Avian peptidase isozymes: tissue distributions, substrate affinities, and assignment of homology

Peptidase (EC 3.4.13.9 and EC 3.4.13.11) isozymes were examined from 14 tissues of 19 species of birds, representing 8 families and 6 orders, via horizontal starch gel electrophoresis. Peptidases were detected by employing histochemical staining techniques utilizing 13 di-, tri-, and tetrapeptides as substrates. It was determined that there is a minimum of six presumptive structural gene loci encoding peptidases in birds (excluding those encoding cytosol aminopeptidase; EC 3.4.11.1). The products of the peptidase loci were widely expressed in most tissues with the exception of Pep-F, which was restricted to pancreas. Products of most loci have multiple, overlapping substrate affinities (except for the products of Pep-D, which react only with a peptide containing a carboxyterminal proline). By using a combination of differential electrophoretic mobility and substrate preferences, the products of the various peptidase loci can be distinguished from one another. The homologies of the avian peptidases with those in other vertebrates were tentatively established and it is suggested that the nomenclature proposed by Rapley et al.