Dimethylthiourea, a hydroxyl radical scavenger, impedes the inactivation of methionine synthase by nitrous oxide in mice.

Dimethylthiourea (DMTU), a potent scavenger of hydroxyl radicals, was studied to see if it attenuated the inactivation of methionine synthase produced by nitrous oxide in mice. Mice were given i.p. injections of DMTU 0.5-4.0 mg g-1 or saline and, 1 h after injection, were exposed to 66% nitrous oxide in oxygen for periods of 0.5-8 h. At given times after nitrous oxide exposure, higher methionine synthase activities were found in the livers, kidneys and brains of mice injected with DMTU than in the saline-injected animals. These higher methionine synthase activities in the DMTU-treated animals represented a delay in the enzyme inactivation produced by nitrous oxide, as the difference in activities between the DMTU-injected and saline-injected mice decreased with increasing duration of exposure to nitrous oxide. Greater differences in methionine synthase activities between the DMTU- and saline-injected animals were observed with increasing doses of DMTU. The rate of enzyme inactivation following exposure to nitrous oxide was greater in liver and least in brain, and the difference in activities between the two groups varied with the organ examined. DMTU exhibited its greatest effect in the kidney, where methionine synthase activities were nearly doubled in the DMTU 2.0 mg g-1-injected compared with the saline-injected mice after 1-h exposure to 66% nitrous oxide. Following a marked inactivation of methionine synthase by exposing mice to 66% nitrous oxide for 4 h, injection of DMTU 2.0 mg g-1 at the end of exposure to nitrous oxide did not enhance, but impaired, the recovery of enzyme activity. The findings are consistent with the hypothesis that nitrous oxide combines with the vitamin B12 molecule of methionine synthase to form a hydroxyl radical that reacts with an inactivates the enzyme, and that DMTU slows this inactivation by scavenging hydroxyl radicals.