678 



HANDBOOK OF PHYSIOLOGY 



NEURiil'in^iOLGGY I 



008\- K 



T — r 



T 



Lepomia op sin 



o* n*o-b rei inQna^ 



• ■mo- ex rei tnanOi 



J \ 



be formulated : 



FIG. 9 Synthesis of poiphyropsin and iso-porphyropsin. The 

 neo-A and iso-a isomers of retinene; , partially purified, were 

 incubated with opsin from the sunfish, Lcpomis. The difference 

 spectra of the products are shown, measured in the presence 

 of hydroxylamine. The neo-6 isomer yields porphyropsin 

 (Xmai , 522 m^i), the iso-a isomer iso-porphyropsin (Xmax , 

 507 m^)- [From Wald, G., P. K. Brown & P. S. Brown, un- 

 published observations.] 



porphyropsin, with X^ax 507 m^ (hg- 9; Wald, G., 

 P. K. Brown & P. S. Brown, unpublished obser- 

 vations). 



In performing such syntheses it makes no difference 

 whether the opsin is derived from a fresh-water 

 fish, a frog or cattle. All these opsins when mixed 

 with neo-6 retinenej yield porphyropsin, while with 

 neo-/> retinenei they form rhodopsin. The pigments 

 obtained with cattle opsin lie at slightly shorter 

 wavelengths than those obtained with frog opsin: 

 'cattle porphyropsin' lies at X,,,.,^ 517 ni/j, while 

 'frog porphyropsin' has Xmax 5-;-^ ni/i. Cattle and frog 

 rhodopsins display similar differences: the former 

 has Xmax 498 m/i, the latter Xmax 502 m^i. Clearly 

 species differences in the opsin affect the X,„:,x of 

 the visual pigments. 



The opsins of the rods that have been examined are 

 so closely related that they must be regarded as 

 belonging to the same family, the rod opsins or 

 scotopsins. The rhodopsin and porphyropsin systems 

 therefore share entirely the same proteins. Only their 

 carotenoids differ and those only by one double bond 

 in the ring. The porphyropsin system can therefore 



Porphyropsin 



Neo-A retinenco -|- scotopsin .KW-Uans retinene-j -\- scotopsin 



[ (alcohol dehydrogenase, DPN) 



Neo-A vitamin Ao 



-^ All-(ra«i vitamin Aj 



lodopsin 



The first light-sensitive pigment of cone vision was 

 extracted from the chicken retina in 1937. It is a violet 

 pigment (Xmax 562 m/x) called iodopsin. The chicken 

 retina contains a few rods among a large predomi- 

 nance of cones and hence yields a mixture of iodopsin 

 and rhodopsin (58). 



The carotenoids of the iodopsin system are identical 

 with those of the rhodopsin system, even to cis-trans 

 configuration. Only the opsin is different. The cone 

 opsins can be called photopsins. The replacement of 

 .scotopsin by photopsin changes the rhodopsin to 

 the iodopsin system (72) : 



lodopsin 



/ light 



/ \ 



..\\\-lrans retinenei -f photopsin 



Neo-A retinenei + photopsin 



(alcohol dehydrogenase, DPN) 



Neo-A vitamin Ai — 



" All-trans vitamin .^i 



From the light-adapted chicken retina one can 

 extract a colorless carotenoid-free mixture of the 

 proteins of rod and cone vision, scotopsin and pho- 

 topsin. On incubating this, or a wholly bleached 

 extract of chicken retinas, in the dark with neo-A 

 retinenei, one obtains a mixture of rhodopsin and 

 iodopsin indistinguishable from that extracted from 

 the dark-adapted chicken retina (fig. 10). 



Just as iso-a retinenei yields iso-rhodopsin when 

 incuijated with rod opsin, it yields a similarly dis- 

 placed pigment, iso-iodopsin, on incubation with 

 cone opsin. The Xm.ix of iso-iodopsin is at about 

 515 m/i. The remaining isomers of retinene are in- 

 active (fig. 1 1). 



Cya7}opsin 



Rod opsin combines with nto-b retinenei to yield 

 rhodopsin, or with neo-6 retineneo to yield por- 



