Dimers of porcine skeletal muscle lactate dehydrogenase produced by limited proteolysis during reassociation are enzymatically active in the presence of stabilizing salt.

Lactate dehydrogenase from pig skeletal muscle is known to be a "dimer of dimers" stabilized in its tetrameric state by an N-terminal sequence of 20 aminoacid residues. Limited proteolysis of dimeric intermediates of association with thermolysin prevents association of the enzyme to its tetrameric structure. The resulting stable "dimers" are still capable of binding to a Procion Green dinucleotide affinity column. This may indicate that the dimeric intermediates contain the dinucleotide fold of the native enzyme. Since this structural feature is preserved after thermolysin treatment, affinity chromatography may be applied to separate the "dimer" on a preparative scale. Endgroup analysis and peptide mapping of the proteolytic dimer indicates that the reaction product consists of intact chains lacking the "N-terminal arm", apart from "nicked subunits" with fragments of 18 000 and 12 000 molecular weight. According to its hydrodynamic and conformational characteristics (sedimentation velocity and circular dichroism), the "dimer" does not differ significantly from the native enzyme in the backbone structure of its subunits. Its sedimentation properties resemble those of other dimeric dehydrogenases. The spectral data are similar to those observed for the dimeric intermediates that form during reconstitution after acid denaturation. Under standard test conditions, the proteolytic "dimer" (as well as the dimeric intermediate on the pathway of folding) do not show enzymatic activity. However, in the presence of "structure-making ions" like 2 M ammonium sulfate, about 40% of the native catalytic function is restored in the dimeric state. As shown by activity transport measurements in the ultracentrifuge, both the native tetramer and the proteolytic "dimer" maintain their quaternary structure in this solvent.(ABSTRACT TRUNCATED AT 250 WORDS)