The exhalation of l4CO2 after the administration of [dirnethylamin~-'~C]aminopyrine to an organism is assumed to reflect the demethylation of aminopyrine by hepatic mixed-function oxidase activity. The formaldehyde formed as a result of the demethylation of aminopyrine is then sequentially oxidized to formic acid and COz. The last step in the pathway, i.e., formate oxidation, is dependent upon tetrahydrofolate; thus, factors which alter hepatic tetrahydrofolate potentially may modify '"C-aminopyrine metabolism to l4COZ in vivo. Exposure of rats to nitrous oxide (NzO) produces a significant reduction in hepatic tetrahydrofolate as a result of the inhibition of 5-methyltetrahydrofo1ate:homocysteine methyltransferase activity (E.C. 2.1.1.13). In the present study, exposure of rats to NZ0/Oz (1:l) for 4 hr prior to the administration of ''C-aminopyrine (40 or 400 pmoles per kg) produced a 60% reduction in the peak rate of '"COZ exhalation and a 45% decrease in the total l4COZ exhaled within 2 hr. In control experiments, exposure of rats to nitrogen/Oz (1: 1) produced no effect on '"C-aminopyrine metabolism to l4CO2. Administration of methionine (1.3 mmoles per kg) 30 min prior to 14C-aminopyrine administration reversed the inhibition of '"CO2 exhalation and reduction in hepatic tetrahydrofolate observed in NzO-exposed animals. Aminopyrine (400 pmoles per kg) administration to air-breathing rats did not affect the level of urinary formate, but exposure to NzO produced a 40-fold increase. Aminopyrine administration to NzO-exposed rats produced a 75% increase in urinary formate as compared to rats treated with NzO alone. The first-order rate constant for aminopyrine elimination from the plasma, apparent volume of distribution, apparent plasma clearance and the level of hepatic cytochrome P-450 were not affected by NzO exposure. NzO exposure inhibited '"CO2 exhalation after [ Nmethyl-'"Clmorphine administration to a similar degree as that observed after aminopyrine administration. These results indicate that factors which alter the level of hepatic tetrahydrofolate, e.g., nutritional status, NzO exposure or certain drugs, may modify the metabolism of '"C-aminopyrine to l4COz in uiuo without affecting the demethylation of aminopyrine. Thus, such factors must be controlled when the exhalation of '*CO2 after ''C-aminopyrine administration to an organism is used as an in vivo measure of hepatic drug metabolism.
Demethylation is an important route of metabolism for many drugs and is primarily catalyzed by one or more of the forms of hepatic microsomal cytochrome P-450. The formaldehye released as a product of the demethylation reaction is then sequentially oxidized to formic acid