Oxygen microsomal mixed function oxidations

The predicted stoichiometry in microsomal mixed function oxidations for NADPH substrate oxygen is 1 1 1 325), In the absence of substrate NADPH oxidase is measured and in the presence of substrate this background oxidase activity is present though oxygen consumption increases 326, 326). As the level of substrate increases the expected stoichiometry is approached 326, 328). The addition of potential substrates which cannot be hydroxylated, such as perfluoro-n-hexane, leads to increased oxygen consumption and this has been termed uncoupling 327). It has been demonstrated that some of the electrons lost from the system are... 

The microsomal fraction of liver contains a monooxygenase system consisting of cytochrome P-450 (EC 1.14.14.1), NADPH-cytochrome P-450 reductase (EC 1.6.2.4), and phospholipid. This system catalyzes the hydroxylation of a large number of both foreign and endogenous compounds in a mixed-function oxidation reaction using molecular oxygen and NADPH or NADH as electron donors ... 

The experimental results given above clearly indicate that cytochrome P-450 is the oxygen-activating catalyst of a wide variety of mixed-function oxidations in tissues of vertebrate animals. Spectrophoto-metric measurements and determinations of the photochemical action spectra did not reveal any significant differences between the pigments from different sources. Yet the substrate specificity of these mixed-function oxidase systems is pronounced, as shown by studies on the induction of microsomal hydroxylase activity toward different xenobiotics (27) and on the affinity of different C-21 methyl corticosteroids toward... 

The renal cytochrome P-450 enzyme system is involved in oxidative reactions in which an atom of molecular oxygen is inserted in an organic molecule. The flavoprotein NADPH-cytochrome P-450 reductase is an essential component of the mixed-function oxidase systems (MFO). Microsomal membranes appear to be particularly subject to attack by reactive oxygen radicals due to their high content of unsaturated fatty acids and the presence of the cytochrome P-450 system [40]. Cephaloridine-induced peroxidation of membrane lipids is decreased by the cytochrome P-450 inhibitor cobalt chloride [31], suggesting a role for a cytochrome P-450 reductase in the P-lactam-induced generation of reactive oxygen species and subsequent peroxidation products. 

Most of the microsomal reactions can be classified as oxidations by what are referred to as mixed-function oxidases utilizing molecular oxygen and cofactors. The key enzyme is an iron-hemecytochrome P-450, a flavoprotein dependent in its reduction and reoxidation on the NADPH to NADP reaction. The 450 notation is based on the 450 nm absorption peak the enzyme exhibits on reaction with carbon monoxide. Thus, drug interactions with this enzyme system can be evaluated by measuring absorption spectra changes. 

Oxidations. Most oxidative processes take place in liver microsomes and are catalysed by mono-oxygenase enzymes known as mixed-function oxidases. These processes require reduced nicotinamide-adenine dinucleotide phosphate, molecular oxygen and a complex of enzymes in the endoplas-matic reticulum. The terminal oxidizing enzyme is cytochrome P450, a hemoprotein. The notation P450 refers to the ability of the reduced (ferrous) form of the hemoprotein to react with carbon monoxide, yielding a complex with absorption peak at 450 nm. For each molecule... 

Cinnamic acid 4-hydroxyla a (EC 1.14.13.11) a mixed function monooxygenase, present in plants, which catalyses an early reaction in Flavonoid (see) biosynthesis, i.e. the insertion of an atom of oxygen into cinnamic acid to form 4-hydroxydnnamic acid (4-coumaric acid) with concomitant oxidation of one molecule of NADPH. The enzyme is a cytochrome P4S0 system associated with the microsom fraction, and is specific for the trans isomer of cinnamic acid. During the hydroxylation, hydrogen at position 4 (experimentally tritium in position 4) is retained, i.e. there is an NIH shift (see). In vitro, a thiol, e.g. 2-mercaptoethanol, is required for activity. (P.R.Rich C.J.Lamb Ear. J.Biochem. 72 (1977) 353-360)... 

In addition to direct oxygenation, e.g. by aryl hydrocarbon hydroxylase, oxidative N- or 0-dealkylation is another process catalyzed by components of the Cytochrome P-450 System (mixed-function oxidases). Reduction also occurs in this system NADPH-cytochrome P-450 reductase has an activity similar to microsomal nitroreductase, i.e. transformation of aronaatic nitro compounds into the corresponding arylamines takes place. The oxidation may be followed by other enzymic reactions, e.g. epoxides are hydrated to vicinal diols by microsomal epoxide hydratase or they are coupled with glutathione by glutathione-S-epoxide transferase. 

The polychlorinated cyclodienes Aldrin 400) and Heptachlor 402) are converted in vitro by rabbit liver microsomes to their corresponding epoxides (Figure 12). The epoxidase has the characteristics of the mixed-function oxidase system in that it requires both NADPH and oxygen and it is inhibited by SKF 525-A. Dieldrin 401) is detoxified in vivo in the rat through hydrolysis of the epoxy group to a rarzi -dihydrodiol, which subsequently is oxidized to the dicarboxylic acid other metabolites include 9-hydroxydieldrin and a pentachloroketone structure. 

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