IMPORTANCE IN VISION 571 



in the retina. Much of the retinene may be reduced to vitamin A alcohol 

 immediately after it is set free. 



Retinene reductase is a dehydrogenating enzyme which catalyzes the 

 reversible change of both retinene ■ and retinene2 to vitamins Ai and A2, re- 

 spectively. The enzyme which occurs in the retina is similar to that pres- 

 ent in the liver, intestine, and kidney. Cozymase is a required co-factor. 

 For a further discussion of this enzyme system, the reader is referred to an 

 earlier section of this chapter (see page 452). 



(2) The Synthesis of Rhodopsin in Higher Animals 



Rhodopsin can be synthesized in the retina by two types of reaction, 

 namely from retinene plus opsin, or from vitamin A plus opsin. At an 

 early date Kiihne^-^ recognized that a fairly rapid synthesis of visual pur- 

 ple (rhodopsin) takes place by "anagenesis" from the yellow precursors; 

 this reaction can proceed to some degree in the isolated retina, and even in 

 solution. The second pathway for the synthesis of rhodopsin, which 

 Kiihne^-* recognized as a slower one, is called "neogenesis" ; this takes 

 place from colorless precursors. It was believed that this reaction could 

 occur in the living retina only when the cooperation of the pigment epithe- 

 lium was available. 



a. The Synthesis of Rhodopsin from Retinene. Hecht and co-workers^^° 

 and also Chase and Smith^^^ have confirmed the observations of Kuhne^^^ 

 that rhodopsin can be regenerated in the dark in solutions which have been 

 completely bleached. However, the extent of this regeneration is usually 

 only 5 to 10%, with a maximum of 15%. On the other hand, Wald and 

 Brown^^" were able to effect a 70% regeneration of bleached rhodopsin solu- 

 tions to rhodopsin, when replaced in the dark, by supplementing them with 

 high concentrations of retinene. The reaction is a spontaneous one which 

 does not require an external source of energy. The bleaching of rhodopsin, 

 with the consequent cleavage to opsin and retinene, is the reaction which 

 requires energy, usually in the form of light. 



b. The Synthesis of Rhodopsin from Vitamin A. The synthesis of 

 rhodopsin from vitamin A and opsin has been observed only in living tissues. 

 The difficulty appears to be in oxidizing vitamin A to retinene under in 

 vitro conditions, since the equilibrium between vitamin A and retinene 



^^^ W. Kiihne, Chemische Vorgdnge in der Netzhaut, in L. Hermann, Handbuch der 

 Physiologie, Vol. 3, Part 1, F. C. W. Vogel, Leipzig, 1879, 235-342. 



^^ S. Hecht, A. M. Chase, S. Shlaer, and C. Haig, Science, 84, 331-333 (1936). 

 931 A. M. Chase and E. L. Smith, /. Gen. Physiol, 23, 21-39 (1939-1940). 

 "2 G. Wald and P. K. Brown, Proc. Nat. Acad. Sci., U.S., 36, 84-92 (1950). 



