Quercetin 7-O-glucoside suppresses nitrite-induced formation of dinitrosocatechins and their quinones in catechin/nitrite systems under stomach simulating conditions.

Foods of plant origin contain flavonoids. In the adzuki bean, (+)-catechin, quercetin 3-O-rutinoside (rutin), and quercetin 7-O-ß-D-glucopyranoside (Q7G) are the major flavonoids. During mastication of foods prepared from the adzuki bean, the flavonoids are mixed with saliva and swallowed into the stomach. Here we investigated the interactions between Q7G and (+)-catechin at pH 2, which may proceed in the stomach after the ingestion of foods prepared from the adzuki bean. Q7G reacted with nitrous acid producing nitric oxide (˙NO) and a glucoside of 2-(3,4-dihydroxybenzoyl)-2,4,6-trihydroxy-3(2H)-benzofuranone. (+)-Catechin reacted with nitrous acid producing ˙NO and 6,8-dinitrosocatechin. The production of the dinitrosocatechin was partly suppressed by Q7G, and the suppression resulted in the enhancement of Q7G oxidation. 6,8-Dinitrosocatechin reacted further with nitrous acid generating the o-quinone, and the quinone formation was effectively suppressed by Q7G. In the flavonoids investigated, the suppressive effect decreased in the order Q7G≈quercetin>kaempferol>quercetin 4'-O-glucoside>rutin. Essentially the same results were obtained when (-)-epicatechin was used instead of (+)-catechin. The results indicate that nitrous acid-induced formation of 6,8-dinitrosocatechins and the o-quinones can be suppressed by flavonols in the stomach, and that both a hydroxyl group at C3 and ortho-hydroxyl groups in the B-ring are required for efficient suppression.

Inhibition of Toxoplasma gondii and Plasmodium falciparum infections in vitro by NSC3852, a redox active antiproliferative and tumor cell differentiation agent.

We searched the National Cancer Institute (NCI) compound library for structures related to the antitumor quinoline NSC3852 (5-nitroso-8-quinolinol) and used a computer algorithm to predict the antiprotozoan activity for each of 13 structures. Half of these compounds inhibited Toxoplasma gondii tachyzoite propagation in human fibroblasts at < or =1 microM. The active compounds comprise a series of low-molecular-weight quinolines bearing nitrogen substituents in the ring-5 position. NSC3852 (EC(50) 80 nM) and NSC74949 (EC(50) 646 nM) were the most potent. NSC3852 also inhibited Plasmodium falciparum growth in human red blood cells (EC(50) 1.3 microM). To investigate the mechanism for NSC3852's anti-T. gondii activity, we used chemiluminescence assays to detect reactive oxygen species (ROS) formation in freshly isolated tachyzoites and in infected host cells; the absence of ROS generation by NSC3852 in these assays indicated NSC3852 does not redox cycle in T. gondii. Inhibitors of enzyme sources of free radicals such as superoxide anion, nitric oxide (NO), and their reaction product peroxynitrite did not interfere with the anti-T. gondii activity of NSC3852. However, inhibition of T. gondii tachyzoite propagation by NSC3852 involved redox reactions because tachyzoites were protected from NSC3852 by inclusion of the cell permeant superoxide dismutase mimetic, MnTMPyP, or N-acetylcysteine in the culture medium. We conclude that the Prediction of Activity Spectra for Substances (PASS) computer program is useful in finding new compounds that inhibit T. gondii tachyzoites in vitro and that NSC3852 is a potent T. gondii inhibitor that acts by indirect generation of oxidative stress in T. gondii.

Protein kinase G regulates the basal tension and plays a major role in nitrovasodilator-induced relaxation of porcine coronary veins.

BACKGROUND AND PURPOSE: Coronary venous activity is modulated by endogenous and exogenous nitrovasodilators. The present study was to determine the role of protein kinase G (PKG) in the regulation of the basal tension and nitrovasodilator-induced relaxation of coronary veins. EXPERIMENTAL APPROACH: Effects of a PKG inhibitor on the basal tension and responses induced by nitroglycerin, DETA NONOate, and 8-Br-cGMP in isolated porcine coronary veins were determined. Cyclic cGMP was measured with radioimmunoassay. PKG activity was determined by measuring the incorporation of 32P from gamma-32P-ATP into the specific substrate BPDEtide. KEY RESULTS: Rp-8-Br-PET-cGMPS, a specific PKG inhibitor, increased the basal tension of porcine coronary veins and decreased PKG activity. The increase in tension was 38% of that caused by nitro-L-arginine. Relaxation of the veins induced by nitroglycerin and DETA NONOate was accompanied with increases in cGMP content and PKG activity. These effects were largely eliminated by inhibiting soluble guanylyl cyclase with ODQ. The increase in PKG activity induced by the nitrovasodilators was abolished by Rp-8-Br-PET-cGMPS. The relaxation caused by these dilators and by 8-Br-cGMP at their EC50 was attenuated by the PKG inhibitor by 51-66%. CONCLUSIONS AND IMPLICATIONS: These results suggest that PKG is critically involved in nitric oxide-mediated regulation of the basal tension in porcine coronary veins and that it plays a primary role in relaxation induced by nitrovasodilators. Since nitric oxide plays a key role in modulating coronary venous activity, augmentation of PKG may be a therapeutic target for improving coronary blood flow.

Overexpressed beta-catenin blocks nitric oxide-induced apoptosis in colonic cancer cells.

Beta-catenin plays an important role in colonic tumorigenesis whereas inducible nitric oxide synthase and nitric oxide are elevated in colonic inflammation. Resistance of colonic epithelial cells to the induction of apoptosis may contribute to tumor development. Nitric oxide can stimulate apoptosis and, paradoxically, is implicated in the development of colon cancer. Our hypothesis was that beta-catenin could increase the resistance of colonic cancer cells to nitric oxide-induced apoptotic cell death. Here we show, using a beta-catenin overexpression system, that increased cytosolic beta-catenin renders colonic epithelial cells more resistant to nitric oxide-induced apoptotic cell death, independently of nitric oxide-induced accumulation of p53. Furthermore, we show that this occurs through inhibition of nitric oxide-induced release of cytochrome c from mitochondria and by blocking both the nitric oxide-induced suppression of the antiapoptotic protein, Bcl-xL, and the phosphorylation of Akt. We contend that increased nitric oxide production, such as that which occurs in chronic colonic inflammation, may select the cells with oncogenic mutant beta-catenin regulatory genes and contribute to human colonic carcinogenesis and tumor progression.

[Inhibition of grape procyanidin on the expression of SSTR-2 mRNA in the hepatic cells induced by N-nitroso compounds].

OBJECTIVE: To study the effect of grape procyanidin (GPC) on the expression of somatostatin receptor 2 mRNA induced by N-nitroso compounds in the hepatic cells of rats. METHODS: NaNO2 was fed to Wistar rats to induce hepatocytes' mutation, and GPC was given in dose of 100mg/kg and 10mg/kg respectively through mouths to the rats of the two experimental groups at the same time. After eight weeks, situ hybridization was applied to measure the expression level of SSTR-2 mRNA in rats' hepatic cells. RESULTS: The positive cells rate of SSTR-2 mRNA expressed in the injury comparison group and the high-dose GPC group were 19.89% and 7.83% respectively and the difference between the two groups had statistical significance (P < 0.05). CONCLUSION: GPC had inhibitory effect on N-nitroso compounds-induced abnormal expression of SSTR-2 mRNA in hepatic cells of rats.

Hydrophilic antimutagens in fermented defatted soybeans with Neurospora intermedia (D-oncom).

The author has previously reported on the higher antioxidative activity in vivo of defatted soybean oncom (D-oncom), fermented defatted soybeans with Neurospora intermedia, in comparison with that of defatted soybeans (DSB). In this paper, the hydrophilic antimutagenicity of D-oncom against N-nitrosodimethylamine (NDMA) was investigated. Water-extract of D-oncom had a stronger antimutagenicity than that of DSB. The main antimutagenic fraction of D-oncom was anionic, and stable against heating at 37 degrees C. Its antimutagenicity was about one sixth of that of ascorbic acid. Actually the fraction scavenged superoxide anions. Therefore, the antimutagenicity of D-oncom against NDMA might be involved in the reduction of oxidative stress by scavenging superoxide anions. Nonetheless the main antimutagenic fraction was not polyphenol, peptide nor nucleotide, its molecular weight being distributed about 5000-12,500.

NMDA receptor antagonists ketamine and PCP have direct effects on the dopamine D(2) and serotonin 5-HT(2)receptors-implications for models of schizophrenia.

Ketamine and PCP are commonly used as selective NMDA receptor antagonists to model the putative hypoglutamate state of schizophrenia and to test new antipsychotics. Recent findings question the NMDA receptor selectivity of these agents. To examine this further, we measured the affinity of ketamine and PCP for the high-affinity states of the dopamine D(2) and serotonin 5-HT(2) receptor and found that ketamine shows very similar affinity at the NMDA receptor and D(2) sites with a slightly lower affinity for 5-HT(2) (0.5 microM, 0.5 microM and 15 microM respectively), while PCP shows similar affinity for the NMDA and 5-HT(2) sites, with a slightly lower affinity for the D(2) site (2 microM, 5 microM and 37 microM respectively). Further, ketamine and PCP in clinically relevant doses caused a significant increase in the incorporation of [(35)S]GTP-gamma-S binding in CHO-cells expressing D(2) receptors, which was prevented by raclopride, suggesting a partial agonist effect at the D(2) receptor. Thus, ketamine and PCP may not produce a selective hypoglutamate state, but more likely produce a non-selective multi-system neurochemical perturbation via direct and indirect effects. These findings confound the inferences one can draw from the ketamine/PCP models of schizophrenia.

Inhibition of the nitrosothiol production of cultured osteoarthritic chondrocytes by rhein, cortisol and diclofenac.

OBJECTIVE: Nitric oxide (NO degrees ) is a free molecule produced by NO synthases which acts as a mediator in inflammatory processes. NO degrees can react with thiol groups of proteins to produce nitrosothiols. Increased concentrations of these bioactive compounds have been found in sera and synovial fluids from patients with osteoarthritis (OA). The aim of this study was to assess the ability of human osteoarthritic chondrocytes to synthesize nitrosothiols and to compare the in vitro effects of rhein, cortisol and diclofenac on nitrosothiol and nitrite production. METHODS: Osteoarthritic chondrocytes were incubated for 24 h with 1 ng/ml of recombinant human interleukin-1beta (IL-1beta) in the presence or absence of rhein (1.3x10(-5) M, 6.5x10(-6) M, or 1.3x10(-6) M), cortisol (10(-5) M) or diclofenac (10(-5) M or 10(-6) M). Nitrite levels were measured in cell supernatants by the Griess method; nitrosothiol levels were determined in supernatants and cellular lysates by fluorimetry. RESULTS: At the basal level, nitrosothiols represented 80% of the total of nitrite and nitrosothiol production. After IL-1beta stimulation, NO degrees production was highly increased in the supernatants (45-fold increase in nitrite, 60-fold increase in nitrosothiols) as well as in cell lysates (35-fold increase in nitrosothiols). Rhein caused a dose-dependent decrease in nitrosothiol and nitrite production. In comparison, diclofenac (10(-5) M) moderately decreased nitrite and nitrosothiol levels in the supernatants but had no effect on lysate nitrosothiol. Cortisol had no significant effect on NO degrees production. CONCLUSIONS: The IL-1beta stimulation increased nitrosothiol production by osteoarthritic chondrocytes. These results demonstrate the need to measure nitrosothiol as well as nitrite production. Rhein inhibited the IL-1beta induced NO degrees production, and may be a suitable treatment for osteoarthritis.

Reversal of nitric oxide-, peroxynitrite- and S-nitrosothiol-induced inhibition of mitochondrial respiration or complex I activity by light and thiols.

Nitric oxide (NO) and its derivatives peroxynitrite and S-nitrosothiols inhibit mitochondrial respiration by various means, but the mechanisms and/or the reversibility of such inhibitions are not clear. We find that the NO-induced inhibition of respiration in isolated mitochondria due to inhibition of cytochrome oxidase is acutely reversible by light. Light also acutely reversed the inhibition of respiration within iNOS-expressing macrophages, and this reversal was partly due to light-induced breakdown of NO, and partly due to reversal of the NO-induced inhibition of cytochrome oxidase. NO did not cause inhibition of complex I activity within isolated mitochondria, but 0.34 mM peroxynitrite, 1 mM S-nitroso-N-acetylpenicillamine or 1 mM S-nitrosoglutathione did cause substantial inhibition of complex I activity. Inhibition by these reagents was reversed by light, dithiothreitol or glutathione-ethyl ester, either partially or completely, depending on the reagent used. The rapid inhibition of complex I activity by S-nitroso-N-acetylpenicillamine also occurred in conditions where there was little or no release of free NO, suggesting that the inhibition was due to transnitrosylation of the complex. These findings have implications for the physiological and pathological regulation of respiration by NO and its derivatives.

Inhibitory effects of citrus essential oils and their components on the formation of N-nitrosodimethylamine.

Twenty-eight kinds of citrus essential oils and their components were studied for inhibitory effects on the formation of N-nitrosodimethylamine (NDMA). The reaction mixture consisted of dimethylamine and sodium nitrite adjusted at pH 3.6, in addition to essential oils and an emulsifying agent. The quantification was determined by high-performance liquid chromatography monitored at 220 nm. All of the essential oils inhibited the formation of NDMA in the range of 20-85%. The oils of ujukitsu (Citrus ujukitsu Hort. ex Shirai), yuzu (C. junos Tanaka), mochiyu (C. inflata Hort. ex Tanaka), and ponkan (C. reticulata Blanco cv. F-2426) inhibited the formation of NDMA much more effectively than other citrus oils. The inhibitory proportions of components of citrus essential oils such as myrcene, alpha-terpinene, and terpinolene were as high as 80%.

The ectoenzyme gamma-glutamyl transpeptidase regulates antiproliferative effects of S-nitrosoglutathione on human T and B lymphocytes.

Expression of the ectoenzyme gamma-glutamyl transpeptidase (GGT) is regulated on T lymphocytes. It is present at a low level on naive T cells, at a high level on activated T cells, and at an intermediate level on resting memory T cells. GGT cleaves the glutamyl group from glutathione, which is the first step in the uptake of extracellular glutathione. In vitro, purified GGT also metabolizes the naturally occurring nitrosothiol, S-nitrosoglutathione (GSNO). Because of this relationship, the effects of cellular GGT on the metabolism of and cellular response to GSNO were tested. The GGT-negative lymphoblasts Ramos and SupT1 were transfected with cDNA for human GGT. In the presence of cells lacking GGT, GSNO is extremely stable. In contrast, GGT-expressing cells rapidly metabolize GSNO leading to nitric oxide release. The nitric oxide causes a rapid (<2-h) inhibition of DNA synthesis. There is a concomitant decrease in the concentration of intracellular deoxyribonucleotides, suggesting that one effect of the nitric oxide generated from GSNO is the previously described inactivation of the enzyme ribonucleotide reductase. GSNO also caused a rapid, GGT-dependent cytostatic effect in Hut-78, a human T cell lymphoma, as well as in activated peripheral blood T cells. Although DNA synthesis was decreased to 16% of control values in anti-CD3-stimulated Hut-78, the production of IL-2 was unchanged by GSNO. These data show that GGT, a regulated ectoenzyme on T cells, controls the rate of nitric oxide production from GSNO and thus markedly affects the physiological response to this biologically active nitrosothiol.

CYP2E1-mediated mechanism of anti-genotoxicity of the broccoli constituent sulforaphane.

The broccoli constituent sulforaphane (1-isothiocyanate-4-methylsulfinylbutane) has previously been shown to protect rats against 9,10-dimethyl-1,2-benz[a]anthracene tumorigenesis, thought to be due, at least in part, to induction of phase II detoxification. We investigated the ability of sulforaphane to also inhibit the phase I enzyme cytochrome P450 isoenzyme 2E1 (CYP2E1), which is responsible for activation of several carcinogens, including dialkylnitrosamines. Using the p-nitrophenol hydroxylation assay in microsomes from livers of acetone-treated Sprague-Dawley rats, sulforaphane was shown to be a potent competitive inhibitor of CYP2E1 with a Ki of 37.0 +/- 4.5 microM. In view of this result, we studied the capacity of sulforaphane to inhibit the genotoxicity of N-nitrosodimethylamine (NDMA). Sulforaphane at concentrations of > 0.8 microM inhibited the mutagenicity of NDMA (4.4 mg/plate) in Salmonella typhimurium strain TA100 after pre-incubation for 45 min with cytosol extract from livers of Balb/c mice pre-treated with acetone. Unscheduled DNA synthesis induced by NDMA (33.5 microM) in mouse hepatocytes was inhibited in a dose-dependent manner by sulforaphane at 0.064-20 microM. Sulforaphane was unable to inhibit mutagenicity of sodium azide (5 micrograms/plate), a direct acting mutagen, in the Salmonella assay. It was not itself genotoxic in hepatocytes, as measured by unscheduled DNA synthesis, or mutagenic in the strain of Salmonella employed and cytotoxic only at high concentrations (> or = 0.5 mM). These findings suggest that inhibition of CYP2E1 by sulforaphane may offer chemoprotection against carcinogenic substrates of this enzyme.

Copper chelation-induced reduction of the biological activity of S-nitrosothiols.

1. The effect of copper on the activity of the S-nitrosothiol compounds S-nitrosocysteine (cysNO) and S-nitrosoglutathione (GSNO) was investigated, using the specific copper chelator bathocuproine sulphonate (BCS), and human washed platelets as target cells. 2. Chelation of trace copper with BCS (10 microM) in washed platelet suspensions reduced the inhibition of thrombin-induced platelet aggregation by GSNO; however, BCS had no significant effect on the anti-aggregatory action of cysNO. BCS inhibited cyclic GMP generation in response to both cysNO and GSNO. 3. The effect of BCS was rapid (within 30 s), and could be abolished by increasing the platelet concentration to 500 x 10(9) l-1. 4. In BCS-treated platelet suspensions, the addition of Cu2+ ions (0.37-2.37 microM) led to a restoration of both guanylate cyclase activation and platelet aggregation inhibition by GSNO. 5. The anti-aggregatory activity of GSNO was reduced in a concentration-dependent manner by the copper (I)-specific chelators BCS and neocuproine, and to a smaller extent by desferal. No effect was observed with the copper (II) specific chelator, cuprizone, the iron-specific chelator, bathophenanthroline sulphonate, or the broader-specificity copper chelator, D-penicillamine. 6. In both BCS-treated and -untreated platelet suspensions, cys NO was more potent than GSNO as a stimulator of guanylate cyclase. In BCS-treated platelet suspensions there was no significant difference between the anti-aggregatory potency of cysNO and GSNO; however, in untreated suspensions, GSNO was significantly more potent than cysNO. Thus, when copper was available, GSNO produced a greater inhibition of aggregation than cysNO, despite being a less potent activator of guanylate cyclase. 7. The breakdown of cysNO and GSNO was measured spectrophotometrically by decrease in absorbance at 334 nm. In Tyrode buffer, cysNO (10 microM) broke down at a rate of 3.3 microM min-1. BCS (10 microM)reduced this to 0.5 microM min-1. GSNO, however, was stable, showing no fall in absorbance over a period of 7 min even in the absence of BCS.8. We conclude that copper is required for the activity of both cysNO and GSNO, although its influence on anti-aggregatory activity is only evident with GSNO. The stimulatory effect of copper is unlikely to be explained solely by catalysis of S-nitrosothiol breakdown. The enhancement by copper of the anti-aggregatory activity of GSNO, relative to cysNO, suggests that copper may be required for biological activity of GSNO which is independent of guanylate cyclase stimulation.

Differential effects of hydroxocobalamin on relaxations induced by nitrosothiols in rat aorta and anococcygeus muscle.

In aortic rings, hydroxocobalamin (30 microM) reduced the relaxant actions of S-nitrosocysteine (0.1-3 microM), S-nitrosoglutathione (0.1-3 microM) and S-nitroso-N-acetylpenicillamine (SNAP, 0.01-3 microM), but did not affect the relaxant action of S-nitroso-coenzyme A (0.1-3 microM). In anococcygeus muscles, hydroxocobalamin (30 microM) had little effect on relaxations produced by nitrosocysteine (0.1-3 microM) and SNAP (0.01-1 microM), and enhanced those produced by nitrosoglutathione (0.1-3 microM) and nitroso-coenzyme A (0.1-3 microM). Since hydroxocobalamin is thought to act like haemoglobin by sequestering NO, some of the effects of hydroxocobalamin were compared with those of haemoglobin. Haemoglobin (10 microM) inhibited relaxations of aortic rings produced by nitrosocysteine and nitrosoglutathione and relaxations of anococcygeus muscles produced by nitrosocysteine, nitrosoglutathione and SNAP. Thus the effects of hydroxocobalamin on nitrosothiol-induced relaxations differ between the rat aorta and anococcygeus muscle, and depend on the exact nature of the nitrosothiol; however, the effects of haemoglobin did not differ qualitatively between the two tissues. Since hydroxocobalamin reduced relaxations of rat anococcygeus muscles elicited by NO, but not those elicited by nitrergic nerve stimulation or nitrosothiols, the nitrergic transmitter more closely resembles a nitrosothiol than free NO. Of those tested, the best correspondence was with nitrosocysteine; however, there were some differences between it and the transmitter.

Chromosomal aberrations and sister-chromatid exchanges induced by N-nitroso-2-acetylaminofluorene and their modifications by arsenite and selenite in Chinese hamster ovary cells.

The frequencies of chromosomal aberrations (CA) and sister-chromatid exchanges (SCE) in Chinese hamster cells were significantly increased by the direct-acting mutagen N-nitroso-2-acetylaminofluorene (N-NO-AAF) at the concentration of 0.1 mM. N-NO-AAF was prepared by nitrosation of the protohepatocarcinogen 2-acetylaminofluorene. The induced CA, which included chromatid breaks, chromatid exchanges, chromosome breaks, and chromosome ring formation were significantly potentiated by the presence of sodium arsenite (10 microM), but not by hydroxyurea (20 mM) or cytosine arabinoside (25 microM). On the other hand, the clastogenic effect of N-NO-AAF was effectively inhibited by sodium selenite (100 microM). Arsenite (10 microM) was shown to be moderately active in CA induction which was partially blocked by the presence of selenite (10 nM). N-Nitroso compounds such as N-nitroso-N-methylurea, N-nitroso-N-ethylurea and N-methyl-N'-nitro-N-nitrosoguanidine were equally or more active in the induction of CA and SCE in CHO cells when compared with N-NO-AAF. The cell cycle was significantly delayed by the intervention of N-NO-AAF.

Reduced bactericidal activity in neutrophils from scorbutic animals and the effect of ascorbic acid on these target bacteria in vivo and in vitro.

Actinomycetes, involved in oral and periodontal diseases, cause serious infections in immunocompromised hosts. Severely scorbutic guinea pig leukocytes killed only 12% of phagocytosed actinomycetes, had distorted nuclear morphology, had 16 times less ascorbate, and had no chemotactic responses in vitro. Ascorbate reversed these indices and also prevented nitrosamine formation by oral organisms. Degranulating leukocytes release lactoferrin and ascorbate that chelate iron, essential for microorganisms. Ascorbic acid, 2,2'-bipyridine and 1,10-phenanthroline were bactericidal to several bacterial pathogens at millimolar concentrations. Iron alone reversed this effect. In in vivo experiments an Actinomyces viscosus monoflora was implanted in rhesus monkeys. Plaque and serum samples showed decreased (by six orders of magnitude) bacterial counts and decreased actinomycete antibody titers in animals given 1 g ascorbate/d. Removing ascorbate returned counts and titers to preascorbate concentrations. Fifteen marmosets, receiving twice daily topical applications of ascorbate or water, had comparatively lower gingival, calculus, and plaque indices and only slightly lowered actinomycete counts.

[The role of UDP glucuronosyl- and sulfotransferases in the butylhydroxytoluene suppression of the mutagenic action of carcinogenic nitroso compounds and cyclophosphane]. / O roli UDF-gliukoronozil- i sul'fotransferaz v podavlenii butilgidroksitoluolom mutagennogo deistviia kantserogennykh nitrozosoedinenii i tsiklofosfana.

Treatment with butylated hydroxytoluene (BHT) was shown to stimulate the activity of UDP-glucuronosyltransferase and to inhibit that of sulfotransferase in liver of Wistar male rats. Addition of UDP-glucuronic acid to incubation medium in Ames' test using BHT-pretreated subfractions of rat liver resulted in decreased mutagenicity of nitrosodiethylamine, nitrosomorpholine and cyclophosphamide. Further treatment with 3'-phosphoadenosine-5'-phosphosulfate failed to affect mutagenic activity of the promutagens tested. However, an increase in mutagenicity of nitrosomorpholine and cyclophosphamide was observed in application of liver subfractions from intact animals. It was concluded that BHT-induced inhibition of active metabolite production as well as increased production of their glucuronides are responsible for inhibition of mutagenicity of the agents tested. Simultaneous decrease in the yield of sulfates potentiated this effect for nitrosomorpholine and cyclophosphamide.

Inhibitors of endogenous nitrosation. Mechanisms and implications in human cancer prevention.

Although the proof that N-nitroso compounds (NOC), a versatile class of carcinogens in animals, are also carcinogenic in man is lacking, humans are exposed through ingestion or inhalation to preformed NOC in the environment and through the endogenous nitrosation of amino precursors in the body. Activated macrophages can synthesize nitrate, nitrite and nitrosating agents that can form NOC. A number of bacterial strains isolated from human infections can produce NOC enzymatically from precursors at neutral pH. As a consequence endogenous nitrosation may occur at various sites of the body such as the oral cavity, stomach, urinary bladder, lungs, and at other sites of infection or inflammation. Since the demonstration by Mirvish et al. (1972) showing that ascorbate can reduce tumor formation in animals following feeding of nitrite plus amine, numerous substances to which humans are exposed have been identified and shown to inhibit formation of NOC in vitro, in animal models and in humans. Such inhibitors of nitrosation include vitamins C and E, phenolic compounds, and complex mixtures such as fruit and vegetable juices or other plant extracts. Nitrosation inhibitors normally destroy the nitrosating agents and thus act as competitors for the amino compound that serves as substrate for the nitrosating species. Independently, epidemiological studies have already established that fresh fruits and vegetables that are sources of vitamin C, other vitamins and polyphenols have a protective effect against cancers at various sites and in particular gastric cancer. Although the evidence that endogenously formed NOC are involved in human cancers is far from conclusive, it is suggestive and justifies preventive measures for reducing exposure to NOC. This article briefly reviews (i) the chemistry of NOC formation and inhibition, (ii) the studies in experimental animals which showed that inhibition of endogenous NOC synthesis leads to a reduction of toxic, mutagenic and carcinogenic effects, (iii) recent studies in humans where the degree of inhibition of endogenous NOC synthesis was directly quantified and lastly (iv) the contribution of nitrosation inhibitors to human cancer prevention.

Mechanisms of inhibition of N-nitroso compounds-induced mutagenicity.

In this review we describe the mechanisms of the inhibitory effects of various chemical agents towards the mutagenicity of N-nitroso compounds, including direct-acting mutagens such as N-nitroso derivatives of alkylureas, alkylnitroguanidines and alkylurethanes, and promutagenic nitrosamines. Possible mechanisms by which the inhibitors may exert their effects outside and inside the target cells include chemical and enzymatic deactivation of the mutagen, inhibition of metabolic activation of nitrosamines, scavenging mutagenic products, inhibition of cellular uptake, induction of detoxifying mechanisms, protecting nucleophilic centers in DNA and modulating DNA repair.