Modulation of the activity of NAD malic enzyme from solanum tuberosum by changes in oligomeric state

The effects of pH, NaCl, and malate2- on the equilibrium between dimeric and higher-molecular-weight forms of NAD malic enzyme from Solanum tuberosum var. Chieftain have been analyzed by monitoring the kinetic changes associated with disaggregation [S. D. Grover and R. T. Wedding (1982) Plant Physiol. 70, 1169-1172]. At pH values above 7.0 the enzyme was disaggregated to the dimeric, high-Km(malate) form by preincubation with NaCl, with a half-maximal effect at 25 mM. At low pH the enzyme remained in the low-Km(malate) (tetramer or octamer) form. Malate protected against disaggregation to the high-Km form in preincubation, and this effect was half-maximal at 6 mM. At pH 7.3, in the absence of malate, half-maximal disaggregation occurred at 580 nM enzyme. Varying the enzyme concentration in the assay led to kinetic changes which fit equations based on an associating enzyme model [B. I. Kurganov (1967) Mol. Biol. (Moscow) 1, 17-27]. This analysis confirmed that the dimer has intrinsic activity, with Vm somewhat lower than that of the tetramer but a Km(malate) that was 9-fold higher than that of the tetramer. Malate decreased the Kd for disaggregation of the enzyme during assay approximately 20-fold, with a half-maximal effect at 3 to 4 mM. In contrast, high NaCl concentrations in the assay increased the Kd for disaggregation in a manner which was competitive with the effect of malate on Kd. The physiological significance of these aggregation state changes is discussed.