Antiserum to purified human erythrocyte pyruvate kinase was used to identify immunologic relationships among erythrocyte pyruvate kinases from several phylogenetically distant mammals: humans, rhesus monkey, beef, rat and two marsupials opposum and phalanger. Each of the enzymes tested shared partial identity with the human enzyme and with the enzyme of every other species by gel diffusion analysis. No two species shared complete identity. Thus each species shares antigenic determinants with the human enzyme which are either missing or unreactive in the other species. All of the enzymes were inactivated by the antibody with the relative efficiency of inactivation being in general agreement with previously established phylogenetic relationships. The one exception was the phalanger which showed a closer relationship to man than expected. We conclude that mammalian pyruvate kinases are related in amino acid sequence and three dimensional structure as predicted by their similar kinetic properties. We further conclude that during evolution from the common ancestor each species has undergone a mutational loss of antigenic determinants so that each bears only a percentage of the ancestral determinants and that these are shared at random with the human enzyme which is presumed to have undergone a similar mutational process. On this basis we suggest that erythrocyte pyruvate kinase has probably evolved at a faster rate than previously thought.
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