rhodopsin (redirected from metarhodopsin)

rhodopsin /rho·dop·sin/ (ro-dop´sin) visual purple; a photosensitive purple-red chromoprotein in the retinal rods that is bleached to visual yellow (all-trans retinal) by light, thereby stimulating retinal sensory endings.

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rho·dop·sin (r-dpsn)

n.

A thermolabile protein that is sensitive to red light and is found in the external segments of the rods of the retina of the eye. Also called visual purple.

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rhodopsin

[rōdop′sin]

Etymology: Gk, rhodon, rose, opsis, vision

the purple pigmented compound in the rods of the retina, formed by a protein-opsin-and a derivative of vitamin A, retinal. Rhodopsin gives the outer segments of the rods a purple color and adapts the eye to low-density light. The compound breaks down when struck by light, and this chemical change triggers the conduction of nerve impulses. Brief periods of darkness allow the opsin and the retinal to reconstitute the rhodopsin, which accounts for the short delay a person experiences in adapting to sudden or drastic changes in lighting, as when moving out of bright sunlight into a darkened room or from darkness into bright light. Closing the eyes is a natural reflex that allows reconstitution of rhodopsin. Compare iodopsin. See also visual purple.

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rhodopsin (rōdop´sin),

n a purple light-receptive pigment found in the retina and consisting of opsin and retinal. Rhodopsin helps the eye adjust to drastic changes in environmental lighting.

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rhodopsin

visual purple: a photosensitive purple-red chromoprotein in the retinal rods that is bleached to visual yellow (all-trans-retinal) by light, thereby stimulating retinal sensory endings. Lack of rhodopsin results in night blindness. Vitamin A is the primary source of rhodopsin.

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rhodopsin 

Visual pigment contained in the outer segments of the rod cells of the retina and involved in scotopic vision. When light stimulates the retina, the chromophore of the pigment molecule '11-cis' retinal (which is vitamin A aldehyde) isomerizes to 'all-trans' retinal. This leads to other chemical transformations which carry on even in the absence of light. The first stage is prelumirhodopsin, then lumirhodopsin and finally metarhodopsin (of which there are two types). This last transformation may lead to the breakdown of the molecule into retinal and opsin. The molecule is regenerated by recombining retinal and opsin with some enzymes. The absorption spectrum of rhodopsin has a maximum around 498 nm. The isomerization from '11-cis' to 'all-trans' also gives rise to the process of transduction in which the membrane potential covering the pigment molecules in the outer segment changes towards a hyperpolarization of the cell. This is the first step in the nervous response to a light stimulation of the retina. Syn. visual purple (not used any more); erythropsin. See dark adaptation; bleaching; receptor potential; absorption spectrum; transduction.