Lactate dehydrogenases of Atlantic hagfish: physiological and evolutionary implications of a primitive heart isozyme.

Isozymes of lactate dehydrogenase from heart and muscle of Atlantic hagfish show less functional divergence than those from other fishes and higher vertebrates. The enzyme from hagfish heart (B4) displays a higher Michaelis constant for pyruvate and lower substrate inhibition at moderate pyruvate concentrations than heart isozymes from other species. These properties support the hypothesis that the ancestral vertebrate lactate dehydrogenase was a muscle (A4)-type enzyme and also suggest a role for the B4 enzyme in the unusual physiology of hagfish cardiac tissue which functions under sustained hypoxic conditions.