Nicotine, enzymic oxidation

Munafo M, Clark T, Johnstone E, Murphy M, Walton R (2004) The genetic basis for smoking behavior a systematic review and meta-analysis. Nicotine Tob Res 6 583-597 Murphy SE, Raulinaitis V, Brown KM (2005) Nicotine 5 -oxidation and methyl oxidation by P450 2A enzymes. Drug Metab Dispos 33 1166-1173... 

No differences were observed5 in the enzymic oxidation of putrescine and TV-methylputrescine by plant extracts of a cultivar of N. tabacum that had a high nicotine content and one with a high content of nornicotine (7). Thus a high nornicotine (7) content cannot be attributed to direct oxidation of putrescine, and this supports evidence which shows nornicotine (7) to be a demethylation product of nicotine. 

The major human urinary metabolites of nicotine are cotinine, nicotine A -oxide, and fra s-3 -hydroxycotinine (113). CYP2A6 appears to be the major enzyme responsible for formation of an iminium ion that is the first step in the C-5 oxidation of nicotine to cotinine and also the subsequent... 

The results (Leete and Chedekel, 1974) indicate that 48% of [2- H](-)-nomicotine and 52% [2- C](+)-nomicotine is incorporated from the labeled nicotine. Thus if (+)-nornicotine is formed from (-)-nicotine, the transformation must involve the loss of the hydrogen from C-2 however, almost the same [ HA C]ratio occurs as in the administered mixture of [2 - H](-)-nicotine and [2 2- C](+)-nicotine, which indicates that in N. glauca (+)- and (-)-nicotine are demethylated at similar rates. If the demethylation had been stereospeciflc for (-)-nicotine, the resultant nomi-cotine would have its [ HA C]ratio doubled. This led the authors to propose a scheme for the formation of (+)-nicotine and (-)-nicotine (Figure 6.11). This mechanism accounts for the partial racemization of the nornicotine derived from (-)-nicotine a Cope elimination of nicotine N -oxide (a) would involve one of the hydrogens at C-3, which would provide the unsaturated compound (b). Elimination of water from this hydroxy amine yields the Schiff base (c), which upon hydrolysis yields formaldehyde or other Cl metabolite and the primary amine (d). Cyclization of this intermediate yields (+)- and ( )-nomicotine. Leete and Chedekel presume that these steps are enzyme mediated, and it is to be expected that the final cyclization would yield a preferential amount of (+)- or (-)-nornicotine however, the mechanism which yields (+)-nornicotine from (-)-nicotine remains unknown. 

These studies were confirmed by tracer experiments showing that nitrogen of nicotinic acid (formed by Neurospora) is derived from 3-hydroxyanthranilic acid (478). Experiments with doubly labeled tryptophan demonstrate that tryptophan is probably the only source of quinolinic acid in rat metabolism (645) and that carbon atom 3 of tryptophan, the precursor of the carboxyl carbon of 3-hydroxyanthranilic acid, becomes carboxyl carbon in nicotinic acid (310,340,341,373). In vitro studies of the enzymic oxidation of 3-hydroxyanthranilic acid confirm its relationship to quinolinic acid (498) and show that picolinic acid may also form from it (539,540) but nicotinic acid synthesis under... 

A successful case study for asymmetric nitrogen oxidation was reported for a series of (hetero)aromatic tertiary amines. High diastereoselectivity was observed for the enzyme-mediated oxidation of S-(—)-nicotine by isolated CHMOAdneto to give the corresponding ds-N-oxide [215]. The stereoselectivity of this biooxidation was complementary to the product obtained by flavin M O (FM O) from human li ver (trows-selective [216]) as well as unspecific oxidations by FMOs from porcine and guinea pig liver. 

This enzyme [EC 1.5.1.13], also known as nicotinate hydroxylase, catalyzes the reaction of nicotinate with NADP+ and water to produce 6-hydroxynicotinate and NADPH. This iron-dependent flavoprotein will also oxidize NADPH. 

An alternative pathway for synthesis of quinoli-nate from aspartate and a triose phosphate exists in bacteria and in plants and provides the major route of nicotinic acid synthesis in nature. In E. coli the reaction is catalyzed by two enzymes, one an FAD-containing L-aspartate oxidase which oxidizes aspartate to a-iminoaspartate.228 The latter condenses with dihydroxyacetone-P to form quinolinate (Eq. 25-13).229 There are at least two other pathways for synthesis of quinolinic acid as well as five or more salvage pathways for resynthesis of degraded pyridine nucleotide coenzymes.224/230/231... 

Riboflavin, like nicotinic acid, forms an oxidation enzyme and, as such, acts as an oxygen carrier to the cell. The structure of riboflavin is ... 

Several of the B vitamins function as coenzymes or as precursors of coenzymes some of these have been mentioned previously. Nicotinamide adenine dinucleotide (NAD) which, in conjunction with the enzyme alcohol dehydrogenase, oxidizes ethanol to ethanal (Section 15-6C), also is the oxidant in the citric acid cycle (Section 20-10B). The precursor to NAD is the B vitamin, niacin or nicotinic acid (Section 23-2). Riboflavin (vitamin B2) is a precursor of flavin adenine nucleotide FAD, a coenzyme in redox processes rather like NAD (Section 15-6C). Another example of a coenzyme is pyri-doxal (vitamin B6), mentioned in connection with the deamination and decarboxylation of amino acids (Section 25-5C). Yet another is coenzyme A (CoASH), which is essential for metabolism and biosynthesis (Sections 18-8F, 20-10B, and 30-5A). 

These enzymes catalyze the two-electron oxidation of purines, aldehydes and pyrimidines, sulfite, formate and nicotinic acid in the general reaction shown in equation (49). These enzymes show some differences in properties. Xanthine oxidase, xanthine dehydrogenase and aldehyde oxidase all have relatively low redox potentials and a unique cyanolyzable sulfur atom, and so will be discussed together. 

Toxicologically it is of interest that the FMO enzyme is responsible for the oxidation of nicotine to nicotine F-N-oxide, whereas the oxidation of nicotine to cotinine is catalyzed by two enzymes acting in sequence CYP followed by a soluble aldehyde dehydrogenase. Thus nicotine is metabolized by two different routes, the relative contributions of which may vary with both the extrinsic and intrinsic factors outlined in Chapter 9. 

The first committed step in TA and nicotine biosynthesis is catalyzed by putrescine JV-methyltransferase (PMT) (Fig.7.4).82 A PMT cDNA isolated from tobacco showed extensive homology to spermidine synthase from mammalian and bacterial sources.83 A-Methylputrescine is oxidatively deaminated to 4-aminobutanal, which undergoes spontaneous cyclization to form the reactive A-methyl-A1-pyrrolinium cation. Although the enzymes involved are unknown, the A-methyl-A1-pyrrolinium cation is thought to condense either with acetoacetic acid to yield hygrine as a precursor to the tropane ring, or with nicotinic acid to form nicotine. 

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