This presentation is orginaly uploaded to http://kpkmedicalcolleges.tk by Dr.Suleman

1. LENS & CATARACT
Prof. Naimatullah Khan Kundi
Head, Department of Ophthalmology
Khyber Teaching Hospital
Peshawar

2. Lens & Cataract
(Biochemistry)
 Molecular Biology

Lens proteins conc. 33% structural proteins

Fibers Proteins:
1. Crystallins - Water soluble group 86% (a, b, r)
2. Water insoluble fraction

3. Lens & Cataract
(Biochemistry)
 Molecular Biology

Crystallins: Intracellular (within the plasma membrane of the
lens epithelium and fiber cells)

Water insoluble fraction associated with fiber plasma
membranes

Brown cataract (Brunescent) – Amount of insoluble proteins correlate with
degree of opacification (90% of nuclear proteins insoluble)

4. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism
 Glucose – (hexokinase)

G6P – Enters

Anaerobic Glycolysis

Pentose Phosphate Pathway / HMP shunt

5. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism

Anaerobic Glycolysis:

More active

Provides most of the high energy phosphate bonds for
lens metabolism

6. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism

HMP Shunt / Pentose phosphate pathway:

Less active pathway of G6P utilization in the lens

5% of lens glucose metabolized by this route

This pathway stimulated in the presence of high levels of
glucose

7. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism (Cont’d)
 Sorbital pathway

Aldose reductase has a key role in the development of
sugar cataract

Normally < 4 % of lens glucose converted to sorbital

When glucose increased in the lens, sorbital pathway
activated relatively more than glycolysis and sorbital
accumulates

8. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism (Cont’d)
 Sorbital pathway (Cont’d)

Sorbital – s. dehydrogenese  fructose:

(This enzyme has low afinity, hence sorbital
accumulates + poor permeability of the lens to
sorbital)

9. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism (Cont’d)
 Sorbital pathway (Cont’d)
 Sorbital + Fructose:

Increased Osmotic pressure within the lens, drawing
in water

Energy dependent pumps of the lens are
overwhlemed

Result: Lens fibers swelling  disruption of the
normal cytoskeletal architecture  lens

10. Lens & Cataract
(Biochemistry)
 Carbohydrate Metabolism (Cont’d)

Galactose

Galactose is also substrate for aldose reductase
producing galactital (Dulcitol)

Galactose is not substrate for alcohol dehydrogenese
and thus rapidly accumulates

11. Lens & Cataract
(Biochemistry)
 Oxidative Damage and Protective
Mechanisms
 Free radicles generated as a result of:
 Normal metabolic activities
 External agents (Radiant energy)

12. Lens & Cataract
(Biochemistry)
 Oxidative Damage and Protective
Mechanisms
 These highly free radicles damage the lens fibers
 Oxygen tension in the lens decreases,
 So free radicles may not involve molecular
oxygen
 Instead the free radicles may react directly with
molecules

13. Lens & Cataract
(Biochemistry)
 Oxidative Damage and Protective
Mechanisms
Protective lens enzymes against free radicles /
oxygen damage:
 Glutathione peroxidase
 Catalase
 Superoxide Dimutase
 Vit. E & C present in the lens also protect against
damage (free radicle scavengers)