CORNEAL NUTRITION AND METABOLISME

PREPARED BY :ANIS SUZANNA BINTI MOHAMAD

A 123369

OVERVIEW:1. Metabolism

1. Definition2. Importance of metabolism

2. Nutritional1. Definition2. Condition for metabolism

3. Metabolism in cornea1. Pentose shunt2. Glycolysis3. Krebs cycle

4. Clinical condition related to metabolism

WHAT IS METABOLISM???

• A series of chemical processes by which energy is obtained and utilized to provide for the normal function of a tissue.

• 2 types of metabolism:– Aerobic metabolism

• Metabolism with O2

– Anaerobic metabolism• Metabolism without O2

WHAT ARE THE IMPORTANCE OF THE

ENERGY TO THE CORNEA???

1) Maintain of the cornea

transparency

2) Corneal dehydration

WHAT IS NUTRITION???

• Nutrition means the intake of nutrients and their subsequent absorption and assimilation by the tissues.

• Corneal epithelium consumes O2 10x greater than stroma

• May be a role for intact corneal innervations• Why it is important???

– to maintain the high metabolic rate of the epithelium

METABOLISM OF GLUCOSE IN CORNEA

• In the corneal metabolism, we must take consideration of about:

– Glucose

– Oxygen (O2)

– Amount of ATP that will be produce

– Condition of the environment

• Glucose mostly comes from aqueous humor• Tear and limbal capillaries also contribute

minimal amount of glucose and O2

• Glucose also will derive from corneal glycogen in epithelium.– 1 mol of glucose will be converted to the pyruvic acid and

produce 2 mol lactic acid and 2 mol of ATP. – In the Krebs cycle, 1 mol of glucose will utilize the pyruvic

acid and O2 to produces 36 mol ATP.

1) Glucose

• Epithelium and endothelium will consume the oxygen

• The oxygen used will come from different structures such as:– Epithelium capillaries of the limbus / limbal vasculature precorneal tear film (155 mmHg)– Endothelium aqueous humor (40 mmHg)

2) Oxygen (O2)

Besides the O2,the capillaries of the limbus / limbal vasculature, precorneal tear film and aqueous humor also provides:

glucose

Amino acids

Vitamins and other nutrients

Type of metabolism Type of cycle Amount of ATP produced

Anaerobic metabolism Glycolysis 2 ATP

Pentose shunt Not as much as in aerobic condition

Aerobic metabolism Krebs cycle 36 ATP

3) Amount of ATP that will be produces

• Aerobic condition– i.e. : Normal and healthy environment which

adequate oxygen• Anaerobic condition

– i.e. : eye are not enough oxygen when plastic contact lens glued to the stroma

– In hypoxic and normoxic conditions (lack of O2)

4) Condition of the environment

There are three processes and cycles in cornea:

1)Pentose shunt2)Glycolysis3)Kreb cycle or TCA or citric acid

cycle

1) Pentose shunt

• The pentose phosphate pathway (hexose monophosphate shunt) is used to metabolize five-carbon sugars; one ATP and 2 NADPH molecules are produced from oxidation of one glucose molecule.

• Produces intermediates for nucleic acid synthesis and some amino acids.

1) Pentose shunt This process will happen in hypoxic or normoxic

condition.

Glucose is diverted to hexose monophosphate (HM) shunt

Therefore, regulating levels of nicotinamide adenosine dinucleotide phosphate (NADPH)

NADPH will convert hexose to pentose

The process will utilize in nucleic acid synthesis

2) Glycolysis • Glycolysis is the process conversion of

glucose, by a series of ten enzyme-catalysed rxns, to lactic acid.

• The oxidation of glucose will yield pyruvic acid as the end-product.

• The two major types of glucose catabolism in cornea:– respiration glucose is completely broken down– fermentation glucose is partially broken down.

2) Glycolysis

1 •Glucose derived from the aqueous humor or from epithelial glycogen stores

2 •Then, glucose is converted to pyruvate by the anaerobic Embden-Meyerhof pathway •i.e. glycolysis

3 •The process will yielding 2 mol of ATP per glucose molecule.

3) Krebs cycle

• General meaning:• A complex cycle of enzyme-catalyzed rxns, occurring

within the cells of all living animals in which acetate, in the presence of O2, is broken down to produce energy in the form of ATP and CO2.

• E.g. : complex cycle in cornea. Under aerobic conditions, pyruvate is then oxidized in the tricarboxylic acid to yield H2O,CO2, and 36 molecules per cycle.

Overall equation for glycolysis and Krebs cycle from glucose to lactate:• Overall equation for glycolysis:

• It occurs in corneal epithelium• Energy in the form of ATP is generated by the

breakdown of glucose into lactic acid (glycolysis) and into CO2 and H2O (Krebs cycle)

• Overall equation for Krebs cycle:

Glucose 2 lactic acid (lactate) + 2ATP + 2H20

Glucose + O2 36ATP + H20 + CO2

Clinically…….

• Under hypoxic condition, such as contact lens wearer • Incraesing amounts of pyruvate are converted

dehydrogenase to lactate • Diffuses from the epithelium into stroma, inducing

epithelial in stromal edema.• The epithelial and stromal edema leads to:

– Haloes and rainbow formation– Increased glare sensitivity– Decreased contrast sensitivity

Stromal edema

Examination demonstrates corneal- ring infiltrate, central epithelial defect and stromal edema.

Stromal edema

During contact lens wear:

• O2 requirement is very important• Cornea can maintain a deturgescent state with

O2 levels as low as 25 mmHg• For RGP lens which is small diameter• Good lens movement to allow exchanged of

O2ted tears from periphery • The action also accomplished action of lid

blink.

Comparison between RGP CL and soft CLRigid gases permeable CL Soft CL

• For larger CL, i.e. soft CL especially for those with extended wear lens, adequate supply of O2 is important to reach the cornea by diffusion thru the lens.

• EW contact lens may alter the epithelial metabolism

Their used has been associated with:

®decreased rate of mitosis®Reduced the O2 uptake and glucose

utilization®Smaller numbers of intercellular

desmosomal connection between the cell

Impact of soft contact lens wear on the cornea.

The consequences of low O2 in cornea during soft CL wear:

At top line:(a) corneal edema(b) limbal redness(c) vascular response(d) epithelial microcysts(e) endothelial polymegethism.

At bottom are RGP benefits:

(a) no corneal edema(b) no increase in limbal redness(c) no blood vessels encroaching the cornea(d) few epithelial microcysts(e) no change in number or size or corneal

endothelial cells.

Questions:

• What happen to the glucose catabolism in cornea when it is in hypoxic condition?

• What are the differences between glycolysis and Krebs cycle?

Thank you for lend me your ears.