GEORGE WALD 



rib 



ing only in iliat the latter possesses an extra double bond in its 

 ytf-ionone ring. From these by oxidation the corresponding aldehydes, 

 retinenei and retinene^., are derived, as shown in above structures. 



There are also two main families of vertebrate opsins, one found 

 in rods, the other in cones. The two retinenes join with the two kinds 

 of opsin to yield the four major pigments of vertebrate vision (44-47) : 



Vitamin Ai 



DPN* 



DPNII 



(alcohol dehj'drogenase) 



DPN+ 



llelinciici 



+ rod opsin 

 + cone opsin 



light 



^ Rhodopsin 



light 



~ lodopsin 



Approx. 



Xinax (ni/x) 



500 



562 



I + rod opsin ' '^ , Porphyropsin 522 



Vitamin Aj ^ 



DPN-H 



Retincne2 



+ cone opsin - 



light 



T Cyanopsin 



620 



Vitamin Aj, having been oxidized to retinenei by the enzyme 

 alcohol dehydrogenase and DPN, combines with the opsin of the 

 rods to yield the red pigment, rhodopsin, or with the opsin of the 

 cones to yield the violet pigment, iodopsin. Similarly vitamin A2, 

 having been oxidized to retinenco by the same enzyme system, joins 

 with rod opsin to yield the purple photosensitive pigment, porphy- 

 ropsin, or with cone opsin to yield the blue pigment, cyanopsin. On 

 exposure of any of the photosensitive pigments to light, these re- 

 actions are reversed, ending in the production of a mixture of an 

 opsin and a vitamin A. 



All the invertebrate eyes so far examined contain similar light- 

 sensitive pigments, formed by the combination of retinenei with a 

 variety of opsins (48) . Vitamin Ao and retinenes have not yet been 

 found in any invertebrate eye; nor as yet the alcohol dehydrogenase 

 system that reduces retinene to vitamin A. 



The operation of a typical vertebrate system, the rhodopsin sys- 

 tem, is shown in more detail in Fig. 1 (27) . On exposure to light 

 rhodopsin yields a transient product, red or orange-red in color, called 

 lumirhodopsin (54) . This is the only photochemical reaction in 

 visual excitation; all that follows consists of ordinary thermal ("dark") 

 reactions. Lumirhodopsin is stable below about — 50°C, but if 

 warmed to about — 20°C, goes over to a second intermediate, still 

 orange-red in color, called metarhodopsin. This is stable below about 

 -»-15°C, but if warmed above this temperature and in the presence 

 of water, it hydrolyzes to a mixture of retinene and opsin. The 



