METABOLISM AND PHARMACOKINETICS OF N-NITROSONORNICOTINE IN THE PATAS MONKEY

This article is available online at http://dmd.aspetjournals.org ABSTRACT: N-Nitrosonornicotine (NNN) is present in significant quantities in tobacco and tobacco smoke and is believed to play an important role as a cause of cancer in people who use tobacco products. Biomarkers of NNN uptake in humans such as urinary metabolites would be useful for assessing cancer risk. Previous studies, carried out almost exclusively in rodents, have characterized urinary metabolites of NNN, but none of these would be suitable as a biomarkers of NNN uptake in humans. Therefore, we studied NNN metabolism in the patas monkey. Monkeys were treated intravenously wit

Comparative Levels of O6-Methylguanine, Pyridyloxobutyl-, and Pyridylhydroxybutyl-DNA Adducts in Lung and Liver of Rats Treated Chronically with the Tobacco-Specific Carcinogen 4-(Methylnitrosamino)-1-(3-pyridyl)-1-butanone

The tobacco-specific nitrosamine 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) is a lung carcinogen in rats and may be a cause of lung cancer in smokers. NNK is metabolized by cytochromes P450 to intermediates that react with DNA forming methyl, pyridyloxobutyl (POB), and pyridylhydroxybutyl (PHB) adducts, which are critical in carcinogenesis. The methyl adduct O6-methylguanine (O6-methyl-G) has miscoding properties, but there are no reports on levels of this adduct in rats treated chronically with NNK in the drinking water, nor has its levels been compared with those of POB- and PHB-DNA adducts. We used liquid chromatography-electrospray ionization-tandem mass spectrometry-selected reaction monitoring to quantify O6-methyl-G in lung and liver DNA of rats treated with a carcinogenic dose of 10 ppm of NNK in the drinking water and sacrificed after 1, 2, 5, 10, 16, and 20 weeks. The maximal level of O6-methyl-G in lung DNA, 2550 ± 263 fmol/mg DNA, was reached at 5 weeks and was significantly greater (P < 0.05) at that point than all other adducts (measured previously) except O2-[4-(3-pyridyl)-4-oxobut-1-yl]thymidine. Overall levels of O6-methyl-G in lung were intermediate between those of total POB- and PHB-DNA adducts. In liver, the wave of O6-methyl-G peaked at 2 weeks while that of total POB-DNA adducts peaked at 10 weeks, and levels of total PHB-DNA adducts were low throughout. The results of this study demonstrate that substantial amounts of O6-methyl-G are formed at various time points in lung and liver DNA of rats treated chronically with NNK, supporting its role in carcinogenesis

Inhibition of human cytochrome P450-catalyzed oxidations of xenobiotics and procarcinogens by synthetic organoselenium compounds

The effects of synthetic chemopreventive organoselenium compounds 1,2-, 1,3-, and 1,4-phenylenebis(methylene)selenocyanate (o-, m-, and p-XSC, respectively), benzyl selenocyanate (BSC), and dibenzyl diselenide (DDS) and inorganic sodium selenite on the oxidation of xenobiotics and procarcinogens by human cytochrome P450 (P450 or CYP) enzymes were determined in vitro. Spectral studies showed that BSC and three XSC compounds (but not sodium selenite or DDS) induced type II difference spectrum when added to the suspension of liver microsomes isolated from β-naphthoflavone-treated rats, with m-XSC being the most potent in inducing spectral interactions with P450 enzymes; m-XSC also produced a type II spectral change with human liver microsomes. o-, m-, and p-XSC inhibited 7-ethoxyresorufin O-deethylation catalyzed by human liver microsomes when added at concentrations below 1 μM levels, but BSC and DDS were less effective. All of these compounds inhibited the oxidation of model substrates for human P450s to varying extents. We studied the effects of these compounds on the activation of procarcinogens by recombinant human CYP1A1, 1A2, and 1B1 enzymes using Salmonella typhimurium NM2009 tester strain for the detection of DNA damage. The three XSCs were found to be very potent inhibitors of metabolic activation of 3-amino-1,4- dimethyl-5H-pyrido[4,3-b]indole, 2-amino-3,5-dimethylimidazo[4,5- f]quinoline, and 2-aminoanthracene, catalyzed by CYP1A1, 1A2, and 1B1, respectively. The potency of inhibition of m-XSC on CYP1B1-dependent activation of 2-aminoanthracene was compatible to those of α-naphthoflavone. These inhibitory actions may, in part, account for the mechanisms responsible for cancer prevention by organoselenium compounds in laboratory animals

Evaluation of Nitrosamide Formation in the Cytochrome P450-Mediated Metabolism of Tobacco-Specific Nitrosamines

<i>N</i>′-Nitrosonornicotine (NNN) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) are carcinogenic tobacco-specific nitrosamines believed to play a vital role in the initiation of tobacco-related cancers. For their carcinogenicities to be exhibited, both NNN and NNK must be metabolically activated by cytochrome P450s, specifically P450 2A6 and P450 2A13, respectively. Prior research has focused on α-hydroxylation, which leads to the formation of several DNA adducts that have been identified and quantified <i>in vivo</i>. However, some studies indicate that P450s can retain substrates within their active sites and perform processive oxidation. For nitrosamines, this would oxidize the highly unstable α-hydroxynitrosamines to potentially more stable nitrosamides, which could also alkylate DNA. Thus, we hypothesized that both NNN and NNK are processively oxidized <i>in vitro</i> to nitrosamides by P450 2A6 and P450 2A13, respectively. To test this hypothesis, we synthesized the NNN- and NNK-derived nitrosamides, determined their half-lives at pH 7.4 and 37 °C, and monitored for nitrosamide formation in an <i>in vitro</i> P450 system with product analysis by LC/NSI⁺-HRMS/MS. Half-lives of the nitrosamides were determined by HPLC-UV and ranged from 7–35 min, which is more than 40 times longer than the corresponding α-hydroxynitrosamines. Incubation of NNN in the P450 2A6 system resulted in the formation of the nitrosamide <i>N</i>′-nitrosonorcotinine (NNC) at low levels. Similarly, the nitrosamide 4-(methylnitrosamino)-1-(3-pyridyl)-1,4-butanedione (CH₂-oxo-NNK) was detected in low amounts in the incubation of NNK with the P450 2A13 system. The other possible NNK-derived nitrosamide, 4-(nitrosoformamido)-1-(3-pyridyl)-1-butanone (CH₃-oxo-NNK), was not observed in the P450 2A13 reactions. CH₂-oxo-NNK readily formed O⁶meGua in reactions with dGuo and calf thymus DNA. These results demonstrate that NNC and CH₂-oxo-NNK are novel metabolites of NNN and NNK, respectively. Though low-forming, their increased stability may allow for mutagenic DNA damage <i>in vivo</i>. More broadly, this study provides the first account of a cytochrome P450-mediated conversion of nitrosamines to nitrosamides, which warrants further studies to determine how general this phenomenon is in nitrosamine metabolism

Climate change and its effect on conservation and use of plant genetic resources for food and agriculture and associated biodiversity for food security

We review the impacts and implications of climate change for the management and use of plant genetic resources for food and agriculture and associated biodiversity (PGRFA-AB). Climate change is predicted to bring about increased temperatures across the world in the range of 1.6°C to as much as 6°C by 2050. Although rainfall is predicted to increase globally, some areas will receive less annual rainfall, while others may receive much more. The timing of rains and crop growing periods will also change. The frequency and duration of extreme weather events are also predicted to increase, although uncertainty exists about the expected degree of changes. These predicted changes in climate are expected to have fairly widespread impacts on agriculture, with poor countries in the south highlighted as being particularly vulnerable, having already weak economies and limited institutional capacities to adapt. More specifically to PGRFA-AB, evidence suggests important risks to wild biodiversity, including crop wild relatives. Changes in climate are also likely to place new pressures on conservation of landraces of crop species