RHODOPSIN 75 



chemical basis, no one photosensitive substance could be entirely respon- 

 sible for color vision — at least three such substances are required by the 

 long-popular Young-Helmholtz theory, and even more were demanded 

 by some other theories of color vision. Rhodopsin might be one of these 

 — but where were the others? The resuscitation of Schultze's ideas in 

 the form of the Duplicity Theory made it necessary to abandon rho- 

 dopsin as a color-vision photochemical, for it was finally made certain 

 that some vertebrates have none of it, and that it never occurs in cones. 

 Still, there were those who believed that vision as such — brightness- 

 vision both photopically and scotopically, apart from hue perception — 

 necessitated rhodopsin. These workers argued that there must be in- 

 visible traces of the substance in cones in order to account for their 

 light-sense; and this idea has been very long a-dying. 



Rhodopsin is still widely regarded as the absolutely essential photo- 

 chemical substance for rod activity. Even this is an unnecessary belief, 

 since rhodopsin may be nothing more than a sensitizer, so powerful that 

 its action masks that of another, essential, material so completely that the 

 brightnesses of lights are directly related to their effects upon rhodopsin. 



The substance is a reddish pigment whose chemical nature is not yet 

 completely known. It is released from the rod outer segment by sub- 

 stances which lower surface tension, such as bile salts, saponin, digitonin, 

 sodium oleate and salicylate, and snake venom. It forms a precipitate 

 with platinic chloride — an insoluble yellow compound which can be seen 

 in the rods in permanent microscopic preparations made of retinse which 

 are kept in darkness for an hour or so before preservation. 



Rhodopsin is commonly described nowadays as a hydrocarbon con- 

 jugated with a protein, through a belief that vitamin A — essentially a 

 hydrocarbon — is an important constituent (r. /'.). The molecular weight 

 of rhodopsin is about 270,000. This and other features make it clear 

 that most of the molecule is proteinous; but of course to say that rho- 

 dopsin is essentially a protein is like saying that dynamite is essentially 

 fuller's earth. The business part of the molecule — its 'chromophoric' 

 (color-bearing) group — is neither a hydrocarbon nor a protein, though 

 it may be derived indirectly from a portion of the vitamin A molecule. 

 The latest information* is that the rhodopsin molecule contains a pro- 

 tein, 'provisual red', and probably a third substance. The chromophore, 

 provisual red, can be split into a fatty acid and 'visual red'; the latter in 



'''Kindly supplied b>- Dr. Arlington C. Krause in advance of his own publication thereof. 



