| Abstract: | Immunological homology was shown between the active site regions of pig and rabbit aminopeptidases N and between those of the corresponding aminopeptidases A. However, no homology was detectable between the aminopeptidases N and A (EC 3.4.11.-) in a given species. The dimeric structure of pig aminopeptidases did not significantly modify their catalytic properties in aqueous solution compared to those of the monomeric rabbit enzymes. Only a slight difference in binding conditions was noted in the case of aminopeptidases N. Aminopeptidase A activity towards acidic substrates was enhanced by physiological concentrations of Ca2+ while that towards neutral substrates was considerably reduced. Therefore, acidic amino acid residues in proteins and peptides may be assumed to be mostly split off in vivo by aminopeptidase A, neutral residues by aminopeptidases N and basic residues by both enzymes. The respective specificity of aminopeptidase A and N for acidic and neutral amino acid residues was found to be mainly due to a more productive binding mode of the substrate rather than to a better affinity. |
