Postsynthetic modification of human enolase isoenzymes

The original form of beta beta enolase (EC 4.2.1.11) in tissue is modified to two more electrophoretically distinct forms when incubated with human serum. The three postsynthetic forms are designated beta beta 3, beta beta 2, and beta beta 1, in order of increasing anodal mobility and increasing modification. Serum and carboxypeptidases A and B all produce identical modifications of beta beta enolase but exhibit very different pH-activity profiles. A purified human serum protein previously named "modifying protein," which is responsible for the modification of creatine kinase-M and alpha-enolase subunits, modifies beta beta enolase and also has a pH-activity profile identical to that for serum. Thus we conclude that the modifying protein is not identical to either carboxypeptidase A or B; it may, however, be an as-yet-undescribed carboxypeptidase. With increased modification, both alpha alpha and beta beta enolase decrease in apparent activation energy; gamma gamma enolase shows no evidence of modification, and its apparent activation energy remains stable. Measurement of activation energy is an easy tool for screening for postsynthetic modifications in an enzyme.