The isolation and characterization of the multiple forms of human skeletal muscle triosephosphate isomerase

1. Human skeletal muscle triosephosphate isomeras (D-glyceraldehyde-3-phosphate ketol-isomerase, EC 5.3.1.1) was isolated and resolved by DEAE-cellulose chromatography into three major forms, A, B, and C, which comprise 97% of the total activity. The relative distribution was 25, 46 and 29% respectively. 2. The A and C forms are homodimers, alpha alpha and beta beta, and form B is the heterodimer, alpha beta. Reassociation studies from guanidinium chloride have indicated that A, B, and C are not conformers. Although these studies revealed the existence of two different chains, the amino acid analysis showed no significant variance. Since no differences were obsrved in Ouchterlony and Mancini tests or in immunotitration, the three fors are assumed to be immunologically identical. 3. The three forms have the same specific activity, Michaelis constants, pH optimum, activation energy, inhibition by metabolites and heat stability. Only with increasing ionic strength did the V and Km values differ. 4. The two poypeptide chains (alpha and beta) appear to be identical (amino acid composition, molecular weight and antigenity), and since the electrophoretic banding pattern changed with cell aging, it is concluded that the multiple forms of trisephosphate isomerase are the consequence of minor post-synthetic alteration(s) of form A.