Cytochrome reactions catalysed

The link with the final electron acceptor, O2, is the enzyme cytochrome c oxidase which spans the inner membrane of the mitochondrion. It consists of cytochromes a and a3 along with two, or possibly three, Cu atoms. The details of its action are not fully established but the overall reaction catalysed by the enzyme is ... 

Trager, W.F. (1988). Isotope effects as mechanistic probes of cytochrome P450-catalysed reactions. In Synthesis and Application of Isotopically Labelled Compounds Proceedings of the Third International Symposium T.A. Baillie and J.R. Jones (Eds.) Amsterdam Elsevier 333-340. 

Table 2.2 Schematic summary of the diverse reactions catalysed by cytochrome P450. Reprinted with permission from Sato et al., 1996. Copyright (1996) American Chemical Society. 

The precursor, 7-dehydrocholesterol is converted by a non-enzymatic reaction to cholecalciferol (calciol). This reaction occurs in skin exposed to sunlight due to irradiation by UV-B light at a wavelength of about 300 nm. Cholecalciferol is transported via carrier proteins to the liver where hydroxylation at carbon-25 occurs in a reaction catalysed by a microsomal cytochrome P450 hydroxylase to form calcidiol. This compound travels to the kidney attached to specific binding proteins, where another cytochrome P450 enzyme, mitochondrial 1-a-hydroxylase, introduces a second hydroxyl group in to the molecule to form the active calcitriol. 

An important point in the regulation of these processes is that all the reactions from mitochondrial NADH, to and including cytochrome c, are near-equilibrium (Figure 9.26(a)) (Appendix 9.8) that is, there is only one reaction in the electron transfer chain that is non-equihbrium - the terminal reaction catalysed by cytochrome oxidase. There is some similarity with the process of glycolysis in which the initial reaction and the terminal reactions are the nonequilibrium reactions (Figure 9.26(b)). 

It was proposed that the rearrangement of littorine to hyoscyamine might occur by a cytochrome P450-catalysed reaction (Fig. 2.3), and that the minor alkaloids 3a-phenylacetoxytropane and 3a-(2 -hydroxyacetoxy)tropane, which are also formed, are side-products of the mechanism (Robins et al, 1995). This particular area of tropane biosynthesis requires further clarification. 

Electrochemical oxidation reactions have been fonnd to mimic cytochrome P450 catalysed oxidation reaction to some extent, and can therefore be used as a tool in performing preliminary drug metabolism studies. Combined electrochemistry-mass spectrometry was explored for drug metabolite studies [77-78],... 

Oxidative reactions catalysed by non-microsomal enzymes are less varied than those mediated by cytochrome P450 mono-oxygenases but are important in the metabolism of some drugs (e.g. isoproterenol, methylxanthines, methimazole, ethanol, chloral hydrate), endogenous substances (adrenalin, histamine) and naturally occurring compounds (vitamin A). Theophylline and caffeine (methylxanthines) simultaneously undergo microsomal cytochrome... 

Let us look at the major types of oxidation reaction catalysed by the cytochromes P-450 system. 

Halogen atoms may be removed from xenobiotics in an oxidative reaction catalysed by cytochromes P-450. For example, the anaesthetic halothane is metabolized to trifluoroacetic acid via several steps, which involves the insertion of an oxygen atom and the loss of chlorine and bromine (figure 4,28). This is the major metabolic pathway in man and is believed to be involved in the hepatotoxicity of the drug. Trifluoroacetyl chloride is thought to be the reactive intermediate (see Chapter 7). 

Here we report studies on the binding of the 33kD protein to the reaction centre. We define the optimal conditions and stoichiometry. Comparisons are made with binding of this protein to the core complex. The effect of the 33kD binding on the properties and the reactions catalysed by the Dl/D2/cytochrome b-559 complex is also reported. 

A large proportion of the phase 1 metabolism of drug molecules occurring in the liver appears to be dependent upon a group of mixed function oxidases found in liver microsomes. These enzymes require a supply of reduced NADP and molecular oxygen and contain a haemoprotein, cytochrome P-450. The reactions catalysed together with examples are given in the final section. 

D-Glucose Oxidases.—The radical produced aerobically in the reaction catalysed by D-glucose oxidase reduced oxidized Cytochrome C, and additional evidence was presented for the formation of hydrogen peroxide in the enzyme-catalysed reaction. ... 

In its reduced form, cytochrome P-4S0 combines with carbon monoxide to form a complex having an absorption maximum at 430 mn (Fig. 4) For this reason, the oxidative reactions catalysed by cytochrome P-430 are inhibited by carbon monoxide. Since monochromatic light of 430 nm causes photodissociation of the cytochrome P-430-CO complex, however, strong light of this wave-length partially reverses the inhibition of the oxidative reactions by carbon monoxide. In fact for... 

FIGURE 11.12 Role of NADPH in hydroxylation reactions catalysed by cytochrome P-450 enzymes... 

The determination of lactate is important in medicine, because certain disorders are related to lactic acidosis, and in industry, for the control of milk and other food products. A lactate sensor can be constructed by coupling amperometric transducers with various enzymes, such as lactate dehydrogenase, cytochrome b2, lactate oxidase and lactate monooxygenase. Lactate dehydrogenase requires NAD as a cofactor, whereas cytochrome b2 uses potassium ferricyanide, which is much cheaper [173]. Cytochrome b2 catalyses the following reaction ... 

Isoenzymes Structurally related enzyme proteins that catalyse very similar or identical reactions (e.g., monoamine oxidase A and B, cytochrome P-450). 

Cytochrome c oxidase is a copper protein, which, in the respiratory electron-transfer chain of mitochondria and many bacteria, catalyses the reduction of molecular oxygen to water, according to the reaction ... 

Figure 6.1 Pathways involved in glucose oxidation by plant cells (a) glycolysis, (b) Krebs cycle, (c) mitochondrial cytochrome chain. Under anoxic conditions. Reactions 1, 2 and 3 of glycolysis are catalysed by lactate dehydrogenase, pyruvate decarboxylase and alcohol dehydrogenase, respectively. ATP and ADP, adenosine tri- and diphosphate NAD and NADHa, oxidized and reduced forms of nicotinamide adenine dinucleotide PGA, phosphoglyceraldehyde PEP, phosphoenolpyruvate Acetyl-CoA, acetyl coenzyme A FP, flavoprotein cyt, cytochrome e, electron. (Modified from Fitter and Hay, 2002). Reprinted with permission from Elsevier...

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