Pyruvate kinase,an enzyme subject to regulation inDioscorea alata

Different species of yam tubers were examined for the presence of pyruvate kinase and phosphatase activities, Pyruvate kinase was purified 25 fold with a yield of 50 %. using ammonium sulphate precipitation and ion exchange chromatography on DEAE-Sephadex. Partially purified enzyme showed normal Michaelis-Menten kinetics. However, pyruvate kinase from crude extract of dormant yam tuber showed slight sigmoid response towards phosphoenol-pyruvate and magnesium and to a certain extent ADP. The enzyme is activated by AMP and inhibited by ATP and citrate in both crude and partially purified preparations. Further studies on the effect of energy charge on the enzyme strongly suggest that pyruvate kinase from D. alata is a regulatory enzyme. No evidence was found for the presence of more than one pyruvate kinase in germinating D. alata tuber. With the exception of D. dumentorum, all the other three species of yams studied contain very little or no detectable phosphatase activity during dormancy. However, phosphatase activity increased during germination in all the species, thus excluding the use of sprouting yam tubers for kinetic study of pyruvate kinase.