Properties of the muscle and heart lactate dehydrogenases of the New Zealand hagfish, Eptatretus cirrhatus: Functional and evolutionary implications

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Abstract
Hagfish possess two lactate dehydrogenase (LDH) subunit types that combine to form only the two homotetramers LDHA4 and LDHB4. Similarities in the kinetic properties of these isozymes from Myxine glutinosa have been attributed to the anaerobic nature of the hagfish heart and the failure of LDHB4 to diverge in function since the ancient A‐B gene duplication. The LDHA4 and LDHB4 isozymes purified from the New Zealand hagfish Eptatretus cirrhatus display more divergent properties which are consistent with the more efficient oxygen delivery system and aerobic lifestyle of this species. This, together with recent reinterpretations of LDH subunit evolution, leads to the conclusion that the unusual properties of hagfish LDH isozymes can be attributed to metabolic adaptation rather than to the conservation of features present in the ancestral LDH of vertebrates.