We have examined the hypothesis that the human erythrocyte isozyme of pyruvate kinase (EC 18.104.22.168) is a hybrid of the two isozymes present in liver. Rabbit antiserum against purified human erythrocyte pyruvate kinase inactivates the erythrocyte isozyme and the major liver isozyme from human tissue but does not inactivate the minor liver isozyme. The electrophoetic mobilities of the erythrocyte and major liver isozymes are altered by anti-erythrocyte enzyme antibody while the mobility of the minor liver isozyme is unaffected. Gel diffusion analysis indicates cross-reactivity between the erythrocyte and major liver isozyme but no cross-reactivity with the minor liver isozyme. The hybrid hypothesis would predict cross-reactivity including changes in activity and mobility of all isozymes and we conclude, therefore, that the hypothesis is incorrect.
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