The Disposition of Formaldehyde and Formate Arising from Drug N‐Demethylations Dependent on Cytochrome P‐450 in Hepatocytes and in Perfused Rat Liver

Abstract

While formaldehyde liberation has become a widely‐used parameter in studies of the cytochrome P‐450 system in isolated microsomal fractions, little is known on the fate of formaldehyde generated by the endoplasmic reticulum in intact cells and organs. The metabolism of formaldehyde arising from drug N‐demethylations was studied in isolated hepatocytes and in perfused rat liver using [dimethylamino‐14C]aminopyrine as drug substrate. In the perfused liver, the rates of production of formaldehyde, formate and CO2 were 0.02, 0.24 and 0.10 μmol × min−1× g liver wet weight−1, respectively, when the perfusion medium consisted of Krebs‐Henseleit buffer with lactate and pyruvate as substrates, whereas in the presence of 0.2 mM added l‐methionine the corresponding values were 0.02, 0.08 and 0.27 μmol × min−1× g liver−1. Similarly, a large stimulatory effect (4‐fold) on [14C]O2 production upon addition of methionine was observed with 14C‐labeled formaldehyde and formate. The contribution of the peroxidatic reaction of catalase in formate and CO2 production from formaldehyde is low, due to a limitation of H2O2 production. When an H2O2‐generating substrate, glycolate, was added, the [14C]O2 production from [dimethylamino‐14C]aminopyrine or from [14C]formate was increased. An increase in the mitochondrial and cytosolic NADH redox indicator ratios, 3‐hydroxybutyrate/acetoacetate and lactate/pyruvate, respectively, upon aminopyrine addition is attributed to the reaction of the two formaldehyde dehydrogenases. Conversely, the rates of [14C]O2 production from labeled aminopyrine or formaldehyde, but less from formate, were decreased when mitochondrial (3‐hydroxybutyrate) or cytosolic (lactate, xylitol, ethanol) NAD+ reductants were added. With formate oxidation being the rate‐limiting step in C1‐unit disposition, the predominant reaction is the methionine‐dependent coupled pathway of 10‐formyltetrahydrofolate synthetase and 10‐formyltetrahydrofolate dehydrogenase (Scheme 1), linking drug N‐demethylation to cellular folate and amino acid metabolism. Thus, CO2 production from drug methyl groups may be altered in states of folate or vitamin B12 deficiency or certain states of malnutrition. Copyright © 1978, Wiley Blackwell. All rights reserved