Metabolism of human lens. In the lens the biological energy necessary for the maintenance of the transparency ,synthesis, and ,repair . In the lens the biological energy necessary for the maintenance of the transparency ,synthesis, and ,repair. Glucose from the aqueous and vitreous diffuses into the lens and is rapidly metabolized. CONTINUOUS SUPPLY OF ATP REQUIRED FOR: Active transport of ions and aminoacids Maintenance of lens dehydration Lens transparency Continuous protein synthesis GSH synthesis MAJOR SITE: epithelium Glucose is rapidly metabolized through four main pathways: -Anaerobic glycolysis -Krebs cycle(oxidative) -Hexose monophosphate shunt -Sorbitol pathway

2. In the lens the biological energy
necessary for the maintenance
of the transparency ,synthesis,
and ,repair .

3. HARDING AND CRABBE (1984)
CHANG AND CHYLACK(1985)

4. The avascular lens relies on the
aqueous humor as the major source of
the oxygen , glucose , and other
nutrients needed to support its
normal metabolic activity.

5.  CONTINUOUS SUPPLY OF ATP REQUIRED
FOR:
1. Active transport of ions and
aminoacids
2. Maintenance of lens dehydration
3. Lens transparency
4. Continuous protein synthesis
5. GSH synthesis
MAJOR SITE: epithelium

7.  Glucose is very essential for the normal
working of the lens.
 Lens can survive under incubation
condition in the absence of oxygen ,so
long as an adequate supply of glucose is
available.
(but not if provided with oxygen only)

8. Glucose from the aqueous and
vitreous diffuses into the lens and
is rapidly metabolized.
Glucose is rapidly metabolized
through four main pathways:
1) Anaerobic glycolysis
2) Krebs cycle(oxidative)
3) Hexose monophosphate shunt
4) Sorbitol pathway

9.  It is not as efficient as the aerobic process.
 But obviates the problem of oxygen
starvation in a tissue
 About 85% of lens glucose is metabolized
through this process
 ENZYMES: hexokinase & phosphofructokinase
 END PRODUCT: lactic acid
 1 mole of glucose gives 2 moles of ATP

10.  1 mole of glucose gives 36 moles of ATP
 Limited to epithelium
 Only 3% of lens glucose metabolized by this
pathway
 But generates upto 20% of total ATP needs for
lens

11. It uses glucose-6- phosphate as its
initial substrate and does not generate
any ATP.
It forms pentose and NADPH
PENTOSE is utilized for RNA synthesis
and some of it is re-enters into the
glycolytic pathway
NADPH is to maintain lens glutathione
in reduced state and also neccessory
factor in sorbitol pathway

12.  5% of glucose used by the lens is metabolized
by this pathway
 It does not generate any ATP
 Hence , this pathway has received wide
attention mainly because of its pivotal role
in the development of sugar cataract.
 When sorbitol accumulates within the cells of
lens,it sets up an osmotic gradient that
induces influx of water and lens swelling ,
and ultimate loss of lens transparency
 Discovered by van heyningen (1959)

15.  PROTIEN SYSTHESIS: These are
synthesized from free amino acids
which are actively transported into the
lens from aqueous .
 Site :epithelium and peripheral cortex
 PROTIEN BREAKDOWN: in the lens it is
catalyzed by enzyme peptides and
proteases.

17.  GSH in the lens is to maintain the thiol(SH)
groups of proteins in the reduced state ,thus
prevents HMW protein aggregates.
 Helps in regulate cation transport and
permeability.
 Protects against oxidative damage to lens
 Its metabolism involves in detoxification of H2O2
which is generally present in aqueous humor.
 GST-detoxification
 Almost 12% of total ATP formed by the lens is
utilized for the GSH synthesis
 Enzymes : -glutamyltransferase & 5-prolinase
 Oxidative form—GSSG(glutathione disulphide)
 Reduced form--GSH
𝜸

18. [1] Berman ER. Biochemistry of the Eye.
Springer Science & Business Media; 1991 Apr
30.