THE VISUAL PIGMENTS 

 lODOPSIN 



WALD, BROWN and SMITH (1952) have also given brief details of 

 experiments with iodopsin, the photosensitive pigment (Amax 562 m^) 

 found, together with rhodopsin in the chicken retina (wald, 1937, 

 BLISS, 1946). According to them the bleaching of iodopsin, Hke that 

 of rhodopsin, yields an inactive isomer of retinenci, apparently the 

 ^W'trans isomer. The synthesis of iodopsin from retinene and chicken 

 opsin, however, requires a different isomer of retinene^, namely neo- 

 retinene 6, the same isomer as is required for cattle rhodopsin. The 

 carotenoids of the rhodopsin and iodopsin systems are therefore 

 identical; only the proteins are different. The cone protein [i.e. 

 chicken opsin] can be called photopsin to distinguish it from the 

 scotopsins of the rods.' In a later paper wald (1953a) reports that 

 *just as /^o-retinene a yields /50-rhodopsin when incubated with 

 scotopsin, it yields a similarly displaced pigment, wo-iodopsin, on 

 incubation with photopsin. The Amax of /^o-iodopsin is at about 

 515 m//.' 



CYANOPSIN 



One of the isomers of retinenci («eo-retinene b) combines with 

 *scotopsin' to form rhodopsin; the same isomer with 'photopsin' 

 forms iodopsin. Similarly, one of the isomers of retinencg combines 

 with 'scotopsin' to form porphyropsin ; what would be the result of 

 combining the same isomer of retinencg with photopsin ? In order to 

 answer this question, wald, brown and smith (1953) first exposed 

 a digitonin extract of dark adapted chicken retinae (iodopsin and 

 rhodopsin) to deep red hght. This Hght hardly affected the rhodopsin 

 in the extract but bleached the iodopsin to retinenej and chicken 

 opsin (photopsin). Since the retinenci was produced in the inactive 

 2i\\-trans form there was no danger of recombination with the protein 

 (regeneration of iodopsin). A small amount of c/^i-retinencg in 

 digitonin solution was now added to the extract which was then left 

 in darkness for 8 min to allow combination between the c/^j-retineneg 

 and photopsin to occur. 



At the end of this period the absorption spectrum of the mixture 

 was measured, and again after it had been exposed for 4J min to deep 

 red hght. The difference between these two sets of measurements is 

 shown by the difference spectrum in Fig. 5.10. This is the difference 

 spectrum of a new visual pigment (cyanopsin) formed by the com- 

 bination of c/^i-retinencg and photopsin. Unfortunately wald did 



152 



