Vitamin A Functions

Vitamin A does not act in a uniform manner. The various derivatives found in the body exert their effects through different functional mechanisms:

  • Retinol is a transport form and metabolic intermediate.
  • Retinal is an essential component required for vision.
  • Retinoic acid, in its cis-trans form, and its polar metabolites have pronounced effects on proliferation and differentiation of various tissues such as respiratory epithelium, intestinal mucosa, skin, and various tumor and embryonic cells. Furthermore they inhibit a range of tumor promoters.
  • Retinyl esters are the storage forms of this vitamin. They are predominantly retinyl palmitate, but also stearate, oleate and others. Main storage sites are the liver and other organs that depend on vitamin A's functions (e. g., retina, testes, and lungs).
  • Glucuronidated compounds are the form in which the vitamin is excreted, but have also been found to have biological effects on growth and differentiation in vitro.

latter transports the 11-cis-retinal to the segment disks where it is bound to the apoprotein opsin. This protein-retinal complex, rhodopsin, absorbs light within the 400-600 nm range. Incoming light isomerizes 11-cis-retinal to all-trans-retinal, thereby separating it from opsin. The separation causes a conformational change in the rho-dopsin, which triggers a cascade. A phosphodiesterase binds to a G-protein and is thereby activated, causing cGMP hydrolysis. Declining cGMP levels cause the closing of Na+ channels; the cell becomes hyperpolarized, increasing the potential difference between the inside and outside. As a result the cell releases fewer transmitters, reducing neuronal excitement and that signals "incoming light" to the brain.

The photolyzed rhodopsin is unstable, and splits into opsin and all-trans-retinal. In order to regenerate the rhodopsin, the all-trans-retinal first has to be isomerized to 11-cis-retinal. For this purpose, the all-trans-retinal is reduced, and the retinal is then reintro-duced into the pigment cells' cycle.

Visual function (A) is mostly understood by now, at least as far as the rods are concerned, which are responsible for black-and-white vision at low light intensities. Retinol is absorbed from the outer capillaries, bound to CRBP, isomerized, oxidized to retinal, and transferred to a cellular retinal-binding protein (CRALBP). Retinol can be esteri-fied inside the pigmented epithelial cells to the all-trans- as well as the cisform, making it storable. 11-cis-retinal, bound to CRALBP, enters the inter-photoreceptor matrix where it is transferred to interphotoreceptor retinoid-binding protein (IRBP). The r A. Vision -

Holo-RBP-trans-thyretin complex

Bruch's membrane

All-trans-retinol

Holo-RBP-trans-thyretin complex

Bruch's membrane

All-trans-retinol

Retinyl-acyl-transferase

All-trans-retinyl palmitate

11-cis-Retinyl palmitate

Retinyl-acyl-transferase

All-trans-retinyl palmitate

11-cis-Retinyl palmitate

All-trans-retino

11-cis-RetindTORALBP

All-trans-retino

G Retinol isomerase

lol'CRBP

lol-CR

11-cis-RetindTORALBP

Retinyl-acyl transferase

©11-äs-Reti not deh dehydrogenase

ial*CR

Pigment epithelial cell

11-cis-RetinffFtRALBP

11-cis-Retinal All-trans- '

•IRBP retinol-IRBP

Pigment epithelial cell

11-cis-Retinal All-trans- '

•IRBP retinol-IRBP

Cis Retinyl Palmitate

Interphoto-receptor matrix inol dehydrogenase

Interphoto-receptor matrix

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