Microsomal monooxygenations

Induction of Hepatic Microsomal Monooxygenation in Rainbow Trout ... 

Phase I reactions include microsomal monooxygenations, cytosolic and mitochondrial oxidations, co-oxidations in the prostaglandin synthetase reaction, reductions, hydrolyses, and epoxide hydration. All of these reactions, with the exception of reductions, introduce polar groups to the molecule that, in most cases, can be conjugated during phase II metabolism. The major phase I reactions are summarized in Table 7.1. 

Monooxygenations are those oxidations in which one atom of molecular oxygen is reduced to water while the other is incorporated into the substrate. Microsomal monooxygenation reactions are catalyzed by nonspecific enzymes such as the flavin-containing monooxygenases (FMOs) or the multienzyme system that has cytochrome P450s (CYPs) as the terminal oxidases. 

In the following years cytochrome P450 was also found as an integral part of the 1 l 3-hydroxylase system in adrenal mitochondria Its involvement in the liver microsomal monooxygenation of codeine, acetanilide and cyclohexane was established using the technique of the photochemical action spectrum (Fig. 2). 

Ullrich V, Kuthan H (1980) Autoxidation and uncoupling in microsomal monooxygenations. Devel-opm Biochem 13 267-272... 

Figure 5.49 (a) Total-ion-current trace, and (b) the reconstructed ion chromatogram of mjz 510.2 0.5 (monooxygenated metabolites) from LC-MS analysis of human microsomal incubation of Glyburide. Reprinted with permission from Zhang, H., Henion, J., Yang, Y. and Spooner, N., Anal. Chem., 72, 3342-3348 (2000). Copyright (2000) American Chemical Society. 

In many mammals induction of monooxygenation by polycyclic aromatic hydrocarbons is accompanied by the formation of a hemoprotein not seen to any appreciable extent in non-induced animals. This leads to an alteration in the microsomal hemoprotein populations, a change in the metabolic activity of the microsomes and, hence, possible alterations in the toxicity of other chemicals (27, 8). 

Initial studies designed to obtain a valid subcellular fractionation scheme for rainbow trout liver illustrated the aryl-hydrocarbon (benzo[a]pyrene] hydroxylase activity separated with glucose-6-phosphatase (35). This observation indicated that the trout hemoprotein P-450-mediated monooxygenation system was located within the endoplasmic reticulum (microsomal fraction). 

Frommer U, Ullrich V, Staudinger H, et al. 1972. The monooxygenation of n-heptane by rat liver microsomes. Biochimica Biophysica Acta 280 487-494. 

In the catalytic cycle of CYP, reducing equivalents are transferred from NADPH to CYP by a flavoprotein enzyme known as NADPH-cytochrome P450 reductase. The evidence that this enzyme is involved in CYP monooxygenations was originally derived from the observation that cytochrome c, which can function as an artificial electron acceptor for the enzyme, is an inhibitor of such oxidations. This reductase is an essential component in CYP-catalyzed enzyme systems reconstituted from purified components. Moreover antibodies prepared from purified reductase are inhibitors of microsomal... 

This reaction should not be confused with the monooxygenation of ethanol by CYP that occurs in the microsomes. The alcohol dehydrogenase reaction is reversible, with the carbonyl compounds being reduced to alcohols. 

In monooxygenation, the P-450 hemoprotein receives two electrons from cofactor NADPH or NADH, or both these electrons are received one at a time, usually via reductases (flavoproteins). In most organisms, the electron chain is deeply embedded principally in the endoplasmic reticulum (and to some degree in the inner mitochondrial membrane and nuclear envelope). After sheering of the membrane during homogenization, the endoplasmic reticulum is centrifuged at 100,000 for an hour, and the product is called the "microsomal pellet." The microsomal electron chain contains reductase and P-450. 

Rodrigues, A.D., D. Fernandez, M.A. Nosarzewski, W.M. Pierce, and R.A. Prough (1991). Inhibition of hepatic microsomal cytochrome P-450 dependent monooxygenation activity by the antioxidant 3-tert-butyl-4-hydroxyanisoIe. Chem. Res. Toxicol. 4, 281-289. 

An enzyme from the flowers of Sinningia cardinalis Reichsteinia cardinalis, Gesneriaceae) has hydrolase activity associated with microsomal fractions and requires NADPH as an essential cofactor (hydrolyase activity II). This enzyme converts naringenin (10) and apigenin (5) to eriodictyol (17) and luteolin (4), respectively (Dewick, 1989). The flavone synthase activity of this enzyme was abolished completely by treatment with the cytochrome P-450 inhibitor ancymidol, but the flavonoid 3 -monooxygen-ase activity was not altered. 

---