The anomally of pyridine nucleotide synergism in carbon tetrachloride metabolism

Carbon tetrachloride metabolism was examined in hepatic microsomes isolated from control and phenobarbital-treated Sprague-Dawley rats to determine the mechanism of pyridine nucleotide synergism. An NADPH generator increased metabolism two fold as determined by lipid peroxidation. Addition of NADP to the reaction system did not alter the maximum velocity, but did decrease the Km for NADPH from 61 μM to 7.6 μM in control and from 21 μM to 6.3 mM PB microsomes. Addition of NAD⁺ produced an increase in metabolism similar to NADH. Substrates and competitive inhibitors of nucleotide pyrophosphatase also enhanced CCl₄ metabolism. A high correlation (r=0.947) was indicated between the percent inhibition of nucleotide pyrophosphatase and the percent synergism of NADPH-catalyzed CCl₄ metabolism. Thus, pyridine nucleotiode synergism in CCl₄ metabolism appears to result from the increased availability of NADPH produced by a decreased degradation of the NADPH by the nucleotide pyrophosphatase.