Comparative studies on the dodecameric and hexameric forms of yeast aminopeptidase I

Yeast aminopeptidase I, when purified from autolysates of brewer's yeast, is obtained in two molecular froms a) the enzymatically active dodecameric complex (Mr = 640,000, s20,w = 22 S) and b) inactive hexamers (Mr = 320,000, s20,w = 12 S). Although the amino acid composition of the 12 S protein is very similar to that of the active enzyme,the hexamers behave differently in ionic exchange chromatography and during electrophoresis on polyacrylamide gels. Moreover, the antigenic properties of 12 S and 22 S aminopeptidase forms suggest a considerable degree of structural diversity. Several strains of Saccharomyces cerevisiae did not contain hexameric forms although their 22 S aminopeptidase was immunologically indistinguishable from brewer's yeast aminopeptidase. It is proposed that the hexameric protein is the result of "unproductive" aggregation of aminopeptidase subunits.