THE VISUAL PIGMENTS AND THEIR PHOTOPRODUCTS 



In solution at ordinary temperatures the rates of thermal de- 

 composition of lumi- into meta-rhodopsin and of the latter into acid 

 indicator yellow are very fast. But the regenerative reactions are also 

 fast. Consequently, if a solution of visual purple is bleached by 

 exposing it to an intense light for a very short time, i.e. a time com- 

 mensurate with the lives of lumi- and meta-rhodopsin, then some 

 regenerated rhodopsin and iso-rhodopsin can be expected in the 

 bleached solution. If, on the other hand, the exposure is prolonged 

 (say ten minutes or more) such regenerated photo-pigments are *re- 

 bleached* while second, third and higher order regenerations become 

 successively less because of the continual 'drainage' to acid indicator 

 yellow. Thus, following a prolonged exposure to Hght, solutions 

 can contain only those products shown below the horizontal line 

 in the scheme. In what follows we shall assume that such is the 

 case. 



The final state of an irradiated preparation of visual purple 

 depends on the conditions, e.g. on the freshness of the preparation 

 and (when the preparation is a solution) on the pH. If a neutral, 

 freshly-made solution is bleached it will slowly fade after the exposure 

 period, sometimes to a colourless condition in which only vitamin A 

 is present. If, however, another sample of the same solution is 

 allowed to age for two or three hours, or alternatively, if in the 

 preparation of the solution the retinae were washed with alum, then 

 the enzyme which catalyses the conversion of indicator yellow and 

 retinene to vitamin A is destroyed and consequently, the neutral 

 solution, after bleaching, contains only retinene and, perhaps, some 

 indicator yellow, arden (1954) found that sucrose suspensions of 

 the retinal rods of the frog (which contain visual purple) behaved in a 

 similar way. Thus fresh suspensions bleached to vitamin A, whereas 

 aged suspensions bleached to retinene or, perhaps, indicator yellow. 

 With suspensions, however, the ageing process takes several days, 

 due no doubt, to the protection afforded to the enzyme in its natural 

 environment inside the rod. 



If an alkahne (pH 8-10) solution of visual purple is bleached there 

 is a rapid and almost quantitative conversion to the stable, alkaline 

 form of indicator yellow. A slow 'leakage' to retinene via acid 

 indicator yellow may occur, but this makes little difference to the 

 spectrophotometric properties of the bleached solution since the 

 density spectra of retinene and of alkahne indicator yellow are not 

 very different. 



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