The Nature and Diversity of Catalytic Proteins 77 



laboratory Westhead has demonstrated such incorporation in a living 

 animal, namely, that rabbits fed />fluorophenylalanine show extensive 

 incorporation of this amino acid analog into tissue proteins. There are 

 clear indications, particularly with bacterial systems, that synthesis 

 of proteins which are biologically inactive may be one cause of the 

 toxicity of the amino acid analogs. A few highly purified enzymes 

 containing analogs have been isolated and their properties compared 

 to their normal counterparts. Crystalline Bacillus subtilis a-amylase 

 containing considerable ethionine in place of methionine retains full 

 activity (17), but replacement of around one-sixth of the phenylal- 

 anine residues by p-fluorophenylalanine results in 30 per cent loss of 

 activity (18). Similarly, p-fluorophenylalanine incorporation into 

 penicillinase gives extensive loss of activity (19 I. In contrast. West- 

 head (20) found that rabbit aldolase with replacement of about 

 one-fourth of the phenylalanine residues with p-fluorophenylalanine 

 was indistinguishable from the normal' enzyme in physical, chromato- 

 graphic, and catalytic properties. Variable effects of analog incorpora- 

 tion could logically reflect the importance of the residues being 

 replaced to the catalytic site or essential structure of a given enzyme. 

 The occurrence of such variability is of interest because it suggests 

 that even selectivity for normal amino acids may be subject to error, 

 and enzyme microheterogeneity might logically occur without pri- 

 mary genetic variation. 



The generality of eofaetor function 



The similarity of the actions of vitamin-containing cofactors is quite 

 well documented and understood. Also the role of divalent cations as 

 an activator is well recognized. For example, every enzyme system 

 reacting with ATP has been found to require a divalent cation, 

 usually Mg++, for maximal activity. Some years ago, during a brief 

 stay at the Marine Biological Laboratory at Woods Hole, I had oc- 

 casion to study whether a less well understood eofaetor requirement, 

 namely, that of the monovalent cation K+, also appeared to be a 

 general requirement for one enzyme from a variety of sources (21). 

 The enzyme studied was pyruvate kinase, catalyzing the transfer of 

 a phosphoryl group from phosphoenolpyruvate to ADP. The result - 

 obtained are summarized in Table 4 and show that, in all tissues and 

 species tested, activation by K+ was demonstrable even without 

 dialysis of the tissue extract. With a fresh-water species, Anadonta, 

 which has a characteristically low intracellular salt content, only a 

 weak activation was demonstrable, however, without dialysis. Thus 



