Hydrolysis, biotransformation reaction

Biotransformation reactions can be classified as phase 1 and phase 11. In phase 1 reactions, dmgs are converted to product by processes of functionalization, including oxidation, reduction, dealkylation, and hydrolysis. Phase 11 or synthetic reactions involve coupling the dmg or its polar metaboHte to endogenous substrates and include methylation, acetylation, and glucuronidation (Table 1). 

Hydrolysis of phthalate diesters to the respective monoesters appears to be the first and the major biotransformation reaction in all of these species, but subsequent oxidative metabolism also may occur. 

Hydrolysis of epoxides, esters, amides, and related structures is an important biotransformation reaction that limits the therapeutic activity of many drugs and generates therapeutically active drugs from prodmg structures. In a few cases, hydrolytic reactions can generate a toxic structure. Epoxide hydrolases and esterases are members of the a/(3 hydrolase-fold family of enzymes (Morisseau and Hammock, 2005 Satoh and Hosokawa, 2006). Although their substrate specificities are radically different (e.g., lipids, peptides, epoxides, esters, amides, haloalkanes), their catalytic mechanisms are similar. All of these enzymes have an active site catalytic triad composed of a nucleophilic serine or cysteine residue (esterases/amidases), or aspartate residue (epoxide hydrolases) to activate the substrate, and histidine residue and glutamate or aspartate residues that act cooperatively in an acid—base reaction to activate a water molecule for the hydrolytic step. 

Attempts have been made to apply the structure-activity concept (Hansch and Leo 1995) to environmental problems, and this has been successfully applied to the rates of hydrolysis of carbamate pesticides (Wolfe et al. 1978), and of esters of chlorinated carboxylic acids (Paris et al. 1984). This has been extended to correlating rates of biotransformation with the structure of the substrates and has been illustrated with a number of single-stage reactions. Clearly, this approach can be refined with the increased understanding of the structure and function of the relevant degradative enzymes. Some examples illustrate the application of this procedure ... 

In some cases enzymes can increase the rate of reaction by up to lO times. Carnell and Roberts (1997) have briefly discussed the scope of biotransformations that are used to make pharmaceuticals like penicillins, cephalosporines, erythromycin, lovastatin, cyclosporin, etc., and for food additives like citric acid, L-glutamate, and L-lysine. A very successful transformation by Zeneca has been that of benzene reduction, with Pseudomonase Putida, to dihydrocatechol and catechol the dihydro derivative is used to produce (+/-) pinitol. Fluorobenzene has been converted to fluorodihydrocatechol, an intermediate for pharmaceuticals. The highly stereo selective Bayer-Villeger reaction has been carried out with genetically engineered S-cerevisvae. Hydrolases have allowed enantioselective, and in some cases regioselective, hydrolysis of racemic esters. 

Despite the diverse range of documented enzyme-catalyzed reactions, there are only certain types of transformations that have thus far emerged as synthetically useful. These reactions are the hydrolysis of esters, reduction/oxidation reactions, and the formation of carbon-carbon bonds. The first part of this chapter gives a brief overview by describing some examples of various biotransformations that can easily be handled and accessed by synthetic organic chemists. These processes are now attracting more and more attention from nonspecialists of enzymes. 

See also Biotransformations Microbial oxidations Microbial reductions applications of, 76 396-399 biocatalyst selection in, 76 404-409 biocatalysts in, 76 409-414 for drug metabolite production, 76 398-399 further advances in, 76 414 in hydrolysis, 76 400-401 multiphase reactions in, 777 412-414 scale-up of, 76 414 systematic studies of, 76 398 technique overview for, 76 403-414 timing of substrate additions in, 76 411-412 uses for, 777 400-403 Microbial waxes, 26 203 Microbiocides, triorganotins as, 24 817 Microbiological culture media, agar in, 73 68... 

Another therapeutic class to be briefly discussed is that of the lipid-lowering agents known as fibrates, e.g., clofibrate and fenofibrate (8.5). Here also, the acidic metabolite is the active form clofibrate (an ethyl ester) is rapidly hydrolyzed to clofibric acid by liver carboxylesterases and blood esterases [11], Human metabolic studies of fenofibrate (8.5), the isopropyl ester of fenofibric acid, showed incomplete absorption after oral administration, while hydrolysis of the absorbed fraction was quantitative [12], This was followed by other reactions of biotransformation, mainly glucuronidation of the carboxylic acid group. 

In connection with our own work on the enzyme-catalysed hydrolysis of cyclohexene epoxide with various fungi we made the unexpected observation that the microorganism Corynesporia casssiicola DSM 62475 was able to interconvert the (1R,2R) and (1S,2S) enantiomers of the product, trans cyclohexan-1,2-dioI 25. As the reaction proceeded the (1R,2R) enantiomer was converted to the (1S,2S) enantiomer [20]. If the racemic trans diol 25 was incubated with the growing fungus over 5 days, optically pure (> 99 % e. e.) (1 S,2S) diol 25 could be isolated in 85% yield. Similarly biotransformation of cis (meso) cycIohexan-1,2-diol 26 yielded the (1S,2S) diol 25 in 41 % (unoptimized) yield (Scheme 11). 

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