Substrate-dependent association of lactic dehydrogenase subunits to active tetramer.

Lactate dehydrogenase (LDH) has been reported to be a tetramner of molecular weight near 145,000, with a sedimentation coefficient at infinite dilution of 7.71S.1-' A has shown that appreciable dissociation of the tetramer is observed at concentrations below 0.5 mg per ml and that the dilute enzyme in sodium chloride solution will dissociate into a 5.5S dimer with one half the molecular weight of the tetramer. I Reasoning from this data, Reithel has proposed that the enzyme may be present largely as a dimer under the dilute conditions (<1.0 ,ug/ml) ordinarily employed in an enzymatic assay.4 This would imply an active dimer form. Kaplan and Chilson et al., on the other hand, feel that the tetramer is the active unit and that the dimer is inactive.5' 6 This position is strengthened by the report of an apparent decrease in specific activity of the enzyme at concentrations below about 1.0