Dissociation-association of lactate dehydrogenase isozymes: influences on the formation of tetramers versus dimers of M4-LDH and H4-LDH.

1. A molecular sieve membrane was used to separate active dimer vs active tetramer fractions of M4-LDH and H4-LDH. 2. Dissociation of both enzymes was influenced by enzyme protein concentration and by the concentration of added substrates, pyruvate or lactate. 3. Increasing lactate concentrations increased the fraction of tetrameric enzyme whereas increasing pyruvate (up to saturating levels) had the opposite effect, raising the content of dimer fraction. 4. For H4-LDH, levels of pyruvate that caused substrate inhibition reversed the effect of lower concentrations of pyruvate and reduced the dimer content. 5. The data suggest that dissociation-association of LDH may have functional importance, the dimer having a preferential role in pyruvate reduction and the tetramer a preferred function in lactate oxidation.