Hydroxylation by microsomal

In rats and mice, TBT compounds are hydroxylated by microsomal monooxygenase attack (see Environmental Health Criteria 116). Hydroxylation can occur on either the alpha or beta carbon of the butyl group, that is, alpha or beta in relation to the Sn atom (see Figure 8.5). After hydroxylation, the hydroxylated moiety breaks away to leave behind dibutyltin. 

Until recently, intestinal metabolism via CYP3A4-mediated metabolic pathways was thought to be insignificant because of the lower levels of expression compared with that seen in the liver and slower metabolic rates measured for intestinal microsomes (224). However, similar Km values have been reported for midazolam 1 -hydroxylation by microsomes obtained in the upper intestine and the liver (254,255). This correlation indicates that the upper intestine and hepatic CYP3 A4 are functionally equivalent. Such findings further establish the importance of the intestine in the elimination of orally administered substrates for CYP3 A4-mediated metabolic pathways. Additionally, coadministration of substrates/inhibitors that may alter the function of these proteins (induction, inhibition) could further be responsible for the variability in intestinal absorption (dmg interactions) seen for some dmgs. 

Cyclohexene is readily hydroxylated by microsomal oxidases to the corresponding dihydroxy derivatives. These are then further conjugated and eliminated in urine. 

When healthy volunteers were exposed by inhalation to 100 or 400 ppm tetrahydrofuran in air, the percentage of expired tetrahydrofuran was 25-35%. The elimination half-life of tetrahydrofuran was 30 min in individuals exposed to 200 ppm for 3h. Some tetrahydrofuran is absorbed in the nasal cavity due to its solubility and inspiratory flow rate. Tetrahydrofuran uptake in the nasal tissue is dependent on its reaction with tissue substrates. Some tetrahydrofuran can be metabolized in the nasal cavity. Tetrahydrofuran blood concentrations were higher at 1 h postexposure than immediately after cessation of exposure. In vitro studies indicated that tetrahydrofuran was first hydroxylated by microsomal enzymes. High concentrations (lO molH ) of tetrahydrofuran inhibited the in vitro activity of rat hepatic cytochrome P450 by 80%. Tetrahydrofuran has been noted to enhance the toxic action of a number of compounds and stimulate the rapid absorption of reactive metabolites. Some of the tetrahydrofuran is excreted in the exhaled breath, while the various metabolites of tetrahydrofuran are excreted in the urine. 

Pleuromutilin (Scheme 17.9) is an antibiotic from Pleurotus or Clitopilus basidiomycete strains which kills mainly Gram-positive bacteria and mycoplasms. Metabolism of pleuromutilin and derivatives results in hydroxylation by microsomal cytochrome P-450 at the 2- or 8-position and inactivates the antibiotics. Modification of the 8-position of pleuromutilin and analogs was of interest as a means of preventing the metabolic hydroxylation. Microbial hydroxylation at the... 

Only limited metabolic studies have been carried out on NPIP. It undergoes a-hydroxylation by rat liver microsomes to give 5-hydroxypentanal, a process analogous to the formation of... 

MgCl2 (10 mM) increased the apparent Km (83 to 173 /am) and reduced the Vjnax (3.4 to 2.4 min-1) of triazolam 4-hydroxylation by expressed CYP3A4 [21]. However, both MgCl2 (30 mM) and CaCl2 (30 mM) significantly increased reaction rates of testosterone 6/3-hydroxylation (approximately threefold) and nifedipine oxidation (three- to six-fold) by human liver microsomes (HLMs) or recombinant CYP3A4 (reconstituted with b5 and GSH) [15]. It was suggested that divalent cation stimulation on the activity was related to involvement of b5 in CYP 3A4 reaction. 

Recent studies suggest that many factors may affect hydroxyl radical generation by microsomes. Reinke et al. [34] demonstrated that the hydroxyl radical-mediated oxidation of ethanol in rat liver microsomes depended on phosphate or Tris buffer. Cytochrome bs can also participate in the microsomal production of hydroxyl radicals catalyzed by NADH-cytochrome bs reductase [35,36]. Considering the numerous demonstrations of hydroxyl radical formation in microsomes, it becomes obvious that this is not a genuine enzymatic process because it depends on the presence or absence of free iron. Consequently, in vitro experiments in buffers containing iron ions can significantly differ from real biological systems. 

The constituent of paint, 2-nitropropane, exhibiting genotoxicity and hepatocarcinogeni-city was oxidized by liver microsomes forming nitric oxide, which was identified as a ferrous NO complex [61]. Clement et al. [62] concluded that superoxide may participate in the microsomal oxidation of /Y-hydroxyguanidincs, which produced nitric oxide, urea, and the cyanamide derivative. Caro et al. [63] suggested that the oxidation of ketoxime acetoxime to nitric oxide by microsomes enriched with P-450 isoforms might be mediated by hydroxyl or hydroxyl-like radicals. 

Thus, superoxide itself is obviously too inert to be a direct initiator of lipid peroxidation. However, it may be converted into some reactive species in superoxide-dependent oxidative processes. It has been suggested that superoxide can initiate lipid peroxidation by reducing ferric into ferrous iron, which is able to catalyze the formation of free hydroxyl radicals via the Fenton reaction. The possibility of hydroxyl-initiated lipid peroxidation was considered in earlier studies. For example, Lai and Piette [8] identified hydroxyl radicals in NADPH-dependent microsomal lipid peroxidation by EPR spectroscopy using the spin-trapping agents DMPO and phenyl-tcrt-butylnitrone. They proposed that hydroxyl radicals are generated by the Fenton reaction between ferrous ions and hydrogen peroxide formed by the dismutation of superoxide. Later on, the formation of hydroxyl radicals was shown in the oxidation of NADPH catalyzed by microsomal NADPH-cytochrome P-450 reductase [9,10]. 

Groves JT, McClusky GA. Aliphatic hydroxylation by highly purified liver microsomal cytochrome P-450. Evidence for a carbon radical intermediate. Biochem Biophys Res Commun... 

Palamanda, J., Feng, W.W., Lin, C.C. and Nomeir, A.A. (2000) Stimulation of tolbutamide hydroxylation by acetone and acetonitrile in human liver microsomes and in a cytochrome P-450 2C9-reconstituted system. Drug Metabolism and Disposition, 28 (1), 38-43. 

Court M.H., S.X. Duan, L.M. Hesse, K. Venkatakishnan, and D.J. Greenblett (2001). Cytochrome P-450 is responsible for interindividual variabihty of propofol hydroxylation by human liver microsomes. Anesthesiology 94 110-119. 

CYP2A6 (cytochrome P450 2A6) has been purified from human liver and CYP2A6 cDNA expression systems are available. Many studies have demonstrated marked interindividual variation in the levels of hepatic CYP2A6 protein, mRNA and associated microsomal coumarin 7-hydroxylase activity (reviewed in Pelkonen et al., 1997 Lake, 1999). The role of CYP2A6 in the metabolism of coumarin by human liver microsomes has been confirmed by Sai et al. (1999), who found that a monoclonal antibody to CYP2A6 inhibited coumarin 7-hydroxylation by more than 94%. 

The corresponding reactions of the methyl groups at C-4 on the A ring16" 68 73 are depicted on the left side of Fig. 22-8. Tire 4a methyl group is first hydroxylated by a microsomal (ER) system similar to cytochrome P450 but able to accept electrons from NADH and cytochrome b5 rather than NADPH.173 The two-step oxidation of the resulting alcohol... 

Ingelmann-Sundberg M, Kaur H, Terelius Y, Persson J-O, Halliwell B (1991) Hydroxylation of salicylate by microsomal fractions and cytochrome P-450. Biochem J 276 753-757 Isobe T, Naiki M, Handa S, Taki T (1996) Anal Biochem 236 35-40... 

Hesse LM, He P, Krishnaswamy S, Hao Q, Hogan K, von Moltke LL, Greenblatt DJ, Court MH. Pharmacogenetic determinants of interindividual variability in bupropion hydroxylation by cytochrome P450 2B6 in human liver microsomes. Pharmacogenetics 2004 14 225-238. 

Figure 3 Effect of protein concentration on the inhibition of CYP2C8 (paclitaxel 6a-hydroxylation) by montelukast in human liver microsomes. The inhibitory effect of montelukast (CYP2C8 inhibitor) on the conversion of paclitaxel to 6a-hydroxypaclitaxel declined almost 20-fold when the microsomal protein concentration increased 20-fold due to nonspecific protein binding.

Yamazaki H, Guo Z, Persmark M, et al. Burfuralol hydroxylation by cytochrome P450 2D6 and 1A2 enzymes in human liver microsomes. Mol Pharmacol 1994 ... 

Reinke LA Moyer MJ. 1985. p-Nitrophenol hydroxylation. A microsomal oxidation which is highly inducible by ethanol. Drug Metab Dispos 13 548-552. 

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