811; and of 52.9, 53.2, and 55.8 per cent for Rats 9812,9814, and 9815, respectively. Increased calcification, therefore, went hand in hand with increased growth. The pronounced growth of Rat 9815 with a difference of 77 gm. in weight from its litter mate control, Rat 9819, to- gether with a difference in ash content of bone of 10 per cent, shows that the effect cannot be looked upon as a matter of temporary stimulation.

4 weeks time it is demonstrat,ed that irradiation of the ration had a decided effect; in fact, it was more effective than exposing the animals ta the rays directly. The animals, however, had received

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H. Steenbock and A. Black 411

more radiations; they had been irradiated for 10 minutes 6 days out of the week, while the ration had been irradiated 20 minutes daily.. The experimental precautions used were the same as in the first series.

As a result of the poor demonstration of the effect of irradiation in the second trial, a third was carried out in which the ration was fortified with respect to its vitamin A content by the addition of 0.3 per cent of alfalfa (Chart III). This had the desired effect; the incidence of ophthalmia or infections of the respiratory tract were delayed for at least 2 weeks except in the case of Litter 1. This was an exceptionally poor litter as the young grew poorly or not at all and autopsy revealed large caseous areas in the lungs, suggesting tuberculosis.

As growth in itself can hardly be taken as sufficient evidence of the existence of the antirachitic fa,ctor, we were led to make analy- ses of shaft bones, usually the femurs or humeri, for total ash con- tent. These were dissected free from muscle and connective tissue and then cleaned by rubbing between folds of cheese-cloth. They were then dried for 2 days in a steam oven at 96”C., extracted thoroughly with alcohol in a Soxhlet for 48 hours, then dried and ashed in an electric muffle furnace. In this series the femurs from two rats of the control group gave an average value of 47.2 per cent ash, four with direct radiation 56.6 per cent, and three on the irradiated ration 53.9.

Further evidence of the effectiveness of irradiation of ration is shown in Chart IV where we also used the alfalfa supplement with our synthetic ration to furnish additional vitamin A. It almost appears that with this the animals were furnished with too much of the antirachit,ic factor because the control groups grew more than usual. How much supplement to furnish is always a matter of uncertainty because in spite of the precautions taken with our stock it cannot be foretold absolutely how much our animals have in reserve. In no case, however, were we forced to discard any animals or any results for this reason. All our data on growth that had been collected when these experiments were terminated are presented without selection in this paper.

In this series (Chart IV) we were particularly interested with the rale of air in the activation of the ration because in connection with the therapeutic effect of light attention has been called

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Cm

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CHART IV. Though the controls show an unusual amount of growth the results show, nevertheless, that irradiation of the ration as carried out in an open porcelain dish, or in a stoppered quartz flask in the presence of air or in a stoppered quartz flask in the presence of GOa is effective in acti- vating the ration. Nor is this activation completely dissipated after 24 hours; in fact, it may bc questioned if it is reduced at all because when failure ensued signs of vitamin A deficiency were clearly evident. Number- ing the litters from left to right-all rats in the same vertical line being from one litter, Litters 1 and 2 were started at 21 days, Litter 3 at 20, and Litter 4 at 23 days.

Analysis of femurs for ash gave values as follows: Lot 2612; Rats 9841 to 9844; 49.3, 48.2, 47.0, and 48.1 per cent.

‘I 2607; “ 9820, 9821, and 9823; 53.0, 52.1 “ 49.4 “ “ “ 2610; “ 9832 to 9836; 54.1, 52.7, 53.8 “ 52.8 “ ‘I

412

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H. Steenbock and A. Black 413

repeatedly to the r61e of ozone and oxides of nitrogen produced by the irradiation of the elements of air. If these play a role in the activation of the ration, less activation should result when the ration is irradiated in the presence of CO2 and possibly also when the air is limited in quantity. To determine this the ration was irradiated in quartz flasks of 300 cc. capacity, stoppered with a rubber stopper.

In one case the flask containing 50 gm. of ration was exhausted in a vacuum desiccator with a water pump to 16 mm. pressure. CO2 was then passed in to destroy the vacuum and the process repeated to wash out the air as thoroughly as possible. The flask itself could not be evacuated directly because it was flat bottomed and unable to withstand the pressure. This also prevented us from irradiating the ration in VQCUO. With the air displaced by COZ, the stop-cock with which the rubber stopper was provided was turned immediately, and the flask irradiated for 20 minutes. Unconsumed ration residues were discarded daily.

As seen in Chart IV the effectiveness of the irradiation was not reduced by this procedure. We admit that this does not absolutely demonstrate the point that we were after, because, in the absence of analytical control, it is possible that there was still sufficient air in the flask to play an important role. Nevertheless, it shows that, within the experimental conditions imposed, air is not an important factor,

The activation of the ration does not appear to be a temporary matter as shown in the last lot of the series. There the rats were fed the ration 24 hours after it had been exposed to the ultra-violet rays for 20 minutes. In the interim the ration had been kept in a glass bottle, stoppered with a rubber stopper. It is true that the rats did not grow so well as in some of the other groups, but neither did the rats exposed to the radiations directly, and failure was post- poned until vitamin A deficiency became notable.

Further data on the activation and persistence of the activation of the ration are presented in Chart V. Here we again fed a ration free from the fat-soluble vitamins and, as seen in the chart, we obtained a better control. The first few groups furnish infor- mation on the relative transmissability of the activating rays by Pyrex glass as compared with brown bottle glass. 50 gm. por- tions of the ration, made up in quantity, were irradiated in the one

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414 Fat-Soluble Vitamins. XVII

2345

CHART V. Irradiation of the ration in a 500 cc. Pyrex flask in contrast with a brown glass-stoppered bottle did not interfere appreciably with the activation. In harmony with this observation is the fact that Pyrex glass is somewhat permeable to ultra-violet light while brown bottle glass shuts out even the violet. Exposing the activated ration to a vacuum or heating it for 45 minutes at 96” did not destroy the activity. These rats were all taken from four lit,ters as in the previous group. Litters 1, 2, and 3 were taken at an age of 24 days, Litter 4 at 25 days. At the time of terminating the experiment the rats not only varied in weight but in length, showing that actual growth had taken place. From tip of nose to tip of tail Rats 18 and 19 measured 9: and 9f inches, respectively; Rats 10 and 11 measured 11 and 118 inches, respectively.

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H. Steenbock and A. Black 415

case in a 500 cc. rubber-stoppered Pyrex flask and in the other case in a 150 cc. rubber-stoppered brown bottle for 20 minutes, the containers being rotated from time to time to expose fresh surfaces. Pyrex, well known to transmit some of the ultra-violet rays, did not prevent the activation in contrast with the brown bottle glass.

The last two groups show that exposing the activated ration to a vacuum or heating it for 45 minutes at 96” did not destroy the activation. In the former case 50 gm. of ration irradiated in a porcelain dish as before were transferred to a beaker and placed in a vacuum desiccator. This was evacuated with a water pump to 16 mm. pressure, then the vaccum was destroyed and the evacua- tion repeated. The ration was fed to the animals immediately. The heating was carried out in the porcelain dish in which the ration had been irradiated. This was placed in a copper steam- heated oven.

Irradiation of Rat Livers.

With the fact demonstrated that growth-promoting properties can be conferred upon a ration by exposing it to the radiations from a quartz mercury vapor lamp it occurred to us that possibly the results of Goldblatt and Soames (5) could be explained on the same basis. It will be recalled that they irradiated rats, fed the livers of these rats and found them growth-promoting, while livers from non-irradiated rats were entirely inactive. In their expcri- ments there prevailed the possibility that the growth-promoting property was mobilized from other tissues or that it was actually synthesized by cellular activities. The fact that these possibilities were excluded in our previous experiments on irradiation of ration suggested a repetition and elaboration of the experiments of the aforementioned investigators.

Chart VI presents a duplication of the experiments of Goldblatt and Soames with the same results; i.e., the livers of irradiated rats were found to have been activated. The rats used as a source of liver were young rats not over 60 gm. in weight, raised on the same ration as the test animals. The irradiated rats had been exposed to ultra-violet radiations 10 minutes a day, 6 days out of the week. Rats from either group were used as a source of liver as soon as t’hey showed signs of ophthalmia. They apparently were very low in vitamin A content because in the test animals the incidence of ophthalmia was not deferred beyond the time of

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CHART VI. Liver taken from non-irradiated rats is less effective in promoting growth than that from irradiated rats. The percentage of ash in the bones differed markedly. Rats 9407, 9408, and 9410 gave values of 48.6, 50.0, and 49.3; while Rats 9411 to 9414 gave values of 57.8, 57.8, 54.0, and 53.9, respectively.

CHART VII. Lung tissue, like liver, varies in its content of the acti- vating agent with the exposure to the ultra-violet radiations. Especial attention was focussed on this experiment in the early belief that if the radiations were growth-stimulating by virtue of their effect upon the air, lung tissue ought to be especially active. Rats 9963 and 9964 had 46.7 and 54.4 per cent of ash in their dried alcohol-extracted femurs; Rats 9965 and 9966 a, 53.9 and 55.0 per cent; and Rats 9966 b and 9967, 46.5 and 49.0 per cent, respectively. All the rats were from one litter taken at 21 days of age.

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CHART VIII. This chart shows again, as shown in Chart VI, that liver tissue taken from an irradiated rat, in contrast with that from a non- irradiated rat, is active. When liver tissue was removed from a non- irradiated rat and then irradiated, no more growth was obtained with it than with untreated tissue. Bone calcification, however, was increased. Taking rats in order , lot for lot, from left to right, the percentage of ash in the femurs was as follows:

Lot 2620; 44.6, 49.3, 47.1, 43.3 “ 2621; 56.1, 52.7, 54.4, 51.8 “ 2617; 56.3, 54.1, 56.9, 54.7 “ 2618; 47.9, 46.8, 46.9, 44.5 “ 2619; 55.1, 51.7, 52.0, 53.1

Apparently, failure of growth is not necessarily accompanied by deficient calcification. The rats were taken from four litters, counting from left to right, Litters 1, 2, and 3 were started at 21 days and Litter 4 at 20 days.

417

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418 Fat-Soluble Vitamins. XVII

CHART IX. Muscle like lung and liver tissue from an irradiated rat is activated, and when not activated by irradiat’on of the living animal, it can be activated subsequently. This is shown by increased growth and also increased calcification of bone, when such muscle was fed. Again taking the rats in order from left to right, lot for lot, the percentage of ash in the femurs was as follows:

Lot 2623; 46.6, 47.2, 46.0, 43.7 “ 2624; 57.7, 55.0, 53.1, 55.6 “ 2625; 52.1, 52.6, 51.6, 51.1 “ 2626; 48.5, 45.2, 51.0, 47.8 “ 2627; 55.3, 57.6, 49.0, 55.2

The rats were taken from four litters, those in the same vertical line being from one litter. Numbering the litters from left to right Nos. 1 and 3 were started at 23 days of age, No. 2 at 22 days, and No. 4 at 21 days.

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H. Steenbock and A. Black 419

its incidence in a control group not receiving any liver. Chart VII shows similar results obtained with lung tissue.

The experiments shown in Chart VIII represent an attempt to determine if non-active liver could be activated by irradiation subsequent to its removal from the rat. To demonstrate this, it was necessary to run sufficient controls so that errors from the side of the test animals as well as from the livers would be excluded. The liver samples, after weighing, were macerated on a glass plate

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CHART X. Livers from irradiated rats dried at 96°C. for 24 hours and kept in the laboratory for months were still active. The rats used were taken from two litters; the six on the left from one litter; and those on the right from the other. They were started at an age of 23 days.

and then exposed to ultra-violet rays for 20 minutes. They were fed to the test animals immediately afterwards. The chart shows that they were inactive as far as growth is concerned, but analysis of the bones for ash, the results of which are detailed in the legend of Chart VIII, show that calcification was promoted just the same. While the rats from Lot 2618, which had been fed livers from non-irradiated rats, had on the average 46.5 per cent of ash in their bones, those from Lot 2619, getting the same livers but irradiated, had 52.9 per cent. Why these animals should not have grown we cannot explain. Calcification evidently took place independently of growth.

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420 Fat-Soluble Vitamins. XVII

Chart IX shows that muscle tissue subjected to the same treat- ment. is activated beyond question. Not only was growth pro- moted but calcification as well. Rats in Lot 2626 on non-irra- diated liver averaged 48.1 per cent ash and those in Lot 2627 on irradiated liver, 54.3 per cent.

Chart X shows the persistence of the activation in the irradiated rat livers left over trom the earlier experiments. They were dried in a steam oven at 96°C. for 24 hours and then put in glass bottles till a sufficient quantity had been accumulated. Some of the material had stood as long as 60 days on a shelf in the labora- tory before being fed. As the chart shows it was still very active.

DISCUSSION.

It is not the intention of the authors to discuss in detail the significance of the data presented in this paper at the present time, because obviously more experimental facts must be obtained. Some time ago we made the assumption that the failure of growth in our young rats in the absence of vitamin A deficiencies was due to the absence of the antirachitic vitamin. We made this assump- tion because of the parallelism in the resumption of growth upon the administration of cod liver oil, free from vitamin A, with the action of ultra-violet light which is a well known antirachitic agent. This assumption was supported by the fact that with the reinitia- tion of growth calcification of bone was increased.

The question now presents itself, are we justified in making the further assumption, in view of the fact that growth-promoting and calcium-depositing properties can be imparted to a ration by irradiation with ultra-violet light, that the acceptance of the exist- ence of an antirachitic vitamin upon its former premises is no longer justifiable, granted, of course, that we accept a vitamin as a compound of biological origin.

If such were justified it would simplify our conceptions materially because it has been difficult though not impossible to conceive how two such apparently different agencies, light and vita- min, should have the same effect. It suggests itself that, in ulti- mate analysis, both light and the antirachitic vitamin may repre- sent the same antirachitic agent-possibly a form of radiant energy. In this connection it is of interest to mention that the authors have conferred growth-promoting properties upon olive oil and lard by irradiation with ultra-violet light. Taken from

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H. Steenbock and A. Black 421

three litters and started at 48 to 63 gm. in weight, young rats on the untreated olive oil at the time of writing weigh from 67 to 73 gm. ; on the irradiated olive oil 92 to 98 gm. On untreated lard they weigh from 74 to 94 gm. and on the irradiated lard from 92 to 125 gm. These weighings were made 3 weeks after the begin- ning of the experiment on a ration carrying but 2 per cent of the fats.

The suggestions carried by the data presented in this paper are obvious. As we realize that we have already indulged extensively in unorthodox speculations we shall withhold further statement of probabilities, and what we believe to be facts, for future pub- lication.

SUMMARY.

By irradiation with the quartz mercury vapor lamp, rat rations can be activated, making them growth-promoting and bone-cal- cifying, to the same degree as when the rats are irradiated directly. This activation takes place when the ration is irradiated in an open dish or in a stoppered Pyrex or quartz flask, filled with air or COS, but not in a brown glass bottle. The activation is not de- stroyed by subjecting the ration to a vacuum, heating it for 45 minutes at 96”C., or letting it stand for 24 hours at room temperature.

Confirming the observations of Goldblatt and Soames, liver taken from irradiated rats is growth-promoting while liver from non-irradiated rats is inactive. The same was found true of lung and muscle tissue. Inactive muscle, exposed, after removal from the body, to the radiations of the lamp was found to have become activated, being both growth-promoting and bone-calcifying. Liver treated the same way also promoted bone calcification. The activity of liver taken from irradiated rats was not destroyed by drying at 96” for 24 hours and then keeping it in the laboratory in a stoppered bottle for 2 months.

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BIBLIOGRAPHY.

1. Steenbock, H., and Nelson, E. M., J. Biol. Chem., 1923, lvi, 355. 2. Steenbock, H., Nelson, M. T., and Black, A., J. Biol. Chem., 1924, lix,

p. ix. 3. Jones, J. H., Steenbock, H., and Nelson, M. T., unpublished data. 4. Hume, E. M., and Smith, H. H., Biochem. J., 1923, xvii, 364. 5. Goldblatt, H., and Soames, K. M., Biochem. J., 1923, xvii, 446. 6. Steenbock, H., Sell, M. T., and Nelson, E. M., J. Biol. Chem., 1923, Iv,

399. 7. Steenbock, H., Science, 1923, Iviii, 449. 8. Steenbock, H., Nelson, M. T., and Black, A., unpublished data.

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H. Steenbock and A. BlackULTRA-VIOLET LIGHT

RATION BY EXPOSURE TOCALCIFYING PROPERTIES IN A

GROWTH-PROMOTING ANDINDUCTION OF

FAT-SOLUBLE VITAMINS: XVII. THE

1924, 61:405-422.J. Biol. Chem. 

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