RETINENE ISOMERASE

Author:

Hubbard Ruth1

Affiliation:

1. From the Biological Laboratories, Harvard University, Cambridge

Abstract

Rhodopsin is formed by the condensation of opsin with a cis isomer of retinene, called neo-b. The bleaching of rhodopsin releases all-trans retinene which must be isomerized back to neo-b in order for rhodopsin to regenerate. Both retinene isomers are in equilibrium with the corresponding isomers of vitamin A, through the alcohol dehydrogenase system. An enzyme is found in cattle retinas and frog pigment layers which catalyzes the interconversion of all-trans and neo-b retinene. We call it "retinene isomerase." It is soluble in neutral phosphate buffer, and precipitates between 20 and 35 per cent saturation with ammonium sulfate. In the dark, the isomerase converts all-trans and neo-b retinene to an equilibrium mixture of 5 parts neo-b and 95 parts all-trans. With opsin present to trap neo-b, the isomerase catalyzes the synthesis of rhodopsin from all-trans retinene. This reaction, however, is too slow to account for dark adaptation. Retinene is isomerized by light, but too slowly to supply the retina with neo-b. In aqueous solution the pseudoequilibrium mixture contains about 15 per cent neo-b. When all-trans retinene is irradiated in the presence of isomerase, the rate of formation of neo-b is increased about 5 times, and the pseudoequilibrium shifted so that the mixture now contains about 32 per cent neo-b. The isomerase is specific for all-trans and neo-b retinene. It does not act on two other cis isomers of retinene, nor on all-trans or neo-b vitamin A. The role of the isomerase in vision appears to be as follows: in the light, as rhodopsin is bleached to opsin and all-trans retinene, the latter is in part converted to the neo-b isomer and stored in the pigment epithelium as neo-b vitamin A. During dark adaptation, the dominant process is the trapping by opsin of neo-b retinene supplied from stores of neo-b vitamin A, and the slow isomerase-catalyzed "dark" conversion of all-trans to neo-b retinene.

Publisher

Rockefeller University Press

Subject

Physiology

Cited by 164 articles. 订阅此论文施引文献 订阅此论文施引文献,注册后可以免费订阅5篇论文的施引文献,订阅后可以查看论文全部施引文献

1. Defective Trafficking of Rhodopsin and Its Role in Retinal Degenerations;International Review of Cell and Molecular Biology;2012

2. Fifty years of dark adaptation 1961–2011;Vision Research;2011-11

3. Chapter 29 Historical aspects of the major neurological vitamin deficiency disorders;Handbook of Clinical Neurology;2009

4. Some Photochemical Transformations of Polyenic Compounds;Advances in Photochemistry;2007-01-05

5. Retinoids and the Chemical Biology of the Visual Cycle;Encyclopedia of Molecular Cell Biology and Molecular Medicine;2006-09-15

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