Biotransformation carboxylesterases

The metabolism of foreign compounds (xenobiotics) often takes place in two consecutive reactions, classically referred to as phases one and two. Phase I is a functionalization of the lipophilic compound that can be used to attach a conjugate in Phase II. The conjugated product is usually sufficiently water-soluble to be excretable into the urine. The most important biotransformations of Phase I are aromatic and aliphatic hydroxylations catalyzed by cytochromes P450. Other Phase I enzymes are for example epoxide hydrolases or carboxylesterases. Typical Phase II enzymes are UDP-glucuronosyltrans-ferases, sulfotransferases, N-acetyltransferases and methyltransferases e.g. thiopurin S-methyltransferase. 

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. 

Ubiquitous glycosylated carboxylesterases (CarbE, EC 3.1.1.1), formerly named ali-esterases, are B-esterases belonging to the multigene enzyme superfamily of a/P hydrolases (Hosokawa and Satoh, 2006 Satoh and Hosokawa, 2006). In principle this class of isozymes plays a major role in pharmacokinetics by hydrolytic biotransformation of exogenous ester-drugs and ester-prodrugs. However, their physiological fimction still remains unclear (Satoh and Hosokawa, 2006). 

The induction of carboxylesterase activity has also been observed in animals exposed to PAHs (Nousiainen et al. 1984). Benzo[a]pyrene, benz[a]anthracene, and chrysene were moderate inducers of hepatic carboxylesterase activity in rats that were intragastrically administered 50,100, and 150 mg/kg/day (100 mg/kg/day for chrysene), respectively, for 4 days. However, rats administered 100 mg/kg/day anthracene or phenanthrene did not exhibit induction of hepatic carboxylesterase activity. Induction of hepatic microsomal enzymes generally results in enhanced biotransformation of other xenobiotics (to either more or less toxic forms). 

Ester and amide hydrolytic reactions are mediated principally by carboxylesterases (GES), though other esterases play a role in the hydrolysis of a limited number of esters. Other key biotransformations include oxidations and conjugation reactions. 

Figure 19.9 Metabolic possibilities for model compounds having representative functionality. Selected phase 1 reactions (1) Hydrolysis of various types of esters, in this case mediated by a carboxylesterase (2) N-dealkylation mediated by certain of the Cytochrome P-450 (CYP) enzymes (3) O-dealkylation mediated by certain of the CYPs and (4) Aromatic hydroxylation also mediated by certain of the CYPs. Depending upon the subtleties of their electronic and steric environments, the relative competitive biotransformation rates for these processes will generally be (1) (2) > (3) (4). Selected phase 2 reactions (5) Formation of a glucuronic acid conjugate (or in some cases a sulfate conjugate) and (6) N-acetylation. In terms of relative biotransformation rates in general (5) >> (6).

The enzyme responsible for the biotransformation of capecitabine to 5 -deoxy-5-fluorocytidine (a precursor to 5-fluorouracil) was evaluated using purified enzyme, cytosol, and microsomes. The purified CES cytosolic enzyme, inhibited by the carboxylesterase inhibitors bis-nitrophenyphosphate and diisopropylfluorophosphate, was identified as belonging to the subgroup CES lAl based on the result of the N-terminal amino acid sequence. 

Linalyl acetate can be found in many plants, however, it is in the highest concentration in the essential oil of Citrus aurantium spp. aurantium (Wichtel, 2002). As an ester, linalyl acetate is hydrolyzed in vivo by carboxylesterases or esterases to linalool (Figure 8.14), which is then further metabolized to numerous oxidized biotransformation products (see metabolism of linalool) (Bickers et al., 2003). 

Using the parent compound depletion method, pyrethroid metabolic rate constants (i.e., Umax and K, hast, etc.) for hydroxylation by cytochrome P450 enzymes or hydrolysis by carboxylesterases were developed by Scollon et al. (2009). The sources of the enzymes were rat and human microsomes. The pyrethroids they studied included bifenthrin, S-bioallethrin, bioresmethrin, p-cyfluthrin, cypermethrin, cis-permethrin, and frans-permethrin. The depletion method considers multiple hydroxylations as a single biotransformation at sites on either the acid or alcohol moieties, or on a combination of both. The metabolic pathways (Tables D1-D15 and E1-E15 of Appendices D and E, respectively) require Umax, Am, and values for the individual hydroxylated and hydrolyzed products. It is interesting that only bioresmethrin and cypermethrin per se were found to actually be hydrolyzed. 

This information was used to cmistruct a PBPK/PD model in the adult male rat (MirfazaeUan et al. 2006). Godin et al. (2006) examined species differences between rat and human liver microsomal carboxylesterases. A significant species difference was noted in the in vitro biotransformation of deltamethrin, due in part to differences in the rate of hydrolysis by human liver microsomes. Godin et al. (2007) identified the rat and human CYP450 isoforms, and rat serum esterases that metabolize deltamethrin and esfenvalerate. Differences in the rates of hepatic oxidative metabolism were related to expression levels (abundance) of the individual P450 isoforms rather than their specific activity. 

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