Reduction of Hepatic Tetrahydrofolate and Inhibition of Exhalation of 14CO2 Formed from [Dimethylamino-14C]Aminopyrine in Nitrous Oxide-Treated Rats

Abstract

The exhalation of 14 CO 2 after the administration of [dimethylamino- 14 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 CO 2 . The last step in the pathway, i.e., formate oxidation, is dependent upon tetrahydrofolate; thus, factors which alter hepatic tetrahydrofolate potentially may modify 14 C-aminopyrine metabolism to 14 CO 2 in vivo. Exposure of rats to nitrous oxide (N 2 O) produces a significant reduction in hepatic tetrahydrofolate as a result of the inhibition of 5-methyltetrahydrofolate:homocysteine methyltransferase activity (E.C. 2.1.1.13). In the present study, exposure of rats to N 2 O/O 2 (1:1) for 4 hr prior to the administration of 14 C-aminopyrine (40 or 400 Mmoles per kg) produced a 60% reduction in the peak rate of 14 CO 2 exhalation and a 45% decrease in the total 14 CO 2 exhaled within 2 hr. In control experiments, exposure of rats to nitrogen/O 2 (1:1) produced no effect on 14 C-aminopyrine metabolism to 14 CO 2 . Administration of methionine (1.3 mmoles per kg) 30 min prior to 14 C-aminopyrine administration reversed the inhibition of 14 CO 2 exhalation and reduction in hepatic tetrahydrofolate observed in N 2 O-exposed animals. Aminopyrine (400 pmoles per kg) administration to air-breathing rats did not affect the level of urinary formate, but exposure to N 2 O produced a 40-fold increase. Aminopyrine administration to N 2 O-exposed rats produced a 75% increase in urinary formate as compared to rats treated with N 2 O 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 N 2 O exposure. N 2 O exposure inhibited 14 CO 2 exhalation after [ N -methyl- 14 C]morphine 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, N 2 0 exposure or certain drugs, may modify the metabolism of 14 C-amino-pyrine to 14 CO 2 in vivo without affecting the demethylation of aminopyrine. Thus, such factors must be controlled when the exhalation of 14 CO 2 after 14 C-aminopyrine administration to an organism is used as an in vivo measure of hepatic drug metabolism.