Evolutionary relationships of vertebrate lactate dehydrogenase isozymes A4 (muscle), B4 (heart), and C4 (testis).

The amino acid sequence variation among seven lactate dehydrogenase isozymes from dogfish muscle, chicken muscle and heart, pig muscle and heart, and mouse and rat testes were compared with respect to the whole lactate dehydrogenase polypeptide chain as well as their four functional domains. The coenzyme-binding domain is more conserved than the substrate-binding domain. The sequence of the loop and helix alpha D region of testicular LDH-C4 isozymes is very different from those of somatic LDH-A4 and LDH-B4 isozymes, while the NH2-terminal arm is extremely variable. The most parsimonious phylogenetic tree among these seven vertebrate lactate dehydrogenase sequences clearly indicates that the LDH-A4 and LDH-B4 isozymes are more closely related to each other than either to the LDH-C4 isozymes.