Organ specificity in the microsomal activation and toxicity of N-nitrosomethylbenzylamine in various species.

The microsomal metabolism of the rat esophageal carcinogen N-nitrosomethylbenzylamine (NMBZA) at the methylene carbon atom to yield benzaldehyde was studied in various organs of a number of species to determine the role of metabolic activation in the carcinogenicity or toxicity of the nitrosamine. In the Sprague-Dawley rat, NMBZA was metabolized by microsomes from liver, lung, and esophageal mucosa. In the F344 rat and rabbit, metabolic activity was present in both liver and esophageal mucosa, the only tissues studied in these species. In contrast, in the Syrian hamster and BALB/cByJ mouse, NMBZA debenzylation was undetectable in the esophagus but occurred at relatively high rates in liver, lung, and kidney. The forestomach mucosa exhibited undetectable levels of activity in the Sprague-Dawley rat and BALB/cByJ mouse, although in the hamster, it was present at a very low level. Administration of a dose of NMBZA acutely toxic to the rat (18 mg/kg i.p.) resulted in significant cellular damage only to the rat esophageal mucosa, no other tissues examined in the rat, hamster, or mouse being affected. These observations, together with the available data on carcinogenicity of the nitrosamine in the rat and rabbit, suggest that in the esophagus, at least, metabolic activation of NMBZA is necessary to elicit its toxic and/or carcinogenic effect. However, NMBZA is also metabolized at a high rate in the liver of all species but is not toxic or carcinogenic in this tissue, suggesting that other factors besides metabolic activation must be involved in the resistance of hepatocytes to the effects of the nitrosamine. Microsomes prepared from human esophageal mucosa from six patients metabolized NMBZA at rates that were either undetectable or approximately 70 times lower than in the Sprague-Dawley rat.

Induction and suppression of N-nitrosomethylbenzylamine activation by microsomes from rat liver and esophagus.

Male Sprague-Dawley rats were pretreated with various chemicals to determine their effects on the microsomal activation of the esophageal carcinogen N-nitrosomethylbenzylamine [( NMBzA ) CAS: 937-40-6; N-methyl-N- nitrosobenzylamine ] in the rat esophagus and, for comparative purposes, in the rat liver. When rats were pretreated with NMBzA , little change in hepatic NMBzA - debenzylase activity was observed. In contrast, NMBzA metabolism in the esophagus was significantly (60-65%) reduced. Similarly, pretreatment of rats with disulfiram [CAS: 97-77-8; bis( diethylthiocarbamoyl )disulfide] caused a 40% decrease in esophageal metabolism, but it had no significant effect in the liver. Pretreatments with the methylenedioxybenzenes safrole [CAS: 94-59-7; 4-allyl-1,2-(methylenedioxy)benzene], isosafrole [CAS: 120-58-1; 1,2-(methylenedioxy)-4-propenylbenzene], and dihydrosafrole (CAS: 94-58-6; 1,2-(methylenedioxy)-4- propylbenzene ) caused a marked induction (twofold to fivefold) of the hepatic metabolism of NMBzA , but again esophageal metabolism was suppressed. The results indicate that esophageal metabolism of NMBzA is either unchanged or suppressed by the various chemical pretreatments, but hepatic metabolism of the nitrosamine is induced by the methylenedioxybenzenes .

Formation of O6-methylguanine in regenerating rat liver by N-nitrosomethylbenzylamine is not sufficient for initiation of preneoplastic foci.

The DNA methylating activities of N-nitrosodimethylamine (NDMA), an initiator of hepatic gamma-glutamyltranspeptidase-positive foci, and N-nitrosomethylbenzylamine (NMBzA), which does not initiate, were studied in regenerating rat liver. Equimolar doses of 14C-labelled NDMA and NMBzA (33.5 mumol/kg) were administered to male Sprague-Dawley rats 18 h after partial hepatectomy. NDMA and NMBzA both produced 7-methylguanine and O6-methylguanine. The results suggest that although the formation of O6-methylguanine may be necessary it is not sufficient for initiation of preneoplastic foci.

Tissue and species specificity of the microsomal metabolism of N-nitrosomethylbenzylamine.

The microsomal activation of N-nitrosomethylbenzylamine (NMBzA) by oxidation at the methylene carbon atom was examined in various organs of a number of species to determine the role of metabolism in the organ-specificity of tumour induction by NMBzA. In Sprague-Dawley rats, NMBzA was metabolized by microsomes from liver, lung and oesophageal mucosa. In Fischer F-344 rats and in rabbits, metabolic activity was present in both liver and oesophageal mucosa, the only tissues studied in those species. In contrast, in Syrian hamsters and in BALB/cBYJ mice, no NMBzA metabolism was detectable in the oesophagus, but it occurred at relatively high rates in liver, lung and kidney. The forestomach mucosa exhibited undetectable levels of activity in all species except the hamster in which it was present at a very low level. In human oesophageal mucosal microsomes from six patients, rates of metabolism of NMBzA were either undetectable or approximately 70 times lower than those in the Sprague-Dawley rats. A comparison of NMBzA metabolism in the different species with the known carcinogenicity of the nitrosamine in rats and rabbits, and our preliminary data on the acute toxicity of NMBzA in hamsters and mice suggests that, in the oesophagus at least, metabolic activation of NMBzA is necessary to elicit its toxic and/or carcinogenic effect. However, in the liver, which in all species has high metabolic activity but which is also resistant to the toxic and carcinogenic effects of NMBzA, other factors besides metabolic activation must be involved.

Esophageal and hepatic microsomal metabolism of N-nitrosomethylbenzylamine and N-nitrosodimethylamine in the rat.

The metabolism of the rat esophageal carcinogen N-nitrosomethylbenzylamine (NMBzA) was studied using microsomes prepared from liver and esophageal mucosa of untreated male Sprague-Dawley rats. NMBzA was extensively metabolized to benzaldehyde, benzyl alcohol, and formaldehyde by hepatic microsomes. The rate of metabolism at the benzyl moiety was 10-fold higher than that at the methyl moiety. Mucosal microsomes metabolized NMBzA to benzaldehyde and formaldehyde at rates one-fifth and one-sixtieth of those in the liver, respectively; benzyl alcohol formation was undetectable. Esophageal metabolism of NMBzA was exclusively located in the mucosa, preferentially in the microsomal fraction, was reduced nicotinamide adenine dinucleotide phosphate dependent, and was inhibited by CO and 2-diethylaminoethyl-2,2-diphenylvalerate. A low level of cytochrome P-450 was detected in the mucosal microsomes. Whereas hepatic metabolism of NMBzA was inducible by phenobarbitone pretreatment, mucosal metabolism was not altered by either phenobarbitone or 3-methylcholanthrene pretreatment. The hepatocarcinogen N-nitrosodimethylamine was extensively metabolized by hepatic microsomes to formaldehyde, but demethylation was not detected in the microsomes from esophageal mucosa, a nontarget tissue. The results indicate that rat esophageal mucosa contains an enzyme system which metabolizes NMBzA at a high rate and exhibits properties typical of cytochrome P-450. This enzyme may play a role in determining which compounds induce tumors in rat esophagus.

Comparative tumor initiating activities of N-nitrosomethylbenzylamine and N-nitrosodimethylamine in rat liver.

The tumor initiating activities of nitrosomethylbenzylamine (NMBzA), an esophageal carcinogen, and nitrosodimethylamine (NDMA), a hepatocarcinogen, were compared in rat liver using modifications of the initiation assays of Tsuda et al. (Cancer Res., 40, 1157, 1980) and Pitot et al. (Nature, 271, 456, 1978). Equimolar doses of NMBzA or NDMA (33.5 mu-mol/kg) were injected i.p. into male Sprague-Dawley rats 18 h after partial hepatectomy. Following selection, foci of preneoplastic hepatocytes were detected by histochemical staining for gamma-glutamyltranspeptidase. Nitrosomethylamylamine (NMAmA), 115 mumol/kg), also an esophageal carcinogen, was tested in the assay of Tsuda et al., and nitrosodiethylamine (NDEA, 33.5 mumol/kg), a hepatic and esophageal carcinogen, was tested in the assay of Pitot et al. Both NDMA and NDEA induced significant increases in the number of preneoplastic foci above background. In contrast, neither NMBzA nor NMAmA increased the number of foci above background. Microsomes from regenerating liver had a lower capacity to metabolize both NDMA and NMBzA compared to microsomes from intact liver, but the decrease in activity was similar for both compounds. Neither NDMA nor NMBzA significantly inhibited the first wave of DNA synthesis in vivo in the regenerating liver. The results demonstrate that in contrast to NDMA and NDEA, NMBzA and NMAmA lack tumor initiating activity in the liver.