EFFECTS OF CHRONIC EXPOSURE TO NITROUS OXIDE ON METHIONINE SYNTHASE ACTIVITY

Hepatic methionine synthase activity has been determined inrats continuously exposed (24h day^–1 and 7 day week^–1)to concentrations of nitrous oxide ranging from 500 to 50000p.p.m. for periods ranging from 24 h to 28 days. The inactivationof this enzyme was dependent upon both time and concentrationof nitrous oxide exposure, but there was no statistical evidencethat exposure continuing beyond 48 h increased the effect. However,the dose-response curve for 24 h was significantly differentfrom the curve for pooled data from exposures lasting 2–28days. These latter data indicated no significant effect with450 p.p.m. and an ED 50 of 5400 p.p.m. Significant inhibitionwas detected at 1000 p. p.m. These results suggest that thelimit of exposure of 25 p.p.m. recommended by the American NationalInstitute of Occupational Safety & Health may be undulyrestrictive.