THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



shown in Fig. 2.12. These measurements, by collins and morton 

 (1950b), extend to the ultra-violet and show that the maximum of the 

 alkahne indicator yellow curve is at c. 365 m^ and confirm that it is 

 isosbestic at c. 400 mfx with the acid curve. 



LYTHGOE (1937) found that indicator yellow was stable between 

 pH 10-0 and pH 6-1 but that outside this range it faded, particularly 

 in strongly acid solutions. In 1948, bliss showed that in acid solu- 

 tion (pH < 7) indicator yellow (Amax 440 vcifj) decomposed in a few 

 hours into retinene (Amax 385 mfx) and protein. Retinene, a sub- 

 stance soluble in fat solvents, was first described by wald (see p. 30) 

 and is the chromophore of indicator yellow. Indicator yellow, like 

 visual purple, is a chromoprotein, i.e. a protein associated with one 

 or more light-absorbing molecular groups called chromophores. 



BLISS (1948) showed also that in conditions where acid indicator 

 yellow and retinene were produced, these substances underwent a 

 further chemical change — if the solutions were fresh — to form the 

 colourless vitamin A (2max = 328 m//). bliss found that this final 

 reaction, which was maximal at pH 6-7 and which did not occur 

 below pH 5-5 nor above pH 8, was probably due to an enzymic 

 factor, present only in fresh solutions, i.e. solutions not more than 

 two hours old. 



summary 



The sequence of reactions which may follow exposure of visual 

 purple to Hght is set out in the scheme on p. 54. 



Iso-rhodopsin, which has Amax = 487 m// when *regenerated' in 

 cattle or rat rhodopsin (497 m/<) solutions, and Amax = 493 m/z 

 when 'regenerated' in frog rhodopsin (502 m^<) solutions (collins 

 and MORTON, 1950c) is shown (because of wald, durell and st. 

 George's experiments) as being produced wholly from meta- 

 rhodopsin (cf. Figs. 2.10 and 2.11). If, in addition, the original 

 rhodopsin can be regenerated from lumi-rhodopsin — a point on 

 which we have no information — then the fact that iso-rhodopsin and 

 rhodopsin are cis-trans isomers suggests that the thermal change of 

 lumi- into meta-rhodopsin is an isomerization process (see Chapter 4, 

 p. 124). 



It is not certain whether the enzymic conversion of indicator 

 yellow to vitamin A proceeds directly or via retinene. On the whole 

 the evidence (bliss, 1948) suggests a direct conversion as shown in 

 the scheme. 



53 



