The effect of nucleoside diphosphate kinase （NDPK） on the activrty of guanine nucleotide regulatory protein （G-protein） mediated phospholipase C（PLC） and on the [35S]GTPTτS binding of G-protein was investigated in this work in order to demonstrate the mechanism behind the regulation of G-protein and its effector PLC by NDPK. The stimulation of PLC in turkey erythrocyte membrane by both GTP and GTPτS indicated that the PLC stimulation was mediated by G-protein. NDPK alone stimulated PLC activity. as well as the stimulation in the presence of GTP and GDP

in a dose-dependent manner. However. NDPK inhibited GTPτS-stimulated PLC. Furthermore

NDPK inhibited [35S]GTPτS binding of purified Gi-protein in a non-competitive manner. A hypothesis implying an important role of direct interaction of G-protein and NDPK in the regulation of their functions is suggested and discussed.