Glucuronidation, biotransformation

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). 

In a study of the metabolism of methyl parathion in intact and subcellular fractions of isolated rat hepatocytes, a high performance liquid chromatography (HPLC) method has been developed that separates and quantitates methyl parathion and six of its hepatic biotransformation products (Anderson et al. 1992). The six biotransformation products identified are methyl paraoxon, desmethyl parathion, desmethyl paraoxon, 4-nitrophenol, />nitrophenyl glucuronide, and /wiitrophenyl sulfate. This method is not an EPA or other standardized method, and thus it has not been included in Table 7-1. 

FIGURE 2.14 Phase 2 biotransformation—conjugation. (1) Glucuronide formation. (2) Sulfate formation. (3) Glutathione conjugation. 

Microbial oxidation of drug substrates occurs in a similar fashion to mammalian oxidative biotransformation. In contrast, microbial cultures rarely catalyze conjugations comparable to those in mammalian system (glucuronidation, sulfation and GSH conjugation). It is thus not surprising that microbial bioreactors are mainly used in the synthesis of oxidative metabolites. 

The concept of microbial models of mammalian metabolism was elaborated by Smith and Rosazza for just such a purpose (27-32). In principle, this concept recognizes the fact that microorganisms catalyze the same types of metabolic reactions as do mammals (32), and they accomplish these by using essentially the same type of enzymes (29). Useful biotransformation reactions common to microbial and mammalian systems include all of the known Phase I and Phase II metabolic reactions implied, including aromatic hydroxylation (accompanied by the NIH shift), N- and O-dealkylations, and glucuronide and sulfate conjugations of phenol to name but a few (27-34). All of these reactions have value in studies with the alkaloids. 

Although the metabolism of several phthalate esters has been studied in vitro, essentially all of the in vivo studies have involved DEHP. A summary of these experiments which involved exposure offish to aqueous - C-DEHP is presented in Table IV (11,12). Tissue C was isolated and separated into parent and the various metabolites by preparative thin layer chromatography on silica gel. Metabolites were hydrolyzed where appropriate and identified by gas chromatography-mass spectroscopy. In whole catfish, whole fathead minnow and trout muscle, the major metabolite was the monoester while in trout bile the major metabolite was the monoester glucuronide. The fact that in all cases the major metabolite was monoester or monoester glucuronide despite the differences in species, exposure level and duration, etc. represented by these data, suggests that hydrolysis of DEHP to monoester is important in the biotransformation of DEHP by fish. 

The selective lampricide, 3-trif1uoromethy1-4-nitrophenol (TFM), is used to control the sea lamprey (Petromyzon marinus) in the Great Lakes (12, 13). Recent studies have shown that in rats TFM is primarily biotransformed to reduced TFM (14). The major metabolite (Figure 1) found in rainbow trout (Salmo gaivdnevi), however, was the glucuronide conjugate of TFM (IS, 16). 

D.W. Fowler, M.J. Eadie, and R.G. Dickinson. Transplacental transfer and biotransformation studies of valproic acid and its glucuronide(s) in the perfused human placenta. J Pharmacol Exp Ther. 249 318-323 (1989). 

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. 

The most significant metabolite of letrozole (3) is its secondary alcohol metabolite (SAM) 23 (Scheme 3.4). Biotransformation of letrozole is the main elimination mechanism, with the glucuronide conjugate of the secondary alcohol metabolite (24) being the prominent species found in urine. However, the total body clearance of letrozole is slow (2.21 L/h). Its elimination half-life is long, at 42 h. Letrozole and its metabolites are excreted mainly via the kidneys. 

Hepatic 0-dealkylation and glucuronide formation appear to be major pathways of biotransformation. Only about 10% of orally administered prazosin is excreted in the urine. Plasma levels of prazosin are increased in patients with renal failure the nature of this interaction is unknown. 

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