Structural relatedness of mouse lactate dehydrogenase isozymes, A4 (muscle), B4 (heart), and C4 (testis)

Three homotetrameric lactate dehydrogenase isozymes, LDH-M(A4), LDH-H(B4), and LDH-X(C4), from DBA/2J mice have been purified by affinity chromatography. The amino acid compositions of the subunits A,B, and C, based on a molecular weight of 36,000, have been determined. The compositional relatedness of these isozymes indicates that subunits A (muscle) and B (heart) are more closely related to each other than to subunit C (testis). Tryptic peptide maps and amino acid compositions of some active site peptides apear to confirm the compositional relatedness among these isozymes. The sequence of the loop region of mouse C subunit seems to be markedly different from all known A and B sequences, and the structural and functional implications are discussed.