Dinitrogen oxide

Endpoint summary

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Absorption rate - inhalation (%):

100

Additional information

Introduction

The discussion below on the toxicokinetics and mechanisms of action of nitrous oxide (N2O) has primarily been drawn from open literature (EIGA 2008, EHC 1997, HSDB 2011, IPCS 1992, MAK 1998).

 

Absorption:

Approximately 5% of inhaled N2O is absorbed by the organism (Eger, 1974). N2O is sparingly soluble in blood and is rapidly distributed. After N2O anaesthesia the gas is transferred rapidly from the blood into the alveoli where it dilutes the oxygen from inhaled air.

 

Distribution and mode of action:

N2O is rapidly distributed, diffuses more rapidly into body cavities such as the cerebral ventricle.

 

- Methionine synthase

Metabolic interaction of N2O results in the oxidation of vitamin B12 (changing the valency of the cobalt from +1 to +3) which is the main route by which toxic effects exhibited by N2O are manifest. As a result the oxidised form cannot function as a co-enzyme for methionine synthase (MS) the effect is to inhibit the activity of this enzyme along with blocking the folate cycle. Serum vitamin B12 seems to be unaffected by N2O exposure, with depletion of MS activity and homocysteine levels and increased rates of deoxyuridine (dU)-suppression being preferred markers of vitamin B12 deficiency.

 

Since all of the known adverse effects of N2O seem to be mediated via the MS pathway it would be reasonable to conclude that exposures which have no effects on this pathway represent an overall NOAEL. From the available data in rats it would appear that inactivation of MS is no greater in the fetus than in the mother at the end stage of pregnancy (GD19). Thus an exposure without effect on the mother at this stage is unlikely to affect the fetus.

 

From the considerations given above, a NOAEL in rats on the basis of the mechanism of action could be concluded to be in the region of 500 ppm as a continuous exposure (Vieira et al., 1980).

 

Comparison of MS inactivation in rats and humans indicates that for a given exposure the rate of inactivation in humans is slower. Although no specific data have been identified on recovery rates limited evidence from haematological studies in which dU-suppression was measured showed that in rats it was slower.

 

For those exposures which result from anaesthetic or analgesic use there is a predictable effect on vitamin B12 and MS activities, with populations who may be vulnerable to those effects, such as those with marginal vitamin B12 deficiency. Furthermore, there is no doubt that some inactivation of MS and vitamin B12 will occur in those subject to N2O anaesthesia during pregnancy since the half-time for depletion of activity has been shown to be less than 1 h. However, the main effects seen in animals at low doses derive from exposures of at least 24 h in duration. Marginal vitamin B12 deficiency in the pregnant women would also tend to increase the possibility of adverse consequences. 

 

No recovery of MS activity occurs during continuous exposure to 50% N2O for 15 d or to low concentrations for 28 d. Although tissue levels of N2O fall rapidly at the end of exposure a parallel rapid recovery of MS activity does not occur. In rats recovery occurs over days, rather than minutes. 

 

The rate of recovery probably differs among species because it depends on the provision of new vitamin B12 and synthesis of new MS holoeznyme.  The oxidation of vitamin B12 is probably irreversible as is the binding of it to the apoenzyme. Administration of vitamin B12 would not speed recovery if this process were limited by the rate at which MS is synthesised. In vitro experiments have shown that oxidised vitamin B12 apparently cannot be displaced from the enzyme.

 

Recovery rates in humans have not been reported as MS activity in WBC is very low and plasma and RBC contain no MS. In order to assess MS activity the sampling of tissue, usually brain or liver is required. Significant differences between mice and rats preclude an extrapolation to humans. The recovery of serum methionine levels after exposure to N2O suggests that recovery may be slow in humans. The effects of intermittent exposure to N2O supports this view (i.e. if recovery were rapid, intermittent exposure would not have a significant effect). 

 

- Interference in purine and pyrimidine synthesis 

Inactivation of MS depletes serum methionine and thus influences a chain of metabolic reactions. An important consequence of this a presumed decrease in 5,10 methylene tetrahydrofolate, the carbon donor required for conversion of deoxyuridine to thymidine. The megaloblastic anaemia, leukopenia and likely the foetotoxicity produced by N2O result from its detrimental effect on DNA synthesis. N2O is known to depress both purine and pyrimidine synthesis and thereby not only impair DNA but also RNA synthesis. The recovery of the capacity to synthesis DNA may require several days. DNA synthesis is in large part dependent on active MS. 

 

- Folate metabolism

Considerable changes in folate metabolism follow the inactivation of vitamin B12 in MS by N2O. The effects of folate metabolism explain most of the consequences of vitamin B12 inactivation.

 

N2O impairs the hepatic uptake of folates in both mice and rats. The principal transport form of folate is methyltetrahydrofolate; exposure to N2O results in increased serum folate levels with folate levels in the liver becoming depleted. Folate levels in other tissues (kidney, brain, bone marrow) decrease less markedly.  Folate in tissue fluids has one glutamic acid residue – which consequently is the form which dietary folate is absorbed. More glutamic acid residues are added intracellularly to produce folate polyglutamates. The most common resulting compound in humans and rats contains 5 residues. In patients with pernicious anaemia the level of folate polyglutamate in RBC decline. In patients exposed to N2O, impairment of hepatic synthesis of folate polyglutamate results.

 

- Cytochrome c oxidase

N2O is known also to reversibly bind to cytochromecoxidase the terminal enzyme in the mitochondrial electron transport system. Consequently, inhibition of this enzyme result in the final step of ATP formation being impeded. Inhibition can therefore lead to uncoupling of the mitochondrial electron transport chain leading to chemical asphyxiation of the cells, and ultimately death.

 

Figure 1: Methionine and folate metabolism (refer to attachment)

 

Metabolism

 N2O is reduced to nitrogen in the reaction with the central Co⁺ion of vitamin B12. After a single passage through the liver the concentration of N2O in the blood decreases by only 0.03%. It has been deduced fromin vitroinvestigations that about 0.004% of the total N2O is metabolised in humans and animals to nitrogen by bacterial reductases in the intestine. During this process OH radicals are probably formed.

 

Excretion:

N2O is hardly metabolised and is therefore exhaled unchanged. Approximately 6.4% of the amount taken up is eliminatedviathe skin.

 

References:

- Eger II, E.I. (1985). Nitrous oxide / N2O. Elsevier Science Publishing Co., Inc. New York, USA. ISBN 0-7131-4461-0

- European Health Criteria (EHC) (1997). Nitrogen oxides (2 nd edition). United Nations Environmental Programme. International Labour Organisation. World Health Organisation. International Programme on Chemical Safety.http://www.inchem.org/documents/ehc/ehc/ehc188.htm

- European Industrial Gases Association (EIGA) (2008). Review of toxicological data on nitrous oxide. MGC 153/08/E. Prepared by P. Branptom.

- MAK collection for occupational health and safety: Nitrous oxide [MAK Value Documentation, 1998]. Published online 30 January 2012:http://onlinelibrary.wiley.com/doi/10.1002/3527600418.mb1002497e0009/full

- HSDB (2011). Nitrous oxide. Hazardous Substances Data Bank.

- International Programme on Chemical Safety (IPCS) INCHEM (1992). Nitrous oxide. PIM 381.http://www.inchem.org/documents/pims/pharm/nitrusox.htm

- National library of medicine HSDB database on nitrous oxide (CASRN: 10024-97-2) retrieved 4 August 2011.http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@rn+@rel+10024-97-2