Kinetic Mechanism and End-Product Regulation of Deoxyguanosine Kinase from Beef Liver Mitochondria

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Initial-rate kinetic measurements with the affinity purified 2-deoxyguanosine (dGuo) kinase from beef liver mitochondria yielded reciprocal plots which converged on the abscissa, with either dGuo or ATP as the varied substrate. The limiting K_m, for dGuo was 4.7 μM, and that for ATP was 780 μM. One product, dGMP, was competitive with both substrates, while the other, ADP, was competitive with ATP and non-competitive with dGuo. Qualitatively identical results were obtained with an alternative substrate, dTTP, and with alternative product inhibitors, dIMP and dTDP. These results are consistent with a random Bi Bi kinetic mechanism, judging from the formation of a dead-end complex of the enzyme, dGuo and ADP. dGTP competes very strongly with ATP (K₁=0.03 μM), but is non-competitive towards dGuo. The more weakly-bound dGDP is competitive with both substrates.