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

The effects of pH, NaCl, and malate 2− 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- K m(malate) form by preincubation with NaCl, with a half-maximal effect at 25 m m. At low pH the enzyme remained in the low- K m(malate) (tetramer or octamer) form. Malate protected against disaggregation to the high- K m(malate) form in preincubation, and this effect was half-maximal at 6 m m. At pH 7.3, in the absence of malate, half-maximal disaggregation occurred at 580 n m 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 V m somewhat lower than that of the tetramer but a K m(malate) that was 9-fold higher than that of the tetramer. Malate decreased the K d for disaggregation of the enzyme during assay approximately 20-fold, with a halfmaximal effect at 3 to 4 m m. In contrast, high NaCl concentrations in the assay increased the K d for disaggregation in a manner which was competitive with the effect of malate on K d . The physiological significance of these aggregation state Changes is discussed.