Abstract: | The effect of SO32? on the activity of PEP-carboxylase and on subsequent malate formation has been studied in leaf extracts of Zea mays. PEP-carboxylase was assayed by incorporation of H14CO3 - into oxaloacetate dinitrophenylhydrazone and by a spectrophotometric method. In contrast to ribulose diphosphate carboxylase, PEP-carboxylase was not inhibited by 10 mM SO32? with respect to PEP. As was the case with ribulose diphosphate carboxylase, the activity of PEP-carboxylase was inhibited non-competitively with respect to Mg2+. However, the Ki value (84.5 mM) was found to be very high. With respect to HCO3?, like ribulose diphosphate carboxylase, PEP-carboxylase was inhibited competitively, but the Ki value (27 mM SO32?) increased by about the same factor (× 9) as the Km, (0·5 mM HCO3?) is decreased. This indicates that the replacement of HCO3? by SO32?, common to both enzymes, is facilitated by decreasing the affinity of the enzyme for HCO3?. At substrate saturating conditions malate formation by the combined action of PEP-carboxylase and endogenous NADH-dependent malate dehydrogenase in leaf extracts was not inhibited by 10 mM SO32?. Although the malate dehydrogenase is inhibited at this SO32? concentration to about 85%, malate formation is unaffected, as PEP-carboxylase is the rate limiting step its turnover rate being only about 8% of NADH-dependent malate dehydrogenase. |