This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Jensen, D. E. Articles by Spiegel, A. Search for Related Content PubMed PubMed Citation Articles by Jensen, D. E. Articles by Spiegel, A.

Species Differences in Blood-mediated Nitrosocimetidine Denitrosation¹

Fels Research Institute, Temple University School of Medicine, Philadelphia, Pennsylvania 19140

Nitrosocimetidine (NC) is the nitrosated derivative of cimetidine (Tagamet), a p.o. administered drug used widely in the treatment of stomach ulcers. NC is capable of methylating DNA in vitro and in cultured cells in a manner similar to that of the laboratory carcinogens 1-methyl-2-nitro-1-nitrosoguanidine and methylnitrosourea (MNU) and gives positive indications in short-term in vitro tests for genotoxicity, generally beld to be prognostic of compound carcinogenic potential. Nevertheless NC has been found to be a weak or non-carcinogen in the rat and mouse model systems and to produce minimal levels of tissue DNA alkylation when dosed p.o. or i.v. to rats. The results from our earlier experiments (D. E. Jensen, Cancer Res., 43: 5258–5267, 1983) indicated that compound denitrosation is the primary fate of NC in the rat and suggested that denitrosation in the blood, mediated by hemoglobin sulfhydryl residues, is perhaps the major detoxification mechanism. We now report that whole blood and hemoglobin isolated from various mammalian species differ in their capacity for NC degradation rate enhancement and for compound denitrosation. The observed whole blood activity in the degradation reaction (rat > mouse/guinea pig > human/hamster) paralleled the hemoglobin activity. The NC half-life in isolated rat blood, 37°C, was found to be about 2 min and in hamster or human blood 27 min. For reference, the MNU half-life in isolated blood is 8 min. Compound denitrosation accounted for at least 75% of the degradation in rat blood and 40 to 55% in human and hamster blood. Parallel NC denitrosation activity was found in the various hemoglobin preparations. The NC degradation rates in the presence of the several hemoglobin species were roughly proportional to the number of sulfhydryls on the hemoglobin tetramers available for reaction with p-chloromercuribenzoate and approximated the rates observed in solutions containing equivalent concentrations of L-cysteine. The percentage of total decomposition due to compound denitrosation in the presence of rat hemoglobin, 95%, was found to be unique relative to the L-cysteine-mediated reactions (about 35%) and the reactions mediated by the other hemoglobins considered. In NC-cysteine reactions studied over the pH range 6 through 10, the denitrosation process never accounted for more than 50% of the total degradation. Chemically blocking the sulfhydryls on human hemoglobin using iodoacetamide deleted the NC degradation rate enhancement. We found no evidence for nitrosylhemoglobin formation. The results from tissue DNA methylation studies in which the yields generated by MNU and by NC in the rat and in the hamster were compared demonstrated that in spite of the relatively long NC half-life in in vitro hamster blood, the compound produces low DNA alkylation yields relative to MNU in the hamster, suggesting that other denitrosating activities besides hemoglobin are active in the intact animal. Determinations of the concentrations of NC and cimetidine in the circulating blood of hamsters at times after i.v. dosing indicated that the in vivo half-life of the compound is somewhat less than 5 min and that the degradation approaches 100% denitrosation.

¹ This investigation was supported by USPHS Grant CA-31503 awarded by the National Cancer Institute, Department of Health and Human Services, and by American Cancer Society Grant SIG-6 awarded to the Fels Research Institute.

² To whom requests for reprints should be addressed.

Received 6/ 4/86. Revised 9/26/86. Accepted 10/ 1/86.

This article has been cited by other articles:

				P.A. Cahill, A.W. Knight, N. Billinton, M.G. Barker, L. Walsh, P.O. Keenan, C.V. Williams, D.J. Tweats, and R.M. Walmsley The GreenScreen(R) genotoxicity assay: a screening validation programme Mutagenesis, March 1, 2004; 19(2): 105 - 119. [Abstract] [Full Text] [PDF]