674 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



T^hodopsin 



l,ghf 



Lumi-rhodopiin 



[.>-20°C. 



Me ta - rhodops in 



alcohof dehydrogenase 

 DPfJ-H 



Vitamin A, i-Opsin -■ =^ Fietinene, -h Opsin 



w 



VtiarrtinAj from 



pigment epithelium 



and circufaiion 



DVN-H 



FIG. 3. Diagram of the rhodopsin system. [From Hubbard 

 & Wald (33).] 



to water, meta-rhodopsin yields retinene and opsin 

 (74). The retinene is then reduced to vitamin A. 

 In the dark, the spontaneous combination of reti- 

 nene and opsin to form rhodopsin promotes the o.xi- 

 dation of vitamin A to retinene. This process is 

 aided Idv the influx of new vitamin A from the pig- 

 ment epithelium which obtains it from the blood 

 circulation, by the provision of DPN, the oxidant of 

 vitamin A, and by respiratory enzymes, which keep 

 DPN oxidized. All these factors acting in concert 

 sweep the system back toward rhodopsin (33). 



It should be noted that light enters this scheme 

 directly at only one point, the conversion of rhodopsin 

 to lumi-rhodopsin. The other reactions follow from 

 this initial act but are themselves 'dark' reactions, 

 i.e. reactions which proceed equally well in light 

 or darkness. 



Judging from figure 3, it should be possible to 

 assemble the rhodopsin system by mixing four sub- 

 stances in solution: vitamin A, opsin, alcohol de- 

 hydrogenase and DPN. The system has, in fact, 

 been put together using highly purified vitamin A, 

 crystalline alcohol dehydrogenase from horse livers 

 and DPN from yeast. The only component that 

 needs to be obtained from the retina, and indeed 

 from the outer segments of the rods, is opsin. This 

 mixture, placed in the dark, forms rhodopsin. 

 Brought into the light it bleaches, and replaced in 

 the dark it synthesizes more rhodopsin. It thus per- 

 forms in solution all the reactions of the rhodopsin 

 system (33). 



However, in making up this mixture, not all 

 vitamin A is effective. \'itamin A, like other carot- 

 enoids, exists in a number of different molecular 



shapes, cis-trans isomers of one another (47, 83, 84). 

 A.\\-trans vitamin A (fig. 4), the predominant isomer in 

 liver and blood (Wald, G. & P. S. Brown, unpub- 

 lished observations), is ineffective in rhodopsin 

 synthesis. Rhodopsin requires for its formation one 

 of the cis isomers of vitamin A (34). 



According to theory, only two of the four side- 

 chain double bonds of vitamin A should be capable of 

 forming stable m-linkages, those marked 9 and 13 

 in figure 4. At the other double bonds, a cis linkage 

 encounters serious steric hindrance, and the molecule 

 must be twisted out of coplanarity. This interferes 

 with resonance and should consequently lead to a 

 lowered stability (42, 43). Only four geometrical 

 isomers of vitamin A or retinene were therefore 

 expected: a\\-trans, g-cis, I'^-cis, and 9, 12,-dicis (fig. 4). 



Five cis-trans isomers of retinene, however, have 

 been identified and crystallized (10, 32, 39, 48): 



CH, 



CH, 



ripC 4 f, t- L. t- *- \ 



1 I 



CH, 



a//-fran3 

 viiamin fi 



^c-^'^X 



H,C C ^C C 



c c 



reiinene 



13 —CIS 



(neo - a) 



CH, 



'3 » 



CH, 



Hx/^^v^N^N- 



H,C C-CH, c^ 



1^, I 



9 -as 

 ftso-a) 



CHj ^C 



CH, 



CH, 



H,C ^c c c 



'1 I t^ I 



H,C C-CHj 



^C-^ CH, 



h'-'^^-h 



9,)3 -d^as 

 (iso - b) 



CH, C 



(4 



FIG. 4. Unhindered geometrical isomers of vitamin .-K. Tiiis 

 molecule can assume the eis configuration only at double 

 bonds 9 and 1 3 without encountering serious steric hindrance. 

 At the other double bonds, groups come into conflict, and the 

 CIS configuration not only bends but twists the molecule. 

 [Modified from Hubbard & Wald (34).] 



