Pyruvate decarboxylase from Zea mays L. I. Purification and partial characterization from mature kernels and anaerobically treated roots

Pyruvate decarboxylase (PDC) was purified from mature, dry maize kernels and from roots of anaerobically treated maize seedlings and partially characterized. PDC was purified to a specific activity of 96 units per milligram protein from kernels and to 41 units per milligram protein from root. The subunit molecular masses were estimated to be 61,000 and 60,000 for kernel PDC and 59,000 and 58,000 for root PDC. The pH optimum for each enzyme was 5.8. Since the pH optimum is nearly one pH unit below the value reported for the cytoplasm of anaerobically metabolizing maize roots (pH 6.7 ± 0.2), we investigated the effects of pH 5.8 and 6.6 on the cooperative kinetics observed for PDC from each source. The maximum Hill coefficients ($\text{n}_{\text{H}}$) were much greater at each pH for the kernel PDC (pH 5.8, $\text{n}_{\text{H}}$ = 2.5 and pH 6.6, $\text{n}_{\text{H}}$ = 3.2) than for the root PDC (pH 5.8, $\text{n}_{\text{H}}$ = 1.4 and pH 6.6, $\text{n}_{\text{H}}$ = 1.8). The cooperative kinetics observed with respect to pyruvate were asymmetric. Potassium inhibited maize PDC and was competitive with pyruvate (root PDC $\text{K}_{i}$ = 16 millimolar and kernel PDC $\text{K}_{i}$ = 10 millimolar).