82 VITAMINS A AND CAROTENES 



light a large part of the vitamin A of the retina escapes into the circulation. ^^ 

 We may assume that during visual activity the retina continuously voids 

 aW-trans vitamin A into the blood stream and continuously selects neo- 

 vitamin A6 out of the circulation by binding it in rhodopsin. These ex- 

 changes keep the visual processes in intimate connection with the metab- 

 olism of vitamin A throughout the body and with an external factor, its 

 supply in the diet. 



We have developed in some detail the biochemistry of the rhodopsin 

 system. The iodopsin system, recently analyzed in the chicken retina, has 

 been found to have precisely the same form.^ The steroisomers of vitamin A 

 and retinene which take part in the iodopsin system are precisely the same 

 as those in the rhodopsin cycle. Only the protein is different. One has only 

 to replace rhodopsin by iodopsin and to substitute photopsin for opsin — or 

 scotopsin — in the diagram of Fig. 25 to have it represent the iodopsin 

 system. 



G. THE GENERAL METABOLISM OF VITAMIN A 



I should like now to consider the general metabolism of vitamin A, 

 emphasizing particularly those aspects of its metabolism into which the 

 visual processes yield some insight. 



It has recently been shown that retinene, fed to rats by mouth or in- 

 jected subcutaneously or intraperitoneally, is reduced to vitamin A as it is 

 absorbed.^* The gut wall and the subcutaneous tissues apparently can 

 accomplish this reduction directly; the site of reduction of intraperitoneal 

 retinene is not known. 



If the reduction of retinene involved a specific enzj^me, the occurrence 

 of this process in the tissues mentioned might be taken as evidence that 

 they encounter retinene in the covu'se of their ordinary metabolism. This 

 would be of some interest, since retinene has never been found outside the 

 retina. We have seen, however, that retinene is reduced by the highly un- 

 specific and widespread enzyme, alcohol dehydrogenase, whose presence 

 in the tissues is sufficiently explained by their exposure to a variety of 

 metabolic aldehydes. The fact that a tissue can reduce retinene, therefore, 

 is no argument that it ever normally encounters this substance. 



It was nevertheless suggested some time ago by Glover et al.^^ that 

 retinene is an intermediate in the oxidation of |8-carotene to vitamin A. In 

 the past few years it has repeatedly been shown that laboratory procedures 

 for oxidizing |3-carotene yield retinene among the products. Hunter and 

 Williams^* obtained very small yields of retinene (0.4 to 0.5 %) on oxidizing 



^^ G. Wald, J. Gen. Physiol. 19, 781 (1935-1936). 



" J. Glover, T. W. Goodwin, and R. A. Morton, Biochem. J. 43, 109, (1948). 



" R. F. Hunter and N. E. Williams, J. Chem. Soc. 1945, 554. 



