Mixed-function oxidases specificity

Brownlee LJ, Evans CH, Hollebone BR. 1986. The relative induction of mixed-function oxidase specific activity to carbon-hydrogen and carbon-carbon 1 bond strengths in polychlorinated derivatives of dibenzo-p-dioxin. J Appl Toxicol 6 67-72. 

The biotransformation systems involved in insecticide metabolism have been studied in the R and S populations to determine any differences which might be potential contributory factors to or results of insecticide resistance. In addition, the possibility of mixed-function oxidase induction has been investigated. Specifically, the studies have encompassed a seasonal study of microsomal mixed-function oxidase (mfo) components, and studies of aldrin, dieldrin and DDT metabolism. 

Hepatic mixed-function oxidase activities demonstrated seasonal trends, with higher specific activities in the cold weather months in both populations with few differences in enzyme activities or cytochrome levels between the two populations. Metabolism of aldrin, dieldrin and DDT was similar between the two populations. R fish have larger relative liver size and, therefore, a greater potential for xenobiotic metabolism. However, biotransformation appears to be of minor importance in chlorinated alicyclic insecticide resistance in mosquitofish barriers to penetration appear to be of greater importance and an implied target site insensitivity appears to be the most important factor in resistance. 

Lu, A.Y.H.and West, S.B. Reconstituted mammalian mixed-function oxidases Requirements, specificities and other properties. Pharmac. Ther. (1978) 2 337-358. 

The marine environment acts as a sink for a large proportion of polyaromatic hydrocarbons (PAH) and these compounds have become a major area of interest in aquatic toxicology. Mixed function oxidases (MFO) are a class of microsomal enzymes involved in oxidative transformation, the primary biochemical process in hydrocarbon detoxification as well as mutagen-carcinogen activation (1,2). The reactions carried out by these enzymes are mediated by multiple forms of cytochrome P-450 which controls the substrate specificity of the system (3). One class of MFO, the aromatic hydrocarbon hydroxylases (AHH), has received considerable attention in relation to their role in hydrocarbon hydroxylation. AHH are found in various species of fish (4) and although limited data is available it appears that these enzymes may be present in a variety of aquatic animals (5,6,7,8). 

It has been well recognized that the mixed-function oxidase system of Bacillus megaterium is involved in steroid hydroxylation (, as already described above. This enzyme system is composed of a NADPH-specific FMN flavoprotein (megaredoxin reductase), an iron-sulfur protein (megaredoxin) and cytochrome P cn. The megaredoxin protein plays an important role as an intermediate component of electron transfer from reduced flavoprotein to cytochrome P en. 

Let us for a moment consider the enzymes responsible for oxidation, the oxidases. Some are very specific as to the type of substrate which they will oxidize others are relatively nonspecific, such as the mixed function oxidases while some have a limited functional specificity such as monoamine oxidase, diamine oxidase and xanthine oxidase. Notice that the specific name for each enzyme will related in some way to its substrate. 

The conversion of cholesterol to bile acids is quantitatively the most important mechanism for degradation of cholesterol. In a normal human adult approximately 0.5 g of cholesterol is converted to bile acids each day. The regulation of this process operates at the initial biosynthetic step catalyzed by an enzyme in the endoplasmic reticulum, la-hydroxylase (fig. 20.18). The 7a-hydroxylase is one of a group of enzymes called mixed-function oxidases, which are involved in the hydroxylation of the sterol molecule at numerous specific sites. A mixed-function oxidase is an enzyme complex that catalyzes hydroxylation of a substrate with a concomitant production of H20 from a single molecule of 02- The 7a-hydroxylase is one of several enzymes referred to as cytochrome P450. 

The hepatic endoplasmic reticulum possesses oxidative enzymes called mixed-function oxidases or monooxygenase with a specific requirement for both molecular oxygen and a reduced concentration of nicotinamide adenine dinucleotide phosphate (NADPH). Essential in the mixed-function oxidase system is P-450 (Figure 1.12). The primary electron donor is NADPH, whereas the electron transfer involved P-450, a flavoprotein. The presence of a heat-stable fraction is necessary for the operation of the system. 

Oxidation is by far the most important Phase I metabolic reaction. One of the main enzyme systems involved in the oxidation of xenobiotics appears to be the so called mixed function oxidases or monooxygenases, which are found mainly in the smooth endoplasmic reticulum of the liver but also occur, to a lesser extent, in other tissues. These enzymes tend to be nonspecific, catalysing the metabolism of a wide variety of compounds (Table 9.2). Two common mixed function oxidase systems are the cytochrome P-450 (CYP-450) and the flavin monoxygenase (FMO) systems (Appendix 12). The overall oxidations of these systems take place in a series of oxidative and reductive steps, each step being catalysed by a specific enzyme. Many of these steps require the presence of molecular oxygen and either NADH or NADPH as co-enzymes. 

Since the major precursor of all of these hormones is cholesterol it becomes clear that, while the total biosynthetic routes are quite complex, they all involve a number of specific hydroxyla-tion reactions involving both the steroid nucleus and the side chain at C-17. Most of these are catalyzed by mixed-function oxidases involving cytochrome P-450 (Figure 1) (13-15). 

As with the mixed-function oxidases involved in xenobiotic metabolism, the substrate specificity of the steroid hydroxylases is dictated, in part, by the existence of multiple forms of both microsomal and mitochondrial cytochrome P-450s and further opportunities for specificity are provided by the distinct localization of the various enzymes in either the mitochondria or the endoplasmic reticulum. 

Vitamin E deficiency is also associated with impaired mitochondrial oxidative metabolism and impaired activity of microsomal cytochrome P450-dependent mixed-function oxidases, and hence the metabolism of xenobi-ofics. There is no evidence that vitamin E has any specific role in electron transport in mitochondria or microsomes. Again, changes in membrane lipids and oxidative damage presumably account for the observed metabolic abnormalities. 

The placenta is both a transport and a metabolizing organ. Transport is accomplished by simple diffusion, facilitated diffusion, active transport across membranes, and by special processes such as pinocytosis, phagocytosis, specific transport molecules, and channels in the barrier . The placenta also contains a full complement of mixed function oxidases located in the microsomal and mitochondrial subcellular fractions capable of induction and metabolism of endogenous and exogenous chemicals. 

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