904 LIGHT AND LIFE 



Denaturation of the opsin by heating (Hubbard) bleaches the pig- 

 ment while releasing most of the retinene still in the W-cis configura- 

 tion. 



Visual excitation, Wald suggests, must depend upon formation of 

 the lumirhodopsin and metarhodopsin forms, since hydrolysis of the 

 latter is far too slow to account for it. In fact, in some invertebrate 

 eyes the bleaching ends with the production of the meta-pigment. 

 The exposure of rhodopsin to bleaching by light leaves some rhodop- 

 sin, and the quantum efficiency of the bleaching appears to be only 

 about .5. Actually, the quantum efficiency is close to unity. But 

 the exposure of lumirhodopsin to light reisomerizes some of the all- 

 trans isomer, just produced, into the W-cis form again. On the other 

 hand, light will not isomerize alUrans retinene in the metarhodopsin 

 intermediate. Wald suggests that it is the special capacity of caro- 

 tenoids to undergo geometrical isomerization that has led to their 

 selection for the particular roles they play as photoreceptors in visual 

 systems. Their straight-chain conjugated systems are readily isomerized 

 by exposure to light. Certainly, it is remarkable that in three phylo- 

 genetically distinct groups of animals, the mollusks, the arthropods, 

 and the vertebrates, where eyes have evolved quite independently, the 

 ll-cis isomer of retinene has come to occupy the same position in visual 

 excitation. Natural selection here seems to have picked on the one 

 geometric isomer of retinene with the largest change of shape and 

 highest photosensitivity, that is, highest quantum efficiency and highest 

 intrinsic rate for photoisomerization to the aW-trans form. 



Visual excitation in the rods poses particularly interesting prob- 

 lems. At the dark-adapted threshold for rod vision (ca. 10-^ milli- 

 lamberts) , a single rod absorbs a quantum about once on the average 

 in 38 minutes. Even at a luminance 1000 times above this threshold, 

 a rod absorbs a quantum less than once in two seconds. Clearly, a 

 single quantum, absorbed by a single molecule of rhodopsin, must 

 be sufficient for visual excitation. The problem is to account for the 

 amplification of effect. Wald suggests such possibilities as the quan- 

 tum's knocking a hole in the rod's outer membrane, which is largely 

 made up of rhodopsin, and thus generating a self-propagating de- 

 polarization; or its uncorking an active site of enzymatic nature 

 previously shielded by the retinene. 



Wald's views that the spectral sensitivity of vision derives from 

 the absorption spectra of the visual pigments, and that photosensitivity 

 and light and dark adaptation are functions of pigment concentration 

 are supported by further evidence. Excellent agreement has been 



