Properties of the pigment layer factor in the regeneration of rhodopsin.

Rhodopsin (visual purple), the photolabile chromoprotein in the retinal rod cells of most vertebrates, bleaches in the light with the release of unstable orange lipides (1) which appear to be precursors (2, 3) of vitamin A aldehyde (retinene) (4, 5). The aldehyde is ultimately reduced to vitamin A alcohol by an enzymatic process (2), which requires reduced diphosphopyridine nucleotide as coenzyme (6). The reduction is reversible, and occurs also in the presence of liver alcohol dehydrogenase (7, 8). Regeneration of Visual Purple in Retina-In the living animal, visual purple is constantly reformed as it is bleached by light. The frog retina, in particular, is capable of considerable recovery in color, even when removed from the optic cup. Kiihne (9) showed that a retina which will no longer regenerate in the dark will do so if it is replaced in the optic cup. Evidently the melanin-containing pigment layers of the cup supply a necessary ingredient in forming visual purple. It is not the pigment itself which is responsible for this, because albino mammals regenerate visual purple normally. Certain physical factors markedly increase the rate or extent of regeneration in the intact retina; e.g., increase in temperature and previous exposure to light. On the other hand, regeneration is slower and less complete in rats fed on a vitamin A-free diet (10, ll), and in the absence of oxygen (12). Regeneration in Solution-In 1878, Ewald and Kiihne reported that visual purple in solution, after being bleached by light, regenerated some of its color in the dark. More recent studies (13) indicate that regeneration occurs maximally at neutrality, and falls off in acid and alkaline solutions. The extractive used also makes a difference in degree of regeneration. Solutions of visual purple in bile salts and in digitonin regenerate; those in sodium desoxycholate show no regeneration (14). The above studies were carried out with visual purple solutions prepared from isolated retinas, and showed marked regeneration only after about 85 per cent of the original visual purple had been bleached. However, almost complete regeneration has been observed in only slightly