Metabolism of lens

1. METABOLISM OF LENS
PRESENTOR- Dr. ANUSA GUPTA
MODERATOR-Dr.(Prof) BABY DEKA

2. lens
 Transparent biconvex
crystalline avascular
structure
 Location – in patellar fossa
in between iris and
vitreous
 Refractive index-1.39
 Refractive power-16 to17
D

3. BIOCHEMICAL COMPOSITION OF LENS
 Water (65%)
 Protien(34%)
Others - Lipids
Carbohydrates particularly
glucose
Electrolytes
Organic phosphates
Ascorbic acid
Glutathione
Amino acids

4. LENS PROTEIN
 Soluble
alpha crystallins- 31.7%
beta crystallins-53.4%
gamma crystallins / albumin – 1.5%
 Insoluble albuminoids – 12.5%
 Mucoproteins
 Nucleoproteins
 Others- glycoprotein,phosphoprotein,lipoprotein etc

5. Glutathione
 Produced from the interaction between glutamate and cysteine
in lens cell
 Glutathione being a tripeptide contains of three amino acids
-Glycine
-cysteine
-glutamic acid
 Glutathione exists in two forms oxidized glutathione (GSSG)
and reduced glutathione ( GSH )
 By the virtue of presence of sulfa-hydryl group (-SH) in the
cysteine fraction of glutathione
 Contributes to the redox systems in the lens microenvironment
and buffers the effects of oxidants along with other enzymes .

7. METABOLISM OF GLUCOSE
 Main source of energy
 Diffuses from – aqueous (main)
- vitreous
 4 main pathways
1. Anaerobic glycolysis
2. Krebs (oxidative pathway) cycle
3. Hexose monophosphate shunt
4. Sorbitol pathway

8. Pathways of glucose metabolism in the
crystalline lens

9. Pathway of glucose metabolism in the crystalline lens

10. Protien metabolism
 Synthesized from free amino acids that are actively
transported into lens from the aqueous
 Formation of peptides require ATP from glucose metabolism
 And RNA template
 Protein breakdown is catalyzed by the enzyme peptidases
and proteases

11. Permeability and Transport mechanism
of lens
 Active and passive (permeability dependent) of lens are
essential to
-provide nutrients for metabolism
-dispose waste products of metabolism
-regulate water and cation balance of lens.
- maintain lens lens transparency

13. Pump and leak mechanism of cation
balance of lens

14. Factors maintaining transparency
 Thin epithelium
 Regular arrangement of
lens fibres
 Little cellular organelles
 Little extra cellular space
 Orderly arrangement of
lens protiens
 Relative dehydration
 Permeability and transport
mechanism
 Avascularity
 Antioxidants

15. MECHANISM OF LOSS OF LENS
TRANSPARENCY
 CORTICAL CATARACT
• INCREASE IN WATER CONTENT
OF THE LENS
• LEAKAGE OF LMW PROTIEN
• CONVERSION OF SOLUBLE
PROTIENS INTO INSOLUBLE
PROTIENS
• DECREASED SYNTHESIS OF
LENS PROTIENS
• AN INCREASED PROTIEN
CATABOLISM
 NUCLEAR CATARACT
• Intensification of age related
nuclear sclerosis associated
with
• Dehydration and
compaction of nucleus
• Associated with increase in
water insoluble protiens

16. FLOW CHART DEPECTING PROBABLE COURSE OF
EVENT IN OCCURRENCE OF CORTICAL CATARACT
?

17. Possible common pathways in human
cataract

18. ROLE OF GLUTATHIONE AND ASCORBIC
ACID IN CATARACTOGENESIS
 Both acts as physiological scavenger of superoxide radicals
 Coverts highly reactive superoxide to less reactive
hydrogen peroxide
 Decreased levels causes oxidative damage leading to
cataract formation

19. Depletion of glutathione during cataract
formation

20. Different types of Metabolic cataract

21. Diabetic cataract
 Due to accumulation of
sorbitol leading to
osmotic overhydration
of the lens
 When glucose is
metabolized by aldose
reductase

22. Course of events in a Diabetic Cataract
Glucose
Aldose
reductase
Sorbitol
Retained within the lens
Osmotic gradient
Vacoule formation
Swelling
Opacification

23. Galactosaemic Cataract
 Associated with inborn
error of galactose
metabolism
 Due to deficiency of
galactose 1 phosphate
uridyl transferase
(GPUT)
 B/L oil droplet cataract

24. Hypocalcaemic (tetanic)
cataract
 Calcium is necessary for
maintaining membrane
integrity
 Deficiency leads to
membrane disruption and
increased prmeabiliaty
leading to cataract
 Zonular cataract in infants
 B/L Cortical cataract in adults

25. Wilsons disease
 Autosomal recessive
inborn error of copper
metabolism
 Characterized by
sunflower cataract
 Also characterized by
Kayser-Fleischer Ring in
the cornea

26. Lowes (oculo cerebro renal)
syndrome
 x- linked recessive inborn
error of copper metabolism
 Ocular features include
 Congenital cataract
 Congenital glaucoma
 Blue sclera

27. MYOTONIC
DYSTROPHY
 Autosomal dominant disorder
due to defect in gene encoding
myosin protein
 Characterized by Christmas
tree cataract.
 And ptosis

28. Cataract in some other metabolic disorder
Disorder Features
Fabrys disease  Spoke like lens opacities
 Cornea verticillata
Mongolian idiocy and hypothyroidism  Posterior subcapsular cataract
Mannosidosis  Spoke like /wheel like posterior
subcapsular opacities

29. REFERENCES
 DISORDERS OF LENS AMD CATARACT SURGERY BY
AK KHURANA
 LENS AND CATARACT BY AMERICAN ACADEMY OF
OPHTHALMOLOGY
 ADLERS PHYSIOLOGY OF THE EYE.
 ALBERT JAKOBIECS PRINCIPAL AND PRACTICE OF
OPHTHALMOLOGY