Microsomal oxidative reactions

The aforementioned six microsomal oxidation reactions are summarized in Figure 8.9. 

Microsomal oxidative reactions constitute the most prominent phase I biotransformation pathway for a wide variety of structurally unrelated drugs (Table 1.4). Some drugs (e.g. amphetamine, diazepam, propranolol, lignocaine) simultaneously undergo more than one type of microsomal-mediated oxidative reaction. Microsomal enzymes are located primarily in liver cells, where they are associated with the smooth-surface (without ribosomes) endoplasmic reticulum (Fouts, 1961). Lipid solubility is a prerequisite for drug access to the... 

Since the results of our experiments with isolated rat liver fractions supported a reaction sequence Initiated by microsomal oxidation of the nitrosamine leading to formation of a carbonium ion, the results of the animal experiment suggested that in the intact hepatocyte, one of the earlier electrophilic intermediates (II, III or V, Figure 1) is intercepted by nucleophilic sites in DNA (exemplified here by the N7 position of guanine) before a carbocation is formed. 

Iron complexes or microsomal nonheme iron are undoubtedly obligatory components in the microsomal oxidation of many organic compounds mediated by hydroxyl radicals. In 1980, Cohen and Cederbaum [27] suggested that rat liver microsomes oxidized ethanol, methional, 2-keto-4-thiomethylbutyric acid, and dimethylsulfoxide via hydrogen atom abstraction by hydroxyl radicals. Then, Ingelman-Sundberg and Ekstrom [28] assumed that the hydroxylation of aniline by reconstituted microsomal cytochrome P-450 system is mediated by hydroxyl radicals formed in the superoxide-driven Fenton reaction. Similar conclusion has been made for the explanation of inhibitory effects of pyrazole and 4-methylpyrazole on the microsomal oxidation of ethanol and DMSO [29],... 

Estabrook, R.W., Hildebrandt, A.G., Baron, J., Netter, K.J. and Leibman, K. (1971) A new spectral intermediate associated with cytochrome P-450 function in liver microsomes. Biochemical and Biophysical Research Communications, 42 (1), 132-139. Pompon, D. and Coon, M.J. (1984) On the mechanism of action of cytochrome P-450. Oxidation and reduction of the ferrous dioxygen complex of liver microsomal cytochrome P-450 by cytochrome b5. Journal of Biological Chemistry, 259 (24), 15377-15385. Hildebrandt, A. and Estabrook, R.W. (1971) Evidence for the participation of cytochrome b 5 in hepatic microsomal mixed-function oxidation reactions. Archives of Biochemistry and Biophysics, 143 (1), 66-79. 

For foreign compounds, the majority of oxidation reactions are catalyzed by monooxygenase enzymes, which are part of the mixed function oxidase (MFO) system and are found in the SER (and also known as microsomal enzymes). Other enzymes involved in the oxidation of xenobiotics are found in other organelles such as the mitochondria and the cytosol. Thus, amine oxidases located in the mitochondria, xanthine oxidase, alcohol dehydrogenase in the cytosol, the prostaglandin synthetase system, and various other peroxidases may all be involved in the oxidation of foreign compounds. 

The most important enzyme involved in bio transformation is cytochrome P-450, which catalyzes many phase 1 reactions. This enzyme is located primarily in the SER (microsomal fraction) of the cell and is especially abundant in liver cells. Cytochrome P-450 primarily catalyzes oxidation reactions and consists of many isoforms (isozymes). These isoenzymes have overlapping substrate specificities. The most important subfamily in humans is CYP3A4, although there is considerable variation in CYP3A4 expression between individuals. 

These reactions have been used for the deblocking of protecting groups. In the case of amines, the formation of metabolites is possible because the anodic oxidation of amines equals the microsomal oxidation to a large extent. 

Under hypoxic conditions, cellular enzymes reduce the benzotriazine di-N-oxide [(reaction (68) P450 reductase Cahill and White 1990 and NADPH may be involved Walton et al. 1992 Wang et al. 1993]. Upon microsomal reduction of tirapazamine the radical formed in reaction (68) has been identified by EPR (Lloyd et al. 1991). Using the pulse radiolysis technique, it has been shown that this radical has a pKd of 6 (Laderoute et al. 1988), and it is the protonated form that undergoes the DNA damaging reaction (Wardman et al. 2003). The rate constants of the bimolecular decay of the radical [reaction (70)] has been found to be 2.7 x 107 dm3 mol-1 s 1. The reaction with its anion is somewhat faster (8.0 x 108 dm3 mol-1 s 1), while the deprotonated radicals do not react with one another at an appreciable rate. From another set of pulse radiolysis data, a first-order process has been extracted (k = 112 s 1) that has been attributed to the water elimination reaction (72), and the tirapazamine action on DNA [reaction (74)] has been considered to be due to the resulting radical (Anderson et al. 2003). 

---