Lens & Cataract (Biochemistry)Molecular BiologyCrystallins:
Intracellular (within the plasma membrane of the lens epithelium
and fiber cells)Water insoluble fraction associated with fiber
plasma membranesBrown cataract (Brunescent) Amount of insoluble
proteins correlate with degree of opacification (90% of nuclear
proteins insoluble)
Lens & Cataract (Biochemistry)Carbohydrate
MetabolismAnaerobic Glycolysis: More active Provides most of the
high energy phosphate bonds for lens metabolism
Lens & Cataract (Biochemistry)Carbohydrate MetabolismHMP
Shunt / Pentose phosphate pathway:Less active pathway of G6P
utilization in the lens5% of lens glucose metabolized by this
routeThis pathway stimulated in the presence of high levels of
glucose
Lens & Cataract (Biochemistry)Carbohydrate Metabolism
(Contd)Sorbital pathwayAldose reductase has a key role in the
development of sugar cataractNormally < 4 % of lens glucose
converted to sorbitalWhen glucose increased in the lens, sorbital
pathway activated relatively more than glycolysis and sorbital
accumulates
Lens & Cataract (Biochemistry)Carbohydrate Metabolism
(Contd)Sorbital pathway (Contd)Sorbital s. dehydrogenese
fructose:(This enzyme has low afinity, hence sorbital accumulates +
poor permeability of the lens to sorbital)
Lens & Cataract (Biochemistry)Carbohydrate Metabolism
(Contd)Sorbital pathway (Contd)Sorbital + Fructose:Increased
Osmotic pressure within the lens, drawing in water Energy dependent
pumps of the lens are overwhlemedResult: Lens fibers swelling
disruption of the normal cytoskeletal architecture lens
opacification
Lens & Cataract (Biochemistry)Carbohydrate Metabolism
(Contd)GalactoseGalactose is also substrate for aldose reductase
producing galactital (Dulcitol)Galactose is not substrate for
alcohol dehydrogenese and thus rapidly accumulates
Lens & Cataract (Biochemistry)Oxidative Damage and
Protective MechanismsFree radicles generated as a result of: Normal
metabolic activities External agents (Radiant energy)
Lens & Cataract (Biochemistry)Oxidative Damage and
Protective MechanismsThese highly free radicles damage the lens
fibersOxygen tension in the lens decreases,So free radicles may not
involve molecular oxygen Instead the free radicles may react
directly with molecules
Lens & Cataract (Biochemistry)Oxidative Damage and
Protective MechanismsProtective lens enzymes against free radicles
/ oxygen damage:Glutathione peroxidaseCatalaseSuperoxide
DimutaseVit. E & C present in the lens also protect against
damage (free radicle scavengers)
