The Origin of Specific Proteins 117 



binations containing two acic nuclei and one med nucleus joined in 

 a common cytoplasm. In this case, the productivity of the two acic 

 nuclei is apparently so great that the med cytoplasm cannot complete 

 the conversion of the acic enzyme, hence an intermediate stage in the 

 conversion accumulates and becomes visible. 



The existence of numerous enzymes in isozymic forms requires us 

 to seek an explanation in terms of physiological utility to the organ- 

 ism. One of the most attractive explanations for isozymes arises out 

 of the work of Stadtman and his collaborators (1961 ) on the enzyme 

 aspartokinase in Escherichia coli. This enzyme catalyzes the phos- 

 phorylation of aspartate by ATP and the resulting aspartyl phosphate 

 serves as a precursor in the synthesis of the two amino acids, lysine 

 and threonine. Both of these amino acids inhibit aspartokinase ac- 

 tivity in extracts of E. coli. The inhibitions are additive and inde- 

 pendent, suggesting that two different forms of aspartokinase are 

 present. And indeed, by ammonium sulfate precipitation two different 

 isozymes were separated; one was inhibited by lysine, the other by 

 threonine. Moreover, when E. coli is grown in the presence of 10 mM 

 L-lysine, the synthesis of the lysine-sensitive isozyme is completely 

 repressed; thus the amount of the isozyme synthesized is adjusted to 

 the physiological recpiirements of the organism. 



Although both isozymes of aspartokinase carry out the same cata- 

 lytic activity, the results of Stadtman et al. suggest that the isozymes 

 may be located in different metabolic pathways, one leading to the 

 production of threonine, the other to lysine. The behavior of the 

 isozymes of aspartokinase suggests the following general hypothesis. 

 Isozymes of any given enzyme carry out the same basic catalytic 

 activity, but as parts of different metabolic pathways. These metabolic 

 pathways are either located in different parts of a cell or give rise to 

 different end products. The gradual emergence during embryonic 

 development of specific isozymic patterns would then reflect the origin 

 of distinct though related metabolic pathways. 



In summary, the origin of specific proteins during development 

 recjuires two basic interacting components — the genes and the cyto- 

 plasm. Genetic and biochemical evidence clearly demonstrates the 

 role of genes in specifying the primary structure of proteins, hut the 

 existence of isozymes also suggests a role for the cytoplasm in modify- 

 ing at least the finer aspects of protein structure. Both the genes and 

 the cytoplasm are brought to a state of active function by as yet un- 

 known mechanisms of cellular differentiation. The most fundamental 

 expression of this differentiation is the appearance of a new specific 

 . protein, which must be regarded as a product involving the collabo- 



