78 The Nature of Biological Diversity 



even in this species, K+ appears to he required for pyruvate kinase 

 activity but in a considerably lower concentration than for the other 

 tissues. 



In addition to the requirement of K+ for pyruvate kinase from 

 sources listed in Table 4, K+ has also been shown to activate pyruvate 



Table 4. K + activation of pyruvate kinase from different sources 



Tissue used 



Activity * 



NoK + 



K + 



Muscle source 



Activity * 



NoK 4 



+K + 



Skate 



Fundulus 



Limulus 



Pecten 



Thyone 

 Phascolosoma 



Torpedo 

 Anadonta t 



5 



2 



41 



8 



45 



65 



390 



39 



* iiimoles pyruvate formed per 100 mg tissue per 10 minutes. 

 1 Dialyzed. 

 source: Ref. 21. 



kinase from yeast (22 ) and from plants (23 ) . The K + activation thus 

 appears to be a rather general phenomenon, suggesting either an 

 irreplaceable role for K + in the reaction mechanism or a close 

 phylogenetic relationship of the various organisms. 



Mechanistic relations of enzymes 

 catalyzing similar reactions 



Enzymologists have quite logically based much of their work on 

 the probability that enzymes catalyzing similar reactions do so by 

 similar mechanisms. Various examples could be cited; a particularly 

 good one comes from the dehydrogenases linked to the cofactor, 

 diphosphopyridine nucleotide. In all instances studied, the reduction 

 of the cofactor occurs with a direct transfer of a hydrogen, probably 

 as a hydride ion, from the substrate to the cofactor ( 24 ) . Another 



