Reversible inactivation of dehydrogenases.

1. The reversible dissociation of lactic and malic dehydrogenases has been studied in detail and compared with the dissociation properties of triosephosphate dehydrogenase, a-glycerophosphate dehydrogenase, and liver alcohol dehydrogenase. 2. Evidence is presented for the existence of an intermediate form of lactic dehydrogenase with catalytic properties altered during reactivation. 3. The technique of reversible dissociation has been used to prepare interspecies hybrids of lactic, malic, and triosephosphate dehydrogenases. 4. Some of the catalytic and immunological properties of hybrid malic dehydrogenases have been examined. 5. The effects of salts on the reactivation of malic dehydrogenases have been investigated and related to previous results on lactic dehydrogenases. It is proposed that the observed salt effects may result from changes in the activity coefficients of the exposed peptide and amide groups. mitochondrial malic dehydrogenases from different species supernatant and mitochondrial malic dehydrogenases from a single species, as well as hybrids of lactic dehydrogenases. Preliminary evidence for the existence of at least two catalytically active forms of lactic dehydrogenase during reactivation from guanidine hydrochloride was also presented. Various aspects of these studies have been examined in further detail and reversible inactivation by lithium chloride, as well as guanidine, acid, and urea, has now also been demonstrated. In our more recent investigations of malic dehydrogenase, particular emphasis has been placed on the effects of salts and reducing agents on reactivation. The nature of acid-dissociated malic dehydrogenase has been examined and the kinetic and immunological properties of hybrid malic dehydrogenases have been studied. In addition, the catalytic and physical properties of the forms of lactic dehydrogenase which are intermediates during reactivation have been further characterized. Also, we have extended our studies of reversible inactivation of dehydrogenases to include a comparison of lactic dehydrogenase and malic dehydrogenase with triosephosphate dehydrogenase, a-glycerophosphate dehydrogenase, and liver alcohol dehydrogenase.