THE STRUCTURE OF VISUAL PURPLE (RHODOPSIN) 



For this reason lythgoe called it "indicator yellow." We find its 

 absorption maxima to lie at about 366 m/Li at pH 9-9-5, 387 m/n at 

 pH 6-7-7 and 393 m^ at pH 4-4-5.' 



Now lythgoe' s indicator yellow is the substance, present in 

 freshly bleached rhodopsin solutions, which has 2max at 365 mfi in 

 alkaline solution and 440 mpi in acid solution. Of the acid form of 

 lythgoe's indicator yellow, wald and hubbard (ibid.) write 'when 

 rhodopsin is bleached at pH about 4 this material appears as an 

 initial product. In light or darkness the 440 m^u maximum slowly 

 moves towards shorter wavelengths, finally coming to rest at about 

 390 m/<, the maximum of acidic retinenci (wald, 1938). The 

 440 mju material therefore is not acidic retinenci-protein but its 

 precursor ; and so is homologous with the 480 m/z precursor of 

 retinenci in neutral solution. In lythgoe's terminology it should 

 be regarded as part of the "transient orange" complex, not as the 

 acidic form of "indicator yellow".' 



The suggestion that acid indicator yellow (Amax = 440 mju) is 

 transient orange (Amax = 480 mju) in an acid medium cannot be 

 accepted, lythgoe himself (1937) had clearly thought of this as a 

 possibility and had devised an experiment to test it. He first bleached 

 an alkahne solution of rhodopsin to the indicator yellow stage (i.e. 

 all transient orange had decomposed to alkahne indicator yellow). 

 On making this solution acid, the 440 m/Li chromogen (acid indicator 

 yellow) was formed. It might, perhaps, be argued that acidification 

 had restored the transient orange. If so, then on making the solution 

 alkaline once more one would expect the reappearance of the 480 m/Li 

 maximum. This, however, does not happen; the absorption spec- 

 trum of the solution reverts to that for alkaline indicator yellow. 

 The change can be repeated any number of times' (lythgoe, 1937). 



Both acid and alkaline forms of indicator yellow eventually suffer 

 hydrolysis to form retinene (Amax = 385 m^) and protein. Through 

 confusing acid indicator yellow (Amax = 440 m/u) with transient 

 orange, wald and hubbard were led to beheve that the Amax for 

 acid indicator yellow is at 390 m/^, the value for a faded solution in 

 which the indicator yellow had been largely hydrolysed to retinene 

 and protein. 



regeneration of rhodopsin 



WALD and BROWN (1950) found that solutions of retinenej and 

 *opsin' (the protein of rhodopsin) will react spontaneously in darkness 



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