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A physiological role for flavin-containing monooxygenase (FMO3) in humans?

Perhaps more often than appreciated, an obvious answer to a problem may be overlooked during the desperate search for a perceived acceptable and probably more complicated solution. For more than half a century, the enzymes of drug metabolism and those of intermediary biochemistry have been thought of, and presented as, two discrete and independent groups rather than as part of a continuum. If a particular enzyme has been shown to entertain both exogenous and endogenous substrates, then this is an ‘unusual’ occurrence, an exception to the rule. Such a stance certainly becomes precarious as more exceptions surface, but the assumption is maintained nevertheless. However, during the past decade, the wisdom of this dichotomy, in the light of experience, has been questioned. It is now recognized that several enzymes—including aldehyde oxidase, dopamine-β-hydroxylase, lipoxygenase, monoamine oxidase, phenylalanine 4-hydroxylase, prostaglandin H-synthase, and xanthine oxidase—all initially and unmistakably established as involved in intermediary metabolism, may also catalyse the biotransformation of foreign compounds (Ioannides 2002; Mitchell & Steventon 2006). Contrariwise, the mono-oxygenases of ‘drug metabolism’ have important rôles within intermediary biochemistry. Mainly steroid structures, cholesterol, bile acids, retinoic acid, vitamin D3, and arachidonic acid derivatives have been proposed as endogenous substrates for the cytochrome P450 isozymes (Gervasini et al. 2004; Guengerich et al. 2005), whilst both biogenic amines (tyramine, phenylethylamine) (Cashman 2002a, 2002b) and sulphur compounds (cysteamine, homocysteine S-conjugates) (Elfarra 1995; Krueger et al. 2002) have been proffered as substrates for the flavin-containing monooxygenases (FMO). However, and in particular with the FMOs, although in vitro investigations demonstrate a catalytic activity with the above substances, there is a lack of evidence that the enzyme and substrate actually meet and interact within the body throwing uncertainty on their potential in vivo rôles. It has been suggested, based on its ability to catalyse the conversion of cysteamine to cystamine, that FMO may be involved in metabolic regulation via the reversible reduction and oxidation of protein thiols (Ziegler 1985) Xenobiotica Xenobiotica, 2010; 40(5): 301–305 2010