Inosinic acid dehydrogenase of sarcoma 180 cells.

Inosinic acid dehydrogenase was partially purified from sarcoma 180 cells. The concentrations of each reactant, inosine 5’-phosphate, nicotinamide adenine dinucleotide, and K+, altered both the apparent K, and Vmaxvalues for each of the other reactants. Enzymatic activity was inhibited by both products of the reaction, xanthosine S-phosphate and reduced nicotinamide adenine dinucleotide; the inhibition produced by XMP was competitive with respect to IMP, whereas the inhibition caused by NADH was not competitive with respect to either IMP, NAD+, or K+. The data are consistent with an enzymatic mechanism involving ordered sequential addition of IMP, NAD+, and K+ to the enzyme to form active enzyme-substrate complexes. Substrate inhibition occurred at NADf concentrations several-fold higher than the optimal concentration of NAD+; the kinetics suggests that formation of an abortive ternary complex between the enzyme, XMP, and NAD+ is involved in producing this substrate inhibition.