Studies on Human Triosephosphate Isomerase

Abstract

Abstract Crystalline triosephosphate isomerase from human erythrocytes can be resolved into three catalytically active forms by isoelectric focusing. Each variant (I, 1 to 5%, pI = 6.7; II, 70 to 75%, pI = 6.5; and III, 20 to 25%, pI = 6.1) refocuses as a single component and is a dimer (mol wt 56,000). Identical electrophoretic patterns are obtained from fresh and aged red cell lysates and are unchanged in the presence of reducing agents or proteolytic inhibitors. The enzyme can be dissociated in guanidinium chloride and reassociated into catalytically active enzyme. Dissociation and reassociation of Components I and III result only in the respective parent species, whereas dissociation and reassociation of Component II gives rise to all three forms of the enzyme. The amino acid compositions and tryptic peptide fingerprints of the components indicate several structural differences in the two types of subunits. The catalytic properties of the three forms are similar but consistent with the designation of Components I and III as homodimers AA and BB and Component II as the heterodimer AB.