340 BIOCHEMICAL GENETICS 



added substrates. Black skin, however, is best able to form melanin when relying 

 upon its own endogenous resources, that is, when incubated in the absence of sub- 

 strates. We therefore conclude that the inherited pigmentary defect in brown skin is 

 in part due to some form of endogenous limitation of substrate. This conclusion was 

 "recently" independently "confirmed" by Sewall Wright in 1916 and 1942. 1416 - 1449 

 (Addendum: Recently we have noted an additional feature of the genetic affliction in 

 brown skin; it cannot form as much total tissue-bound melanin as black skin upon in- 

 cubation in vitro. Thus the genetic damage in the brown genotype may also involve 

 decreased number or effectiveness of the melanin binding sites of the protein matrix.) 



Finally, if we make systematic comparisons to determine the effects of single and 

 combined allelic substitutions at the p and d loci, we find that the amount of tissue- 

 bound melanin formed upon incubation in vitro does not run parallel to the amount of 

 oxidase activity. We are therefore led to suspect that the limiting factor in these color 

 mutants is not one of oxidase activity but rather one or more defects in the terminal 

 phases of the pigment-building process, phases which are not readily measured respiro- 

 metrically. A useful by-product of this study is the discovery of an unsuspected, 

 exceptionally rich source for enzymatic extraction procedures, namely, the pink-eyed, 

 dilute, brown genotype. 



While the methodologic improvements discussed are encouraging, they have not 

 yet helped us solve two important problems. The first concerns the nature of the 

 process for forming yellow pigment ( phaeomelanin) . At the present time no evidence 

 conclusively demonstrates tyrosine or any other likely chromogen as the natural 

 precursor of phaeomelanin. (See, however, Foster 395 and also Fitzpatrick and 

 Kukita. 382 ) A second apparent paradox is the striking albino-like behavior of deeply 

 pigmented skins obtained from animals bearing intermediate alleles at the albinism 

 locus. As long as allele C is lacking, that is, the chinchilla mouse (aac ch c ch ), the skin 

 behaves like the completely unpigmented albino skin toward added tyrosine or dopa. 

 A similar situation in the guinea pig was previously reported. 394 No evidence has yet 

 been obtained to explain this apparent paradox. One possible approach involves the 

 use of paper chromatographic methods for identifying diffusible endogenous melano- 

 genic precursors and intermediates. This method was already proved successful in 

 identifying the unstable, diffusible, intermediate dopa, in an incubation medium 

 containing skin with high tyrosinase activity (Foster and Brown, 397 work supported 

 in part by NIH Grant C-4305 and NSF Grant G-6480). 



I regret that Dr. Silvers failed to mention the excellent studies performed by 

 himself, in conjunction with Dr. Russell, concerning the role of the melanocyte environ- 

 ment in instructing the cell to synthesize either eumelanin or phaeomelanin. At this 

 point I should like at least to draw attention to the pertinent publications. 1202 - 1203, 



1205. 1208 



Dr. Barrett: I have a question that is not too profound, but I have usually 

 wondered about it when thinking of this work. Regardless of which theoretical explan- 

 ation applies, why do we not see in nature some explanation for the fact that in spotted, 



