2l6 G. WALD VOL. 4 (1950) 



pigment rhodopsin. This substance takes part with the carotenoids retinenCi and 

 vitamin Aj in a cycle of reactions of the following form* : 



Rhodopsin 

 (500 m^i) 



^\ 

 (3)/ \Light 



(i)r\ Orange intermediates 



\ \ 

 (2) \ ^ 



Vitamin A^ + protein < Retinene^ + protein 



(325 m/i in petroleum ether) (365 m^ in petroleum ether) 

 (4- SbClg — > 615-620 m/j.) (-}- SbCl3 > 664 m/n) 



Rhodopsin bleaches in the light over unstable orange intermediates to a mixture 

 of yellow retinenej and colourless protein ; the retinenej, is then transformed to colourless 

 vitamin Aj) and both vitamin Aj and retinenej — ^or its orange precursors — recombine 

 with protein to form new rhodopsin (Wald, 1935-36 a, b). 



One has only to separate the retina from contact with the underlying tissues which 

 line the optic cup to abolish the synthesis of rhodopsin from vitamin A^ (reaction 3). 

 According to KOhne this process requires the cooperation of a living pigment epithelium 

 (EwALD AND KiJHNE, 1878, page 255; KiJHNE, 1879). 



When the system is further reduced by bringing rhodopsin into aqueous solution, 

 processes (i) and (2) are usually also eliminated. Nothing then remains but the succes- 

 sion of light and "dark" reactions which transform rhodopsin into retinenej and protein. 



The present paper is concerned primarily with reaction (2), the conversion of reti- 

 nene^ to vitamin A^. This is a slow, irreversible process which goes to completion in 

 the isolated retina in about an hour at room temperature**. In 1942-43 we succeeded 

 in biinging this process into a cell-free brei prepared from cattle retinas; and recently 

 Bliss (1948) has shown that it occurs under certain conditions in freshly prepared 

 rhodopsin solutions. These demonstrations that it can proceed in vitro form a prelude 

 to the present experiments. Their other antecedent is the clarification of chemical rela- 

 tions between retinenC] and vitamin A^, due primarily to the work of Morton and his 

 colleagues in Liverpool. 



Vitamin Aj is the primary alcohol C19H27CH2OH. Ball, Goodwin, and Morton 

 (1948) found that on mild oxidation this is transformed to a product which agrees in 

 spectrum and antimony chloride reaction with retinenei. They have crystallized this 

 product and shown it to be an aldehyde, which they believe to be simply vitamin A^ 

 aldehyde, C19H27CHO. Their analytic data do not establish this formulation unequi- 

 vocally as yet; but all that is now known of retinenci from the work of Morton's 

 laboratory and our own is consistent with the view that it is vitamin A^ aldehyde. We 

 shall accept this as its structure in what follows. 



The wavelength values written below components of this cycle represent maxima in the ab- 

 sorption spectra of these substances in solution, or, when so indicated, of the products which these 

 substances yield when treated with antimony trichloride. 



Designating this as an irreversible process is not intended to exclude the possibility that 

 it is in fact reversible, but with the equilibrium far over toward vitamin A formation. It might for 

 example be possible by greatly increasing the concentration of vitamin Aj in the system to demon- 

 strate a small reversion to retinenCj. 



References p. 228. 



