GEORGE WALD 731 



ami periodically its spectrum was remeasured. The retinene band de- 

 clined coiuiiuioiisly, that of rhodopsin simultaneously rising. At the 

 end of 3 hours, only rhodopsin remained. VVe had purposely pre- 

 pared this mixture with a small excess of opsin, and by this time 

 it had completely removed all the 11-m retinene. The small band 

 at about 350 m^u, in curve 9 is not the last of the retinene, but the 

 /3-band of rhodopsin. 



Now curve 9 in Fig. 3 was redrawn at the right as curve /. The 

 solution was exposed to orange light for 5 seconds, and the spectrum 

 redrawn (2) . Then it was exposed for 5 seconds longer, and the spec- 

 trum drawn again (3) . By such repeated exposures the rhodopsin was 

 bleached, and simultaneously retinene reformed. It looks, however, 

 as though by this transaction we had made some retinene, for the ex- 

 tinction of retinene at the end of bleaching was about 1.5 times that 

 with which \\e began. The reason for this is that, whereas the experi- 

 ment began with \l-cis retinene, it ended with the sill-trans isomer; 

 and this has about 1.7 times the molar extinction of the hindered cis 

 species. 



The rhodopsin system, therefore, is arranged as follows (28) : 



Rhodopsin 



Isomerization ^~~-^^^ 



Neo-b retincno^+ Op^in •<= ^\\\-trans retinene + Opsin 



(Alcohol dehydrogenase. DPX) 

 Isomerization 



Neo-b vitamin A-= :^\\\-trart!< vitamin A 



Rhodopsin on exposure to light bleaches over the orange inter- 

 mediates already mentioned — lumi- and meta-rhodopsin — to a mixture 

 of all-^rfln5 retinene and opsin. In order to regenerate photopigment, 

 the aU-traus retinene must be isomerized to the W-cis configuration. 

 While this is going on, however, much of it is reduced by alcohol de- 

 hydrogenase and DPNH to aU-trans vitamin A; and this in turn 

 must be isomerized to or exchanged for ll-cis vitamin A before it 

 can reform rhodopsin. 



A cycle of cis-trans isomerization is therefore an intrinsic component 

 of the rhodopsin system. The same can be said of every other visual 

 system that has so far been examined. In every case the visual pigment 

 contains as chromophore the hindered 1 \-cis configuration of retinenej 

 or retinene^, and in every case the action of light on the visual pig- 

 ment yields as product the 2i\\-trans configuration, which must be 

 re-isomerized to ll-f/5 for resynthesis to occur. 



