A U-M biochemist may have discovered why women dental assistants who are exposed to high levels of a common anesthetic called nitrous oxide, or laughing gas, have more trouble getting pregnant than women in other occupations.
According to Rowena G. Matthews, professor of biological chemistry and research scientist in the Biophysics Research Division, the culprit could be hydroxyl radicals—destructive molecules that can disable cell proteins and induce mutations in a cell’s DNA or genetic code.
The initial study documenting reduced fertility in female dental assistants exposed to high levels of nitrous oxide (N2O) was published by Andrew S. Rowland of the National Institute of Environmental Health Sciences and his colleagues in the Oct. 1, 1992, issue of the New England Journal of Medicine
“Until now, no one has proposed a mechanism by which long-term exposure to nitrous oxide could interfere with pregnancy,” Matthews says.
In search of such a mechanism, Matthews and former graduate student James T. Drummond traced the complex series of biochemical reactions that occur when nitrous oxide reacts with a common enzyme found in humans, animals and bacteria. Two articles presenting the results of their research were published in the April 5 issue of Biochemistry.
Matthews and Drummond focused on vitamin B12-dependent methionine synthase—an enzyme that plays a vital role in the uptake of folic acid from food. This enzyme is responsible for recycling methionine, an amino acid present in limited quantities in the human diet. “For the past 20 years, scientists have known that long-term exposure to nitrous oxide inactivates methionine synthase and decreases our ability to utilize folic acid and to make methionine,” Matthews explains. “Without folic acid and methionine, people develop serious anemias and degenerative spinal disease.”
Drummond and Matthews showed that methionine synthase from bacteria converts nitrous oxide (N2O) to nitrogen gas (N2), releasing oxygen in a highly reactive form—possibly as hydroxyl radicals—that can damage the enzyme.
Identifying one single point of damage in the massive (137,000 molecular weight) methionine synthase protein was “like looking for a needle in a haystack,” Matthews says. “While we can’t say absolutely that hydroxyl radicals are the culprit, we did find signatures of hydroxyl-induced protein damage.”
Free radicals, such as hydroxyl radicals, are molecules that contain an unpaired electron, which makes them chemically unstable and “anxious” to snatch an electron from any nearby molecule. In doing so, radicals can alter the flexibility of cell membranes, modify proteins so they don’t function properly, and produce mutations in a cell’s DNA or genetic code.
“A single molecule of methionine synthase can break down many molecules of nitrous oxide, and produce a cascade of hydroxyl radicals,” Matthews says. “These radicals could induce spontaneous abortions before women even realize they are pregnant—by producing defects in the developing embryo.”
“We studied the enzyme found in bacteria,” Matthews emphasizes. “Although we have not performed the same studies with human enzyme, we believe the inactivation process will be very similar.”
Matthews emphasizes that the greatest danger of reduced fertility might be to women exposed to chronic levels of nitrous oxide either through their work in operating rooms or dentist’s offices or through long-term use of the gas as a recreational drug.
“Some stores sell small canisters of nitrous oxide for recreational use and sniffing nitrous oxide from whipped-cream dispensers was once popular,” she said. “Knowing what we know now, I wouldn’t recommend that.
“When used for brief periods, nitrous oxide is a safe and efficacious anesthetic. Once you stop breathing the gas, it’s rapidly removed from the body and new methionine synthase is produced,” Matthews said. “Prolonged exposure is the problem, particularly for women working in dental offices where it may be difficult to control levels of exposure to the gas.”
In many operating rooms, Matthews explained, nitrous oxide used for anesthesia is never released into the air, so operating room personnel are not exposed to it.
Matthews’ research is funded by a National Institutes of Health Merit Award—given only to a select group of scientists who have demonstrated long-term productivity in their research.