Further characterization of phosphoinositol kinase isolated from germinating mung bean seeds

Abstract

Phosphoinositol kinase isolated and purified from germinating mung bean seeds has been further characterized. The rate of phosphorylation varies with different inositol phosphates and this is consistent with the K_m and V_max for each of the substrates. The phosphate transfer from ATP has been found to be mediated by a phosphoprotein intermediate. In a particular step of the reaction the immediate product of the reaction has been found to be most inhibitory, other products being less or non-inhibitory. The inhibition has been found to be competitive in nature. The K_is have been found to range between 0.6 and 1 × 10^-4 M. ADP also inhibited non-competitively with respect to IP₅. K_i for this has been found to be 2.3 × 10^-4 M. The purified enzyme migrated as a single protein band on polyacrylamide gel electrophoresis. In the presence of sodium dodecyl sulphate it is dissociated into 3 subunits in the ratio 1 : 1 : 1. The MW of the three subunits are approx. 86 000, 56 000 and 35 000. The MW of the enzyme has been found to be approx. 177 000.