III. BIOCHEMICAL SYSTEMS 



79 



some connections, l)n( tliey are decisive for the reaction of a molecule with 

 an enzyme or with any other type of protein. 



The synthesis of rliodopsin involves two proteins and indeed is a two-step 

 reaction. First vitamin A is oxidized to retinene by cozymase on the enzyme 

 protein, alcohol dehydrogenase; then retinene couples with the protein 

 opsin to form rhodopsin. The first of these reactions has proved to be rela- 

 tively unspecific to\\'ard the isomers of vitamin A and retinene. It is the 

 second reaction, the combination of retinene with opsin, that demands a 

 specific CIS isomer of retinene. 



280 



440 



480 



sao 



320 360 400 



Fig. 23. Absorption spectra of crj-stalline stereoisomers of retinene in ethyl alco- 

 hol. The absorption maximum in each spectrum is marked with a vertical bar. Neo- 

 retinenes a and h and isoretinene a appear to be mono-cfs structures; and isoretinene 

 6 is a di-c?s retinene. Each presumptive cis linkage shifts the absorption maximum 

 5.5 to 7 vcifj. toward shorter wavelengths. W\-trans retinene was first crj'stallized by 

 Ball el al.]' neoretinenes a and h in our laboratory; and isoretinenes a and b in the 

 Organic Research Laboratory of Distillation Products Industries. (From R. Hubbard 

 et al.^) 



We have now examined five crystalline isomers of retinene: all-/mns 

 retinene, first prepared by Ball et al? (1948); neoretinenes a and h, first 

 isolated in our laboratory; and isoretinenes a and h, prepared and made 

 available to us by the Organic Research Laboratory of Distillation Prod- 

 ucts Industries of Rochester, New York (Fig. 23).^- ^*'' "" 



On incubation with opsin, all-/mn.s retinene and neoretinene a are in- 

 active. Neoretinene h yields a rapid synthesis of rhodopsin; the product is 

 indistinguishable from rhodopsin extracted from the retina. Isoretinene a 

 yields about the same amount of light-sensitive pigment, and about as 

 quickly; but the spectrum of the pigment is displaced about 13 m^u below 

 that of rhodopsin, Xmax falling at about 487 m/^t. We have suggested that 



