LIPID MAPS Lipid Metabolomics Tutorial

Fat Soluble Vitamins

Professor Edward A. Dennis

Department of Chemistry and BiochemistryDepartment of Pharmacology, School of Medicine

University of California, San Diego

Copyright/attribution notice: You are free to copy, distribute, adapt and transmit this tutorial or individual slides (without alteration) for academic, non-profit and non-commercial purposes. Attribution: Edward A. Dennis (2010) “LIPID MAPS Lipid Metabolomics Tutorial” www.lipidmaps.org

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Vitamins: The Family Tree

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Preview: Vitamin A - Retinol

Various “retinoids”– retinol, retinal & retinoic acid

– retinyl esters• transport in blood

– beta carotene

Numerous Functions– Vision

• all-trans retinal

• 11-cis-retinal

• Growth & Wound healing

• especially epithelium

– Reproduction

O

CH2

OH

O

H

O

OH

1112

Retinol

All-trans retinal

11-cis retinal (formed by photoisomerization of all-trans retinal)

Retinoic acid

H

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11-cis retinal(formed by photoisomerizationof all-trans retinal)

Retinoic acid

Retinol

All-trans retinal

1112

Vision- How it works…

• Cis-Retinal acts as a cofactor of the protein opsin to form rhodopsin.

• It functions in a similar way in rods and cones. But we’ll use rods as a model.

• Rhodopsin and transducin are embedded in the cell membrane of the outer rod segment.

• Rhodopsin is the photoreceptor. It is an integral membrane protein with 7 membrane spanning segments. When a photon of light hits rhodopsin, it causes the isomerization of cis-retinal to trans-retinal. This activates the receptor, causing it to bind to the heterotrimeric G protein, transducin.

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Retinol from b-Carotene

• Retinal and retinol are readily interconverted

• Most dietary vitamin A comes in the form of b-carotene.– 1 carotene = 2 retinal

– But: inefficient conversion

– Less toxic than retinol• Safer form of the vitamin

– Found in yellow and dark green vegetables

• carrots, doc.

O

H

Cleavage site

Retinalb-carotene

2x

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CleavageSite

2x

Vitamin A Deficiency• Incidence: Rare in the US

– Seen in people who don’t eat well• Worldwide, it is the third most common

nutritional deficiency, accounting for 500,000 cases of blindness annually.

• Symptoms– Loss of acuity & night blindness,

blindness

– Lesions on corneal surface

– Eventually, dermatology problems

• Mechanism: Lack of retinoids– Poor epithelial growth

– Rhodopsin synthesis is impaired

• Treatments: A supplementation– Better diet

Trivia: Ancient Egyptians recognized that night blindness could be treated by eating liver.

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Vitamin A Toxicity

There are three syndromes of vitamin A toxicity:

• Acute Toxicity (very rare)– occurs in adults when >200 mg are ingested

– symptoms include nausea, vomiting, vertigo, and blurry vision.

• Chronic Toxicity (rare)– occurs with long-term ingestion of doses higher than 10

times the RDA.

– symptoms include problems talking, hair loss, hyperlipidemia, hepatotoxicity, bone and muscle pain, and vision problems.

– In postmenopausal women, it has been associated with increased fracture risk.

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Vitamin A Toxicity• Teratogenic Effects:

– Synthetic retinoids can be used to treat severe dermatological conditions including severe psoriasis and acne vulgaris.

– Synthetic retinoids, like acitretin, cause spontaneous abortions and severe life-threatening congenital malformations. • Women treated with retinoids must not get pregnant at

the time of treatment or become pregnant for up to 3 years after treatment.

• Patients receiving treatment with retinoids must not give blood for up to three years after treatment.

– The presence of these drugs in plasma can be demonstrated for up to several years after a person stops taking them. It could be disastrous if an unsuspecting pregnant woman received one in a transfusion, hence the ban.

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Vitamin D - Cholecalciferol

• Vitamin D is a cholesterol-like molecule

• Important to bone and calcium regulation– Acts more like a steroid

hormone rather than a enzyme cofactor

H

H

HO

H

H

H

H

HO

H

H

H

H

HO

H

H

Cholecalciferol

Ergocalciferol

7-dehydrocholesterol

synthesis in skin

Diet

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• Cholecalciferol (D3) has two sources:– Diet

• plants have ergocalciferol (D2), which easily becomes D3

• animal flesh has ready-made D3

– Sunlight (UV)• Converts 7-dehydrocholesterol into

D3 in the skin

synthesis in skin

Cholecalciferol

Diet

Ergocalciferol

Metabolism of Cholecalciferol (D)

Calcitriol is the active form of the vitamin– aka: 1,25 (OH)2-cholecalciferol or 1,25-D

Its precursor (provitamin) is cholecalciferol, native vitamin D– aka: D or D3

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Vitamin D Toxicity

• Vitamin D can build to toxic levels!

– Most dangerous vitamin!

– Messes up serum Ca++ leading to hypercalcemia, hypercalciuria and bone demineralization.

– Intoxication may occur in fad dieters who consume "megadoses" of supplements, in patients on vitamin D replacement therapy for malabsorption, or in children who overdose on supplements.

Trivia: In the 1940s and 1950s, a number of children developed hypercalcemia and some even had hypercalcemic-induced brain injury. This was felt to be a result of the high concentrations of vitamin D in fortified milk products

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Vitamin K - Phylloquinone

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Vitamin K1

• Made by normal intestinal flora– Antibiotics can cause a loss of K!

• Two forms exist– phylloquinone (K1) - plant sources

• spinach, cabbage, cauliflower

– menaquinone (K2) - bacterial sources

• Important in blood clotting

Vitamin K Function

• Important in Hematology

• Catalyzes g -carboxylation

• Required by liver to make clotting factors– Factors II, VII, IX & X

– K = Klotting

– No K = No Klotting

• Inhibited by anticoagulants like warfarin

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Vitamin K Deficiency

• Incidence: Common in certain groups – Malnourished people on antibiotics

– Babies whose intestines are still sterile

– Patients on anticoagulants like warfarin

• Symptoms: Spontaneous bleeding– Hematemesis (bloody vomit)

– Hemarthrosis (blood in joint capsules)

– Spontaneous bruising & bleeding gums

• Mechanism: Lack of vitamin K– Factors II, VII, IX & X depend on K

• Treatments: K supplements

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Warfarin Overdose

• Warfarin has been the standard oral anticoagulant used in a variety of clinical settings.

• Patients treated with warfarin frequently become overly anticoagulated. The most common causes include drug interaction, dietary changes and superimposed illnesses.

• If the clotting time is significantly impaired or if the patient is at a high risk for bleeding Vitamin K is given to reverse the effects.

• Mechanism of Action: Warfarin is similar in structure to vitamin K and interrupts the vitamin K dependent carboxylation cycle by blocking reduction of the inactive K 2,3 epoxide to the active form of the vitamin. Trivia: Warfarin was

originally developed as a rodenticide.

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Warfarin

Vitamin E - Tocopherols

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Vitamin E

• Several variations exist– D-a-tocopherol is most potent

• Lipid-media antioxidants– contrast with vitamin C: a water-media antioxidant

• Found in nuts and meats• No clear deficiency disease• Long term effects and safety of

supplementation are unclear.

Summary of Fat Soluble Vitamins

Vitamin Active Forms Function Sources Disease Toxic?

Retinol ll-cis retinal Vision, growth veggies A deficiency Somewhatmany others

Cholecalciferol Calcitriol Bone and Ca++ UV light Rickets Very!regulation Dairy

Tocopherols same Antioxidant Nuts, meat Atherosclerosis? Not really

Phylloquinones same Blood clotting Colonic flora K deficiency Somewhat

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Isoprene Units - A Common Thread

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Example: Recognizing Isoprene Units

• Retinol=4 x Isoprene

• Look for 5-carbon units

• Double bonds may be removed or rearranged

• C2 connects always to at least 3 other carbons

CH2

OH

Retinol

...

.

12

3

.Isoprene

(2-methyl-1,3-butadiene)

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Isoprene (2-methyl-1,3-butadiene)

Retinol

12

3

What About Vitamin Supplements?

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Review of Lipid MetabolismLIPIDS CARBOHYDRATES PROTEINS

fatty acids glucose amino acids

Acetyl CoA

malonyl CoA citrate acetoacetyl CoA

fatty acid

CO2 + ATP(energy)

HMG-CoA

neutral lipidsphospholipidssphingolipids

cholesterol esters

ketone bodies cholesterol

Oversimplified picture

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Organ Localization

Oversimplified picture

BLOOD TRANSPORT• TG on lipoproteins• FA on albumin• Glucose dissolved• Ketone Bodies dissolved

Liver

Adipose

Intestine

Brain

Muscles

TG

FA

BileGlucose, FA(Ketones)

Glucose(Ketones)

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Intracellular Localization

FA

Acetyl CoA

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Acknowledgement

This tutorial is based on an evolving subset of lectures and accompanying slides presented to medical students in the Cell Biology and Biochemistry course at the School of Medicine of the University of California, San Diego.

I wish to thank Dr. Bridget Quinn and Dr. Keith Cross for aid in developing many of the original slides, Dr. Eoin Fahy for advice in applying the LIPID MAPS nomenclature and structural drawing conventions [Fahy et al (2005) J Lipid Res, 46, 839-61; Fahy et al (2009) J Lipid Res, 50, S9-14] and Masada Disenhouse for help in adopting to the tutorial format.

Edward A. DennisSeptember, 2010La Jolla, California

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