The complete primary structure of human LDH-B (heart), mouse LDH-B and LDH-C (testis) isoenzymes have been determined by both protein and cDNA sequencing. The human LDH-A, human LDH-B and mouse LDH-C proteins have been/are being expressed in E. coli as well as in CHO cells. The cancer- associated lactate dehydrogenase was shown to be tyrosylphosphorylated form of human LDH-A, probably complexed with ras P21 protein. The mammalian LDH-A isozymes were shown by both liquid-binding assay and Western blotting to bind single-stranded DNA. The LDH proteins were also shown to be one of major protein components present in human and mouse centrosomes. Recently, the duck epsilon- crystallin (about 23% of lens proteins) was found by partial protein sequencing to be active LDH-B isozyme. These unexpected findings suggest that the LDH isozymes also function as structural proteins. Their physiological significance is being investigated by site-directed mutagenesis.
