INTRODUCTION TO BIOCHEMICAL SYSTEMATICS 



57 



Bloch: "On the other hand you see no difficulty in assuming entirely 

 separate pathways in the evolution of the specific bile acids?" 



Haslewood: "No difficulty. If you were to tell me, as the result of 

 researches on protein, that the enzymes making cholic acid in the cod 

 are quite different substances from the enzymes making cholic acid in 

 man, I would not be at all surprised." 



When the time arrives at which amino acid sequences of in- 

 dividual proteins can be efficiently analyzed, this procedure will 

 doubtlessly provide some of the answers to questions of enzyme 

 homology versus analogy. At present the procedure is complex and 

 tedious, and only a few laboratories are involved. Serological inves- 

 tigations may provide considerable circumstantial evidence, as has 

 been discussed, for if two similar enzymes behave as a single antigen, 

 they are best considered to be homologous. It is possible, of course, 

 but not highly probable, that the enzymes have evolved independ- 

 ently as serologically identical molecules. There are some interesting 

 possibilities for the study of enzyme systems which appear to be 

 definitely non-homologous. For example, in the squid eye the pros- 

 thetic group of the visual pigment is described as neo-b-retinine 

 (Hubbard and St. George, 1958), similar to the pigment of the ver- 

 tebrate eye. Since the squid eye and vertebrate eye are generally re- 

 garded as one of the classic examples of convergent evolution in 

 structure, the precise molecular configuration of the "enzymes" in- 

 volved in the remarkable correlated biochemical parallelism is of 

 interest. In another similar case Johnson et al. (1960) reported an 

 interacting luciferin-luciferase system between a crustacean {Cypri- 

 dina) and a fish {Apogon). There is reason to believe that the 

 biochemical mechanisms of phosphorescence are similar in the two 

 species and represent another example of convergent biochemical 

 evolution. Surprisingly, the authors seem to interpret the discovery 

 somewhat differently, however, "although the similarities in the 

 luminescent systems of a fish and crustacean could represent a rare, 

 evolutionary coincidence, they as likely indicate that more of a thread 

 of unity exists in the comparative biochemistry of luminescence 

 among diverse types of organisms than has been hitherto supposed." 

 Anyway, serological comparisons of luciferase from the two sources 

 would be of interest as they represent potentially analogous enzymes. 



Dessauer et al. (1962) have compared certain iron-binding 

 proteins (transferrins) of 150 reptiles and amphibians by electro- 

 phoresis. Large differences in migration rates were observed; in some 

 instances there was considerable intraspecific variation, and also 

 multiple transferrins were often found. In some cases the transferrins 



