VISION Biochem Mau

THE EYE: METABOLISM and VISION

MAU

THE EYE

AQUEOUS HUMOR• Anterior Chamber• Bathes the interior of the cornea

(tears bathe the exterior)• Isoosmotic fluid that contains:

– Salts– Albumin– Globulin– Glucose– Other constituents

AQUEOUS HUMOR vs VITREOUS HUMOR

• AQUEOUS HUMOR• For Cornea and Lens:

– Brings Nutrients– Removes end products of

metabolism• VITREOUS HUMOR• Gelatinous mass that helps

maintain the shape of the eye while allowing it to remain somewhat pliable.

THE CORNEA

Cornea derives ATP from Aerobic Metabolism

• Eye is extension of nervous system

• Major metabolic fuel = GLUCOSE

• Cornea is not a homogenous tissue, it consists of:

1. Anterior corneal epithelium

2. Bowman’s membrane

3. Stroma (Substantia Propria) – type I collagen, 90% of corneal thickness

4. Descemet’s membrane

5. Endothelium (Posterior Corneal Epithelium)

A B S D E

CORNEA

• Clear tissue, diffracts light (like lens), no blood vessels

• Permeable to water and oxygen– To maintain clarity– Has a water pump, uses ATP from TC

and CHO metabolism

CORNEA

• Corneal stroma water content must be controlled – to maintain clarity (ATP-driven water pump)

• Clearness due to:• 1. arrangement of collagen molecules

of the stroma.• 2. lack of blood vesicles in the

epithelial layer (due to VEGFR-3)

VEGFR-3 (Vascular Endothelial Growth Factor

Receptor – 3)• Large amount of protein found in Anterior Epithelial Layer

• Prevents growth of blood vessels by:1. binding to growth factors or;2. neutralizing growth factors that are produced to stimulate blood vessel growth.

CORNEA• Obtains its ATP from Aerobic Glucose

Metabolism: Glycolysis and TCA cycle.

• Lactate does not accumulate due to efficient use of Pyruvate (oxidative metabolism) and presence of water and oxygen.

• Glucose metabolism• 30% - glycolysis• 65% - hexose monophosphate pathway

CORNEA

• Highest activity of HMS (PPP) of any mammalian tissue.

• High activity of GLUTATHIONE REDUCTASE that requires NADPH (product of HMS)

CORNEA• Corneal Epithelium is

permeable to atmospheric oxygen.

• Reactions to oxygen can lead to formation of various ACTIVE OXYGEN SPECIES that are harmful to tissues by:

• 1. Oxidizing protein sulfhydryl groups to disulfides

• 2. Lipid Peroxidation, mostly of cellular medium –chain lipids of six carbons or more.

CORNEA

• Reduced glutathione (GSH) – reduces disulfide bonds and lipid peroxidases (back to original native states)

• GSSG (oxidized glutathione) formed by:1. GSH itself2. Active oxygen species (directly)

CORNEA

• Glutathione Reductase uses NADPH to reduce GSSG to 2GSH.

CORNEA

• The pentose phosphate pathway and glutathione reductase help protect the cornea (neutralizing active oxygen species) from oxygenation along with oxygen and water permeability

CORNEA

• Some lipids that are subjected to peroxidation may spontaneously form active aldehydes that react with other tissue components and lead to various pathological conditions.

CORNEA

• The cornea also contains an Isoform of ALDEHYDE DEHYDROGENASE (ALDH3A1) – member of a superfamily of enzymes that uses either NAD+ or NADP+ to inactivate these active aldehydes by oxidizing them to their corresponding acids states.

THE LENS

LENS Consists Mostly of Water and Protein

• Bathed on one side by aqueous humor

• Supported on other side by vitreous humor

• No blood supply, but metabolically active

• AQUEOUS HUMOR – for obtaining nutrients and eliminating wastes.

• Mostly water and proteins

• The majority of vertebrate lens proteins are α-, β-, and ɣ- crystallins

• There are also albuminoids, enzymes and membrane proteins that are synthesized in an epithelial layer around the edge of the lens.

LENS

LENS• Other animals have different

crystallins.

• The most important physical requirement of these proteins is that they maintain a clear crystalline state.

• They are sensitive to changes in oxidation-reduction, osmolarity, excessively increased concentration of metabolites and UV irradiation.

LENS• Structural integrity of the lens is

maintained:•

– for osmotic balance by the Na+/K+ exchanging ATPase

– for redox-state balance by glutathione reductase

– For growth and maintenance by protein synthesis

– Other metabolic processes (peripheral cells of the lens)

LENS

• The primary role of most lens proteins is to function as crystallins, but many are expressed in other tissues and serve other roles such as enzymes and/or have other functional roles.

LENS• α- and β- crystallins are SMALL HEAT

SHOCK PROTEINS (sHSP) or CHAPERONES – help maintain lens proteins in their native, unaggregated states.

• Their highest expression is in eye lens, but they also occur in other tissues such as skeletal and cardiac muscle where they are involved in filament assembly.

LENS

• Mutations in α- and β- crystallins:– CATARACT FORMATION

– Possible MUSCLE WEAKNESS and HEART FAILURE

LENS

• Energy for these processes comes from the metabolism of glucose.

• Glucose metabolism in lens tissue (periphery)

• 85% - glycolysis• 3% - TCA cycle• Remaining (12%) – PPP

LENS

• The central area of the lens, the nucleus or core, consists of lens that were present at birth.

• The lens grows from the periphery and in humans increases in weight and thickness with age and becomes less elastic

LENS

• This leads to a loss of near vision, a normal condition referred to as presbyopia.

• On average, the lens may increase threefold in size and approximately 12-fold in thickness from birth to about age 80

LENS

• CATARACT – the only known disease of the lens, is an opacity of lenses brought about by many different conditions.

• Two most common types of cataracts 1. senile cataracts 2. diabetic cataracts

SENILE CATARACTS OF LENS

Changes in architectural arrangement of the lens crystallins and other lens proteins are due to:1. Age.

2. breakdown of the protein molecules starting at the C-terminal ends, deamidation and racemization of aspartyl residues.

DIABETIC CATARACTS OF THE LENS

• These result from loss of control of osmolarity of the lens due to increased activity of ALDOSE REDUCTASE and POLYOL (ALDOSE) DEHYDROGENASE of the Polyol Metabolic Pathway.

• When glucose concentration in the lens is high, aldose reductase converts some of it to SORBITOL that may be converted to FRUCTOSE by polyol dehydrogenase.

POLYOL PATHWAY

DIABETIC CATARACT of the LENS

• In human lens, the ratio of activities of these ratio enzymes favors sorbitol accumulation, especially since sorbitol is not used by other pathways and it diffuses out of the lens rather slowly.

DIABETIC CATARACT OF THE LENS

• Accumulation of sorbitol increases osmolarity of the lens, affects the structural organization of the crystallins, and enhances the rate of protein aggregation and denaturation.

• Areas where this occurs have increased light scattering capacities, which is the definition of CATARACTS

DIABETIC CATARACT OF THE LENS

• Normally, sorbitol formation is not a problem because the Km of aldose reductase for glucose is about 200mM and very little sorbitol would be formed.

• In diabetics, where circulating concentration of glucose, activity of this enzyme can be significant.

CATARACT OF THE LENS

• Cataracts affect millions of people per year throughout the world, and there are no known cures or preventative measures, especially for the senile type.

Cataract Surgery and Lens Replacement

• The most common remedy is lens replacement; a routine operation in many countries.

• A side effect of cataract and surgical treatment for it can be glaucoma; but this is rare.

THIRD CAUSE OF CATARACTS

• A third cause, especially among young people, is due to inherited mutations in crystallins that function in lens as CHAPERONES.

• When there are mutations in chaperones that interfere with their function, misfolded proteins can occur and result in cataract formation.

THE RETINA

RETINA derives ATP from ANAEROBIC GLYCOLYSIS

• The retina, like the lens, depends heavily on anaerobic glycolysis for ATP production.

• Unlike the lens, the retina is a VASCULAR TISSUE.

• In its center is the MACULA, and in the center of the macula is the FOVEA CENTRALIS, an avascular concave area that contains only cones.

RETINA: FOVEA CENTRALIS

• Center of macula• Avascular concave area• Contains only CONES• Area of GREATEST VISUAL

ACUITY

• Mitochondria are present in retinal rods and cones but not in the outer segments where visual pigments are located.

CLINICAL CORRELATION• MACULAR DEGENERATION and LOSS OF VISION

MACULAR DEGENERATION and LOSS OF VISION

• Many diseases of the eye affect vision, but not all have clear, direct biochemical origins.

• The most serious eye diseases are those that result in blindness.

• GLAUCOMA is the most common and is often associated with DM, the biochemistry of which is fairly well known.

• Glaucoma is treatable and blindness does not have to be a result.


• Macula degeneration leads to blindness and there is no cure.

• The macula is a circular area of the retina, the center of which is the fovea centralis, the area of greatest visual acuity, which contains the highest number of cones.


• Age-related macula degeneration (AMD) may be among the leading causes of blindness of people over the age of 50 and is of two types: DRY and WET

• The dry form gradually develops over time

• The wet form develops rapidly and can lead to blindness within days. It occurs when blood vessels invade and/or rupture under the macula leading to a rapid loss of vision.


• One current therapy under assessment for neovascular age-related macular degeneration is intravitreal RANIBIZUMAB.

• Ranibizumab is a humanized monoclonal antibody fragment that inhibits VEGF (vascular endothelial growth factor)

• In 2006, a Phase 3 trial showed that this treatment is somewhat effective.


• Another therapy involves the use of siRNA to inhibit expression of the gene for VEGF. VEGF promotes excessive blood vesicle growth behind the retina.

• These blood vessels leak and cause blindness. siRNA treatment has been quite effective in treating AMD. This is the first siRNA therapy to be given to patients in clinical trials.

• Thus, advances in basic science and medical research continue to reveal better ways of treating and possibly preventing the onset of AMD.


• Rupture of blood vessels that obscure macula details and result in rapid onset of blindness may be temporary.

• Several cases of sudden visual loss associated with sexual activity, but not with a sexually transmitted disease, have been reported.


• Vision was lost in one eye apparently during, but most often was reported a few days after engaging in, highly stimulatory sexual activity.

• Blindness was due to rupture of blood vessels in the macula area.

• Most patients were reluctant to discuss their ophthalmologist what they were doing when sight loss was first observed.


• Four patients recovered with restoration of vision on reabsorption of blood.

• In one case, blood was trapped between the vitreous humor and the retinal surface directly in front of the fovea.

• The hemorrhage cleared only slightly during the next month, but visual acuity did not improve. That patient did not return for a follow-up examination, but there was no indication during the initial examination that the condition was permanent.


• Since most of the victims of this phenomenon were over the age of 39, it may be a worry more to professors than to students.

• It also may give a new meaning to the phrase, “Love is blind”

VISUAL TRANSDUCTION

VISUAL TRANSDUCTION INVOLVES

PHOTOCHEMICAL, BIOCHEMICAL and

ELECTRICAL EVENTS• Light enters the eye, • and when it reaches the

retinal membrane, • it passes the optic nerve

fibers, • the ganglion neurons, • the bipolar neurons, and • the nuclei of rods and

cones before it reaches • the outer segment of the

rods and cones where • the signal transduction

process begins.

LAYERS OF THE HUMAN RETINAL MEMBRANE

• P• R• C• O• O• H• A• M• I• G• O

Pigment Epithelium

RODS

CONES

Outer Nuclear

Outer Plexiform

Inner NuclearH- Horizontal

B – Bipolar CellsA – Amacrine Cells

M – Muller Cells

Inner Plexiform

Ganglion Cell Layer

Optic Nerve Fiber

VISUAL TRANSDUCTION• The tips of the rods and cones penetrate the

pigmented epithelial layer of the retina.

• The pigmented epithelial layer of the retina is:

• 1. involved in the trans- to cis- retinal recycling phase of the visual cycle, and

• 2. it also absorbs excess light and prevents reflections back into rods and cones where it may cause distortion or blurring of images.

• The CHOROID lies behind the retina and contains blood vesicles that supply nutrients to the retina.

VISUAL TRANSDUCTION• The eye may be compared to a

video camera, which collects images, converts them into electrical pulses, records them on magnetic tape, and allows their visualization by decoding the taped information.

• The eye focuses on an image by projecting that image onto the retina.

VISUAL TRANSDUCTION• A series of events begins, the first

of which is PHOTOCHEMICAL, followed by biochemical reactions that amplify the signal, and finally electrical impulses are sent to the brain where the image is reconstructed – “in the mind’s eye”

• PHOTOCHEMICAL BIOCHEMICAL ELECTRICAL

VISUAL TRANSDUCTION

• In effecting this process, the initial event is transformed from a physical event to a chemical event through a series of biochemical reactions, to an electrical event, to a conscious acknowledgement of the presence of an object in the environment outside the body.

VISUAL TRANSDUCTION• Photons (light) are absorbed by

photoreceptors in the outer segments of rods or cones, where they cause isomerization of the visual pigment, retinal, from the 11-cis-form to the all-trans-form.

• The isomerization causes a conformational change in the protein moiety of the complex and affects the resting membrane potential of the cell, resulting in an electrical signal being transmitted by way of the optic nerve to the brain.

CLINICAL CORRELATION• NIEMANN-PICK DISEASE and RETINITIS PIGMENTOSA

NIEMANN-PICK DISEASE and RETINITIS PIGMENTOSA• There are central nervous system

disorders associated with the Niemann-Pick group of diseases that can become evident by ocular changes.

• Some of these are observed as abnormal macula with gray discoloration and granular pigmentation or granule opacities about the fovea

NIEMANN-PICK DISEASE and RETINITIS PIGMENTOSA• Acute Type I Niemann-Pick disease,

lipidosis with sphingomyelinase deficiency and primary sphingomyelin storage, may show a CHERRY RED SPOT in the retina in up to 50% of these patients.

• Macula halo describes the crystalloid opacities seen in some patients with subacute type I disease. These halos are approximately one-half the disk diameter at their outer edge and are scattered throughout the various layers of the retina. They do not interfere with vision.

NIEMANN-PICK DISEASE and RETINITIS PIGMENTOSA• An 11 y/o girl with type II disease had

more extensive ocular involvement. There was sphingomyelin storage in the keratocytes of the cornea, the lens, the retinal ganglion cells, the pigmented epithelium, the corneal tract, and the fibrous astrocytes of the optic nerve.

• Thus, retinitis pigmentosa may also be a secondary effect of the abnormal biochemistry associated with Niemann-Pick disease.

RETINITIS PIGMENTOSA

Loss of Rods and Cones

RODS AND CONES

RODS and CONES are PHOTORECEPTOR CELLS

• PHOTORECEPTOR CELLS of the eye are the rods and cones.

• Each type has flattened disks that contain a photoreceptor pigment that consists of a protein and a prosthetic group, 11-cis-retinal.


• The pigment is rhodopsin (chromosome 3) in rod cells, and the pigments in cone cells are:– Red (Long)– Green (Medium) or;– Blue (Short)

– Red and green = x chromosome, blue pigment, chromosome 7


• Rhodopsin, as are other photoreceptor pigments, is a transmembrane protein containing 11-cis-retinal, the protein moiety of which is called OPSIN

• The three proteins that form the red, green, and blue pigments of cone cells are different from each other and from the opsin of rhodopsin; but the prosthetic group for all visual pigments is the same.

RODS AND CONES• RHODOPSIN

(~40kDa) contains seven transmembrane α – helices.

• An 11-cis-retinal molecule is attached through a protonated Schiff base to the Ɛ-Amino group of Lys296 on the seventh helix and lies about midway between the two faces of the membrane.

• β-carotene 11-cis retinal

• Opsin + 11-cis-retinal Rhodopsin

RODS AND CONES

RODS AND CONES• 11 Cis Retinal is

derived from Vitamin A and/or β-carotene of the diet.

• • Cleavage of β-

carotene yields two molecules All-trans-retinol, which is isomerized by an enzyme in the pigmented epithelial cell layer of the retina to 11-cis-retinol.

• 11-Cis-Retinol is then oxidized to 11-cis-retinal and transported to the rod outer segment where it binds to opsin.

THE VISUAL CYCLE

RODS AND CONES• The absorption

spectra of 11-cis-retinal and the four visual pigments are shown.

• The wavelength of maximum absorption of 11-cis-retinal is different when it binds to opsin (λmax = 498nm) or to the protein moieties of other visual pigments.

RODS AND CONES• Spectral

differences are due to subtle differences in the chemical environments in which the 11-cis-retinal resides.

• Absorption bands for the pigments reflect spectral areas of their light sensitivity.

1. why is it not advisable for teenagers with near

sightedness to get Lasix surgery?

• LASIK is safe for teens, but it is not recommended for most teenagers. It is best to have LASIK only when you have stopped growing and your eyes have stopped changing, which is frequently in the very late teenage years or during your twenties. If you have LASIK before your eyes stop changing, you will need to come back and have further LASIK after your eyes stop changing

2. why does ortistat or xenical cause night vision

blindness?• Orlistat (Xenical) works in the

gastrointestinal tract, blocking an enzyme that is needed to digest fat. Instead of being absorbed into the body, up to one third of the fat that a person consumes will accumulate in the intestines and be excreted in the stool. Orlistat (Xenical) also blocks the absorption of needed fat-soluble vitamins A, D, E, and K, as well as beta-carotene, so daily vitamin supplements must be taken.

2. why does ortistat or xenical cause night vision

blindness?– Vitamin A (or Vitamin A Retinol, retinal,

and four carotenoids including beta carotene) is a vitamin that is needed by the retina of the eye in the form of a specific metabolite, the light-absorbing molecule retinal, that is necessary for both low-light (scotopic vision) and color vision. Vitamin A also functions in a very different role as an irreversibly oxidized form of retinol known as retinoic acid, which is an important hormone-like growth factor for epithelial and other cells

•

References

• Devlin Biochemistry pp.953-959• Dr. Taylor’s Lecure

Thank You!

VISION Biochem Mau

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