Microbial models of mammalian metabolism: uses and misuses (clarification of some misconceptions)
Introduction
Oxidative metabolism is the most commonly encountered metabolic clearance pathway for drugs in animals and man 1, 2. This metabolism is often carried out by a super-family of enzymes: the cytochrome P450s (CYPs) and many CYPs have now been gene sequenced for man (>40) and other species (>200) [3]. Such enzymes are not specific to animals, but are also found in plants and, more relevantly here, in microorganisms. Microorganisms have been extensively investigated as a tool to provide mechanistic insight or for their biocatalytic properties and there are many reports of drugs which are successfully metabolised by microorganisms: warfarin, propranolol, phenacetin, quinidine, carbamazepine, ibuprofen, phenytoin, diazepam, aminopyrene, theophilline, tamoxifen, sulindac, imipramine, etc. The use of microorganisms appeals as a potentially very useful tool in the design of novel drugs. However; two key questions must be asked: can useful and reliable predictions be made from microbial systems and can they be used as reliable biocatalytic reagents?
Section snippets
Metabolism by mammalian systems
The mammalian CYPs responsible for the metabolism of xeno- and endobiotics are a super-family of membrane bound, heme-containing enzymes. They are found in many organs in low concentrations but reside mainly in high levels in hepatic tissues and intestinal epithelium. A fundamental property of metabolic reactions carried out by CYPs is that they are all based on electrophilic attack (Table 1) [4].
For all CYPs, the different types of `oxidative metabolism' (hydroxylation, de-alkylation,
Applications of microbial CYPs
Since the early days of Smith and Rosazza [14]when the basic principles of microbial modelling of mammalian metabolism were set out, much research has been undertaken to characterise this approach. Most of this research has led to factual reporting of microbial biocatalysis results, and although their predictive capabilities are occasionally mentioned, it seems always to be as a generic or throw away comment usually in the introduction. There are many literature examples of microorganisms being
Are microbial biotransformations predictive of human drug metabolism?
At first sight, there appear to be a lot of commonality between all CYPs. But does this include reliable commonality of metabolic pathways of xenobiotics?
One of the most frequently reported substrate which is metabolised by both mammalian and microbial CYPs is warfarin. This drug has, therefore, been used to examine any overlap of metabolism patterns between mammalian and microbial CYPs.
The human metabolism of warfarin is summarised in Fig. 4, with relevant specificity of individual CYPs.
In
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