Nitrosamine metabolism in kwashiorkor rats

Toxicokinetics and Biliary Excretion of N-Nitrosodiethylamine in Rat Supplemented with Low and High Dietary Proteins

Although biliary excretion is one of the biological elimination processes for foreign compounds, intake of high-protein diets was hypothesized to enhance their detoxification rates. Hence, this study investigates the effect of differential dietary protein intake on toxicokinetics and biliary excretion in rats following exposure to N-nitrosodiethylamine (NDEA) and aflatoxin B1 (AFB1). The animals were divided into five groups. Groups I and II were exposed to low and high dietary proteins following a single intraperitoneal dose of 43 µg NDEA/kg body weight, respectively. Groups III and IV were equally treated after a combined single intraperitoneal dose of 43 µg NDEA plus 0.022µg AFB_I/kg body weight, respectively. Group V was fed with low-protein diets following a single intraperitoneal dose of 0.022µg AFB1/kg body weight. The experiment lasted 35 days. The bile excreted higher amounts of unchanged NDEA than nitrite. The groups placed on high-protein diets (HPD = 64%) eliminated higher amounts of the unchanged NDEA and nitrite than the lower-protein diet (LPD = 8%) groups. Furthermore, the animals fed with high dietary protein (HPD = 64%) depicted short half-life with corresponding increase in elimination rate constant. The presence of AFB₁ heightened the excretion of bound NDEA with AFB₁ than NDEA only. Generally, this study advocates that N-nitrosodiethylamine and the corresponding metabolites follow hepatobiliary system potentiated by high intake of dietary proteins.

N-nitrososdimethylamine is activated in microsomes from hepatocytes to reactive metabolites which damage DNA of non-parenchymal cells in rat liver

The liver carcinogen N-nitrosodimethylamine (NDMA) has to be metabolically activated by specific cytochromes before it can react with cellular macromolecules (e.g. proteins or DNA). Although hepatocytes are believed to be responsible for this activation, the liver tumours originate mainly from non-parenchymal cells (NPC). To investigate their activation capacity we determined NDMA-demethylase activity in isolated microsomes from both liver cell types. The results demonstrate that only hepatocytes have activation capacity. Additional experiments were performed with hepatocytes and NPC using the single cell microgel electrophoresis assay (MGE). DNA damage appears in both cell types following in vivo exposure. Tested in vitro, however, the carcinogens induce DNA damages only in hepatocytes (the cells which activate these compounds). N-nitroso-hydroxymethyl-methylamine could be the responsible metabolite as it is stable enough to be transported from hepatocytes to NPC in an intact liver.

Analysis of the inhibition of N-nitroso-dimethylamine activation in the liver by N-nitro-dimethylamine using a new non-linear statistical method

N-nitro-dimethylamine (NTDMA) is carcinogenic to rats: it induces nasal cavity tumours. It can be demethylated to N-nitromethylamine and formaldehyde and reduced to N-nitroso-dimethylamine (NDMA): a potent liver carcinogen and also of the nasal cavity if activation in the liver is blocked. To explain the mechanism of NTDMA carcinogenicity we compared its demethylation with that of NDMA in liver microsomes from female and male rats, untreated, fasted or treated with ethanol to induce cytochrome P450 2E1 (CYP2E1). Kinetic parameters were analysed by nonlinear statistical methods, which yielded unbiased parameter estimates for the calculated Km and Vmax values. Km for both compounds was very similar in females (24-47 microM) whereas Vmax for NTDMA was consistently higher than for NDMA as substrate: 1.07-4.70 nmol formaldehyde/mg microsomal protein x min and 0.52-2.76 nmol, respectively. In liver microsomes from induced male rats NTDMA was found to be a much more effective inhibitor of NDMA activation (KEI 39.6-73.6 microM) than NDMA of NTDMA demethylation (KEI 224-286 microM). Nasal microsomes can demethylate both NDMA and NTDMA but the kinetics are vastly different. NTDMA is demethylated at a linear rate and approximately 10-fold more effectively than NDMA. The mechanism of carcinogenicity of ingested NTDMA, we propose, is a partial reduction to NDMA in the liver and inhibition of NDMA activation in the liver by residual NTDMA, which enables NDMA to reach the nasal mucosa where it is activated to DNA-alkylating species and the observed tumours are formed.

Caffeine-derived N-nitroso compounds. IV: Kinetics of mononitrosocaffeidine demethylation by rat liver microsomes

The study describes the kinetics of demethylation of mononitrosocaffeidine (MNC), a new asymmetric N-nitrosamine derived from caffeine. The demethylation of its precursor compound caffeidine was also studied. The results presented here suggest (a) that liver microsomes from fasted rats preferentially demethylate the N-methylnitrosamine group in MNC indicating the demethylation by cytochrome P450IIE1, (b) demethylation of MNC shows two apparent Km values, one of 117-166 microM responsible for the demethylation at the N-methylnitrosamino group of MNC, and the other Km of 1.84-2.26 mM for the remaining N-demethylations, (c) in contrast, caffeidine is a low affinity substrate for microsomal demethylation as indicated by a high Km of 14.3-16.3 mM, and (d) the demethylation at amino-N amino-N, and N-1 in both these compounds are mainly catalysed by P450 enzymes induced by Aroclor 1245 in rats.

Alterations in dimethylnitrosamine-induced lethality and acute hepatotoxicity in rats during dietary thiamin, riboflavin and pyridoxine deficiencies

The effects of dietary thiamin, riboflavin and pyridoxine deficiencies on dimethylnitrosamine-induced lethality and hepatotoxicity were investigated in the rat. Development of deficiencies was monitored by growth rate, food intake, ratio of liver weight to body weight and the biochemical parameters (thiamin diphosphate effects for thiamin deficiency, glutathione reductase activity coefficient for riboflavin deficiency and erythrocyte glutamate-oxaloacetate transaminase activity for pyridoxine deficiency). Thiamin deficiency slightly increased the acute toxicity of dimethylnitrosamine as observed by the lowering of the LD50 dose and the greater increase in the serum glutamate-oxaloacetate transaminase and serum glutamate-pyruvate transaminase levels. Riboflavin deficiency, on the other hand, slightly increased the LD50 dose of dimethylnitrosamine and resulted in less dimethylnitrosamine-induced damage to the liver. Pyridoxine deficiency did not affect the lethal dose nor significantly alter the transaminases levels.

Determination of DNA single strand breaks and selective DNA amplification by N-nitrodimethylamine and analogs, and estimation of the indicator cells' metabolic capacities

N-nitrodimethylamine is metabolized oxidatively to N-nitrohydroxymethylmethylamine, which decomposes to yield formaldehyde and N-nitromethylamine. All four compounds and N-nitromethylamine were tested for their ability to induce DNA single strand breaks in hepatocytes and in SV 40-transformed Chinese hamster embryo cell lines. Only the two monoalkylnitramines were positive. They induced single strand breaks in hepatocytes, but were not effective in the other cells. Formaldehyde and N-nitrohydroxymethylmethylamine were toxic to the cells. None of the compounds tested was able to induce selective DNA amplification in the two transformed cell lines. Enzymes involved in drug metabolism were assayed in the hamster cell lines. The activity of UDP-glucuronosyltransferase and cytosolic epoxide hydrolase were not detectable. N-nitrodimethylamine demethylation was low. The content of reduced glutathione and the activities of glutathione transferase and membrane bound epoxide hydrolase were comparable to values obtained in the rat liver.