CYTOCHROME 



31 



muscles which operate the radula of the edible snail b is unusually 

 prominent as compared with a and c. In baker's yeast also band b 

 is as conspicuous as band c. Now to pass to bands x, y and z. Where 

 band c is conspicuous relatively to a and b, band x is conspicuous 

 relatively to z and y, similarly z pairs off with a and y with 6, so that 

 in the bumble-bee z is relatively intense, whilst in the snail's tongue- 

 muscle and in baker's yeast, y is more prominent than is ordinarily 

 the case. 



This pairing of the bands suggests that the spectrum of cytochrome 

 is not that of a single substance but of a mixture of three substances, 

 of which bands a and z represent the spectrum of one, b and y of the 

 second, and c and x of the third. Each substance would then have a 

 two-banded spectrum (Fig. 6) . Let us see what further evidence there 

 may be for this tripartite structure of cytochrome. Is it possible to 



RED BLUE 



Fig. 6. Showing the bands of the three components of cytochrome. 



abolish one pair of bands without abolishing the whole? This may 

 be done. As has been said oxidation of cytochrome under ordinary 

 circumstances abolishes all the bands, but if a suspension of baker's 

 yeast be treated with urethane or alcohol before oxidation the bands 

 disappear seriatim when the suspension is oxidised; of the bands a, 

 b and c, c disappears first (with it x), then a (with z), so that 6, of these 

 three, remains alone with the corresponding part of d. 



Again, if yeast be dried and the dried mass be extracted with 

 water a yellowish opalescent liquid results. This liquid shows only 

 two bands, one of which corresponds to band c, and the other to a 

 portion of the band d of cytochrome. It may be inferred therefore 

 that the aqueous extract of the desiccated yeast contains one and one 

 only of the three constituents of cytochrome. 



Now a fresh light flashes on the constitution of cytochrome — surely 

 this two-banded spectrum of the aqueous extract is a very old friend — 



