Nitrous oxide abuse in the emergency practice, and Review of toxicity mechanisms and potential markers

Abstract

Nitrous oxide (N2O) toxicity is a concern common to several medical fields. Here, retrospective study of four N2O abuses with neurological signs in the emergency practice provides a preliminary basis for a metabolic Discussion/Review. This latter highlights N2O abuse as pathology of DNA/RNA/protein methylations, for instance consistent with impairments of protein arginine methyltransferases involved in myelinogenesis and myelopathy in patients. Basically, pathogenesis starts with oxidation by N2O of coordinated cobalamine cobalt ions at enzyme sites with impairments of vitamin-B12-dependent pathways. Methionine synthase (methylcobalamine) and methymalonyl-CoA mutase (adenosylcobalamine) are inactivated and cofactor-depleted, respectively. The number of impacted pathways (folate cycle, methylation cycle, S-adenosylmethionine-dependent methyltransferases, transulfuration pathway, Krebs cycle fueling by methylmalonyl-CoA, glutathione synthesis) explains the variety of potential research/laboratory markers, and may provide new clues and future angles to explore N2O toxicity. Overall, homocysteine measurements obviously help diagnosis of N2O abuses. Additional markers may include vitamin-B12, methionine, methylmalonate, dimethylglycine, sarcosine, S-adenosylmethionine to S-adenosylhomocysteine ratio, various S-adenosylamino acids, S-adenosylmethionine-dependent cellular methylations, and additional analytes (propionylcarnitine, propionylglycine, cystathionine and derived metabolites, methylated amino acids [eg arginine], betaine).