452 IX. CAROTENOIDS AND VITAMINS A 



proteins break down to the protein and retinene pigment. According to 

 Wald et al.,^"^^-^ these compounds differ from each other as follows: 



Rhodopsin = Opsin (Scotopsin) + Retinenei 

 Porphyropsin = Opsin (Scotopsin) + Retinene2 

 lodopsin = Photopsin + Retinenei 



Retinenei was identified as ordinary vitamin A aldehyde (vitamin Ai 

 aldehyde), while retinene2 was later shown to be vitamin A2 aldehyde. ^^'^ 

 For a more complete discussion of the chemical nature and the functioning 

 of these systems, the reader is referred to a later section of this chapter 

 (see page 569). 



a. The Identity of Retinene Reductase and Alcohol Dehydrogenase. 

 "Retinene reductase" is the name which Wald and Hubbard'"'^ first applied 

 to the enzyme system in the retina which transfers two hydrogen atoms to 

 the aldehyde group of retinene, transforming it to the primary alcohol group 

 of vitamin A. The enzyme was shown to consist of an apoprotein, which 

 works together with the coenzyme, cozymase (coenzyme I, or DPN). 

 The reaction, with DPN as the coenzyme, would be as follows: 



Retinenei reductase 



C19H27CHO + DPN - Ho > C,9H27CH20H + DPN 



Retinenei Dihydro- Vitamin Ai Cozymase 



cozymase 



The Transformation of Retinenei to Vitamin Ai 



Retinene reductase was early recognized to be similar to alcohol dehydro- 

 genase. The latter enzyme is known to occur in a wide variety of tissues 

 including liver, kidney, and intestine. Lutwak-Mann^^a pointed out that 

 this enzyme, together with cozymase as a coenzyme, catalyzes the equilib- 

 rium between various alcohols and their aldehydes. 



Considerable evidence now exists which proves that retinene reductase 

 and alcohol dehydrogenase are the same. For example, BUss^^" reported 

 that crude preparations of liver dehydrogenase catalyze the equilibrium 

 between retinene and vitamin A. Hubbard and Wald^*^ have confirmed 

 this observation, using a crystalline alcohol dehydrogenase prepared from 



226 G. Wald, P. K. Brown, and P. H. Smith, Federation Proc, 11, 304-305 (1952). 



227 K. P. Farrar, J. C. Hamlet, H. B. Henbest, and E. R. H. Jones, /. Chem. Soc, 1952, 

 2657-2668. 



228 G. Wald and R. Hubbard, /. Gen. Physiol., 32, 367-389 (1948-1949). 



229 C. Lutwak-Mann, Biochem. J., 32, 1364-1374 (1938). 



230 A. F. Bliss, Biol Bull., 97, 221-222 (1949); Arch. Biochem. Biophtjs., 31, 197-204 

 (1951). 



231 R. Hubbard and G. Wald, Proc. Nat. Acad. Sci., U.S., 37, 69-79 (1951). 



