Cytochrome P-450 mono-oxygenase

Natnre does not actually make a methylenedioxy group using formaldehyde. Instead, it modifies an existing ortAo-hydroxy-methoxy arrangement. Enzymic hydroxylation of the methoxy methyl converts this substitnent into what is identical to a hemiacetal of formaldehyde, and then acetal formation follows in a process analogous to a chemical synthesis. The hydroxylating enzyme involved is a cytochrome P-450 mono-oxygenase (see Box 11.4). 

Neims AH, Warner M, Loughnan PM, et al. Developmental aspects of the hepatic cytochrome P-450 mono-oxygenase system. Annu Rev Pharmacol Toxicol 1976 16 427. 

MEIJER, A.H., CARDOSO, M.I.L., VOSKUILEN, J.T., DE WAAL, A., VERPOORTE, R., HOGE, J.H.C., Isolation and characterization of a cDNA clone from Catharanthus roseus encoding NADPH cytochrome P-450 reductase, an enzyme essential for reactions catalyzed by cytochrome P-450 mono-oxygenases in plants. Plant J., 1993,4,47-60. 

Matson, R. S., and Fulco, A. J. 1981. Hydroxystearates as Inhibitors of Palmitate Hydroxylation Catalyzed by the Cytochrome-P-450 Mono-Oxygenase from Bacillus megaterium. Biochem. Biophys. Res. Comm., 103, 531-535. 

Aminocyclopropane derivatives are known to possess enzyme inhibitory properties.In particular, aminocyclopropanes are potent inhibitors of cytochrome P-450 mono-oxygenases. A carbon-centered free radical, generated by one-electron oxidation of aminocyclopropane by P-450 to form a nitrogen-centered radical cation and subsequent ring opening, may play an important role in the destruction of the enzyme. 

Reaction mimic for the cytochrome P-450 mono-oxygenase/reductase system [Fe HMeCN)l+/O2/PhNHNHPh/substrate]... 

Table 6-4 summarizes the product yields from the combination of substrate [PhaP, Ph2S(O), PhCH2OH, or 1,4-cyclohexadiene] with O2, PhNHNHPh, and FelKMeCN) in dimethylformamide. 15 The system acts as an effective mono-oxygenase/dehydrogenase, and as such mimics the reaction cycle of the cytochrome P-450 mono-oxygenase/reductase system. 

Table 6-4 Mono-oxygenation and Dehydrogenation of Organic Substrates via a Model System for the Cytochrome P-450 Mono-oxygenase/Reductase Enzymes"...

The cytochromes P-450 mono-oxygenase system is actually a collection of isoenzymes all of which possess an iron protoporphyrin IX as the prosthetic group. The monomer of the enzyme has a molecular weight of 45 000-55 000. The enzyme is membrane bound within the endoplasmic reticulum. Cytochromes P-450 is closely associated with another vital component of the system, NADPH cytochrome P-450 reductase. This is a flavoprotein which has 1 mol of FAD and 1 mol of FMN per mol of apoprotein. The monomeric molecular weight of the enzyme is 78 000. The enzyme transfers two electrons to cytochromes P-450, but one at a time. There only seems to be one reductase which serves a group of isoenzymes of cytochromes P-450, and consequently its concentration is 1/10 to 1/30 that of cytochromes P-450. 

In conclusion, the pulmonary toxicity of 4-ipomeanol seems to be due to metabolic activation of the furan ring, probably by epoxidation, catalysed by the cytochromes P-450 mono-oxygenases found in the Clara cell. The reactive intermediate binds to protein and possibly other macromolecules, initiating cellular necrosis followed by oedema. 

The eicosanoids are locally active hormones (autocoids) that are derived from precursor polyunsaturated fatty acids. The rate-limiting step in the synthesis of eicosanoids is the phospholipase-regulated release of arachidonic add from membranes. Arachi-donic acid metabolism may follow one of three possible pathways. In the first, the cydooxygenase- peroxidase pathway leads to the formation of the prostenoids - prostaglandins and thromboxanes. In the second, the lipoxygenase pathway yields the leuko-trienes and lipoxins. A third pathway, the cytochrome P-450 mono oxygenase pathway is also involved in the metabolism of arachidonic add. 

Modulation of cytochrome P-450 mono-oxygenases and/or NADPH-dependent cytochrome P-450 reductases 538... 

Ohnishi, T. and Y. Ichikawa (1997). Direct inhibitions of the activities of steroidogenic cytochrome P-450 mono-oxygenase systems by anticonvulsants. J. Steroid Biochem. Mol. Biol. 60, 77-85. 

Although the 1 la-hydroxylation reaction became a commercial process, it is not an efficient one. It has a high selectivity, but a very low catalytic intensity. The fungus is first grown as if in a normal fermentation. The steroid, which is very poorly soluble in the aqueous fermentation medium, is added as a fine dispersion to a final concentration between 0.1% and 1% (w/v). The catalyst itself is a cytochrome P-450 mono-oxygenase attached to an intracellular membrane. It is an enzyme complex which also requires the co-factor NADPH to reduce one atom of molecular oxygen to water, while the other oxygen atom is incorporated into the steroid (Scheme 6.4). 

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