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Abstract
A variant of angiotensin-converting enzyme occurs in (male) germinal cells. This testicular isozyme is catalytically similar to the widespread pulmonary-type isozyme, but contains a shorter polypeptide chain and does not appear until puberty. The two proteins differ at their NH2- and COOH-termini, but share many tryptic peptides. All antigenic determinants of the testicular form are represented in the pulmonary molecule whereas the latter contains determinants unrelated to catalysis which are lacking in the testicular species. The data indicate that the testicular isozyme corresponds closely to an internal part of the pulmonary polypeptide which includes its active site. The structural and developmental differences between the two polypeptides are pretranslationally determined since they are demonstrable in a cell-free system programmed by the appropriate mRNAs. Characterization of the molecular mechanisms responsible for the relationship of these isozymes may yield useful information regarding cell-specific protein expression.