676 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



FIG. 7. Synthesis and bleaching of 

 rhodopsin in solution (22.5°C, pH 7.0). 

 Left: A mixture of neo-A retinene and 

 cattle opsin was incubated in the dark, 

 and absorption spectra recorded peri- 

 odically, (/) at 0.3 min., (5) at 2.5, 

 (3) at 5, (^) at 10, (5) at 18, (ff) at 30, 

 (7) at 60, (<9) at 120 and (5) at 180 min. 

 The absorption band of neo-A retinene 

 (Xmax 380 m^i) falls regularly, while that 

 of rhodopsin (X„,ax 498 m^) rises. Righl . 

 The rhodopsin formed at the left (;) is 

 exposed to light of wavelengths >550 

 mil for various intervals, and the spec- 

 trum is recorded immediately after each 

 exposure. The total irradiations are: (2) 



5 sec, (3) 10 sec, (^) 15 sec, (5) 30 

 sec, and (5) 120 sec. The residue was 

 exposed for 45 sec. longer to light of 

 wavelengths >440 mn (7). [From VVald 



6 Brown (70).] 



300 400 SOO 600 400 500 



Wavelength- m/u 



600 



A stored in the eye (primarily in the pigmented 

 layers, choroid and pigment epithelium) is in the 

 form of an ester. Recently a cell-free enzyme system 

 has been prepared from cattle retinas and pigmented 

 layers which esterifies vitamin A in vitro (38). The 

 significance of this reaction for the visual cycle is 

 still obscure. It is, however, noteworthy that the 

 amount of vitamin A ester stored in the dark-adapted 

 eye is roughly equivalent to its rhodopsin content on a 

 molar basis, and that the neo-h isomer constitutes 

 about one-half of this store (37; VVald, G. & P. S. 

 Brown, unpublished observations). 



Porphyropsin 



The rods of vertebrates which li\e in, or better, 

 spawn in fresh water — fresh-water fishes, spaw^ning 

 lampreys, and certain larval and adult amphibia — 

 characteristically contain in place of rhodopsin a 

 purple light-sensitive pigment called porphyropsin 

 (62). Its X„i;,x in aqueous .solution ordinarily is close 

 to 522 niM (fig. 8). On bleaching, it yields a mixture 



of opsin and a new retinene called retinenca, and 

 this in turn is reduced to a new vitamin A called 

 \itamin A-j. It was the analysis of this visual system 

 that led to the discovery of these carotenoids (57, 

 60). The structures of these substances have now been 

 established by total synthesis (13). They differ from 

 \itamin A and retinene' — sometimes called vitamin 

 Ai and retinenei — only in possessing an additional 

 double bond in the ring: 



CH3 



CH3 



2 Throughout this discussion, the terms vitamin A and 

 retinene will be used synonymously with vitamin Ai and 

 retinenei. 



