Winter 2010 Biochem Final

The cornea is made up of 5 layers.

1. Epithelial layer which is 8-10% of total thickness
2. Basal lamina
3. Stroma (90% of the total thickness)
4. Descemet’s membrane
5. Inner endothelial layer
***Not vascularized

Oxygen enters from the pre-ocular tear film, and glucose enters through the stroma

Glucose --> glucose-6-phosphate by hexokinase

-65% of G-6-P --> pyruvate by glycolysis
-35% utilized by the PPP for the primary purpose of generating sufficient NADPH to keep gluthionine and ascorbate reduced

Vitamin A causes epi keratinization, loss of conjunctival goblet cells, spots on the conjunctiva and night blindness

-eventually corneal erosion

Delivered from the tears

1. Controls keratin expression in the epithelium
2. Involved in the synthesis of glycoconjugates
3. Activates and induces creatine kinase

Corneal endothelium which consists of a single layer of hexagonal shaped cells lining the posterior cornea and resting on Descemet's membrane

-also it provides a physical barrier controlling the entry of fluid and dissolved solutes into stroma from the aqueous humor

-90%
-consists mainly of extracellular matrix of collagen and ground substance (proteoglycans, glycoproteins, and protein)
-12-15% collagen

-reduction product fromed from glucose, and is a polyol

Gluc + NADPH + H+ --> Sorbitol + NADP+

-Enzyme is Aldol Reductase

Galactose + NADPH --> Galactitol + NADP+

-galactitol is also known as dulcitol

-sorbitol does not contain an aldehyde or ketone functional group like the monosaccharides we've studied

-involved in the early stages of diabetic cataractogenesis
-also can cause osmotic effects such as the disruption of layers of the eye lens or cornea by drawing too much water into the stroma
*galactitol too

Sorbitol + NAD+ --> fructose + NADH + H+
-Enzyme is Iditol dh (polyol dh)
-fructose be phosphylated and metabolized in the glycolysis pathway

GALACITITOL NOT METABOLIZED

-an extension of ISF,not really secreted
-viscosity due to the [hyaluronic acid]
-normally clear and pale yellow
-[protein] is lower than plasma

-Inflammatory joint diseases lead to an increase in total volume and changes in composition
- [protein] increases by factor of 4
- globulin increases
-albumin decreases
-becomes cloudy increases leukocytes
-fibrinogen appears, and fluid will clot under pathologic conditions

-fills the anterior chamber of eye
-fluid is to maintain intraocular pressure
-nourish the cornea, lens, and iris as well as act as a lens
-2.8 ml/day secreted by the ciliary body via fenestrated caps
-enters the post chamber and flows through the iris to the ant chamber
-leaves the eye at the iridocorneal angle thrugh the canal of Schlemm
-blockage leads to glaucoma treated by Timolol and Acetazolamide

-composition Na, K, Mg, bicarb, gluc, citrate and urea [] similar to plasma
-[protein] is much lower
-[ascorbate] is much higher
-lactate is 1.5x the plasma level

-provides O2 to anterior lens and posterior cornea, only source of O2 when eyes closed
-proteins present 50% is albumin, transferrin, alpha-1 acid glycoprotein, and immunoglobulins with almost no enzymes present

-located in the posterior chamber of the eye
-clear gelatinous material
1. maintains intraocular pressure
2. offers mechanical support as a kind of shock absorber
3. acts as a source and sink of metabolites for retina
4. acts as a lens
-over 98% water and about 1% colloids

-Na, Cl, lactate levels similar to plasma
-protein much lower, ascorbate higher
-phosphate, glucose and free amino acid levels are a few tenths of plasma
-potassium levels are 1.7 that of plasma and used forensically estimate time of death

-hyaluronic acid is secreted by the retina provides the necessary viscosity
-w/ aging vitreous liquefaction occurs--> vitreous as well as retinal detachment, opacifications are imaged by ultrasound

-[electrolyte] greater overall [protein] is less
-proteins include lysozyme, lactoferrin, a tear specific prealbumin, albumin, and immunoglobins
-enzymes necessary for glycolysis the TCA cycle and PPP: amylase peroxidases, plaminogen activator, and lysosoamal hydrolases

-Tay sachs (no hexosaminidase A)
-Fabry's (no alpha-galactosidase)
-Hurler's (no alpha-iduronidase)

-the glucose levels are 2.5/dl and can be used to test comatose pts for hyperglycemia with Clinistix

1. Lubrication and oxygenation of corneal tissues - tears provide most of the O2 of the cornea with eyes open

2. Protection
a. lysozyme breaks B 1-4 linkages in bact cell walls
b. lactoferrin binds iron, reducing bact growth
c. tear-specific prealbumin binds retinol,may have bacteriacidal activ

3. Wetting of the optical surface - a smooth overall surface reduces refraction errors

sensible - when droplets are visible

Insensible - when no visible perspiration is produced