Hydrolytic Metabolism

The presence of fluorine atoms protects a molecule not only from oxidative metabolism but also from proteolysis by disfavoring the formation of cationic intermediates involved in the proteolysis process. This is particularly important for 

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Pure human CESs (hCEl and hCE2), a rabbit CES (rCE), and two rat CESs (Hydrolases A and B) were used to study the hydrolytic metabolism of the following pyrethroids lR-trans-resmethrin (bioresmethrin), 1RS-1runs-permethrin, and 1RS-r/.v-pennethrin [28], hCEl and hCE2 hydrolyzed /ram-pemiethrin 8- and 28-fold more efficiently than m-permethrin (when kcat/Km values were compared), respectively. In contrast, hydrolysis of bioresmethrin was catalyzed efficiently by hCEl, but not by hCE2. The kinetic parameters for the pure rat and rabbit CESs were qualitatively similar to the human CESs when hydrolysis rates of the investigated pyrethroids were evaluated. Further, a comparison of pyrethroid hydrolysis by hepatic microsomes from rats, mice, and humans indicated that the rates for each compound were similar between species. 

NADPH-independent hydrolytic metabolism. The CLint for deltamethrin was estimated to be twice as rapid in humans as in rats on a per kg body weight basis. Metabolism by purified rat and human CESs was used to examine further the species differences in hydrolysis of deltamethrin and esfenvalerate. Results of CES metabolism revealed that hCEl was markedly more active toward deltamethrin than the Class I rat CESs, hydrolase A and B, and the Class II human CES, hCE2 however, hydrolase A metabolized esfenvalerate twice as fast as hCEl, whereas hydrolase B and hCEl hydrolyzed esfenvalerate at equal rates. These studies demonstrated a significant species difference in the in vitro pathways of biotransformation of deltamethrin in rat and human liver microsomes, which was due in part to differences in the intrinsic activities of rat and human CESs. 

Ross MK, Borazjani A, Edwards CC, Potter PM (2006) Hydrolytic metabolism of pyrethroids by human and other mammalian carboxylesterases. Biochem Pharmacol 71 657-669... 

Godin SJ, Scollon EJ, Hughes MF, Potter PM, DeVito MJ, Ross MK (2006) Species differences in the in vitro metabolism of deltamethrin and esfenvalerate differential oxidative and hydrolytic metabolism by humans and rats. Drug Metab Dispos 34 1764-1771... 

Given that hydroxylamine reacts rapidly with heme proteins and other oxidants to produce NO [53], the hydrolysis of hydroxyurea to hydroxylamine also provides an alternative mechanism of NO formation from hydroxyurea, potentially compatible with the observed clinical increases in NO metabolites during hydroxyurea therapy. Incubation of hydroxyurea with human blood in the presence of urease results in the formation of HbNO [122]. This reaction also produces metHb and the NO metabolites nitrite and nitrate and time course studies show that the HbNO forms quickly and reaches a peak after 15 min [122]. Consistent with earlier reports, the incubation ofhy-droxyurea (10 mM) and blood in the absence of urease or with heat-denatured urease fails to produce HbNO over 2 h and suggests that HbNO formation occurs through the reactions of hemoglobin and hydroxylamine, formed by the urease-mediated hydrolysis of hydroxyurea [122]. Significantly, these results confirm that the kinetics of HbNO formation from the direct reactions of hydroxyurea with any blood component occur too slowly to account for the observed in vivo increase in HbNO and focus future work on the hydrolytic metabolism of hydroxyurea. 

Fig. 7.8. The hydrolytic metabolism of cocaine (7.57) to form benzoylecgonine (7.58), ecgo-nine methyl ester (7.59), and ecgonine (7.60). In the presence of ethanol, benzoylecgonine ethylester (7.61, cocaethylene) is also formed enzymatically as discussed in the text.

The presence of fluorine strongly destabihzes a carbocation centered on the jS carbon because only the inductive effect takes place. " The effect on solvolysis or protonation reaction of double bonds can be very important. The destabilization of carbenium and alkoxycarbenium ions plays an importantrole in the design of enzyme inhibitors (cf Chapter 7) and in the hydrolytic metabolism of active molecules (cf. Chapter 3). 

Methylene penems constitute good /3-lactamase inhibitors as their hydrolytic metabolism leads to the formation of 1,4-thiazepines, strengthening the thus-formed acyl-enzyme complex link (see Sections 2.03.6.9 and 2.03.12.4). However, compared to penams, penem sulfones are not as good /3-lactamase inhibitors as their half-lives of hydrolysis are too short, making them labile under physiological conditions <1997BML2217>. 

Different kinds of hydrolytic metabolism are relevant to the deactivation of, e. g. thromboxane A2 [55] (Scheme 4.24), prostacyclin [56[ (Scheme 4.25), and many nucleoside-based pharmaceuticals [57[- The first step of the metabolic deactivation of cortisol is oxidation of the lip hydroxy group to a carbonyl function, presumably by a reaction mechanism involving hydride transfer [58[ (Scheme 4.26). 

Recently Tsu Hui et al. (1976) investigated the metabolism of chlortoluron, fluometuron and metobromuron in human embryonal lung cell culture. More than 9.5% of the original compounds could be recovered. Oxidative metabolism predominated over hydrolytic metabolism. 

An example of inhibition of phase I hydrolytic metabolism, is the inhibition of epoxide hydrolase by valpromide, which increases the levels of carbamazepine , (p.537). Phase II conjugative metabolism can also be inhibited. Examples are the inhibition of carbamazepine glucuronidation by... 

No synergism in vivo (SRTD) was revealed with either piperonyl butoxide (PBO) or DEF-6 (looking for oxidative or hydrolytic metabolism)... 

Acid hydrolase-membrane interaction and maintenance of optimal microenvironment in lysosome-vacuolar apparatus, for hydrolytic metabolism... 

Ross et al. (2006) studied the hydrolytic metabolism of Type 1 pyrethroids (bioresmethrin, IRS fraws-permethrin, and IRS c/s-permethrin) and several Type II pyrethroids (alpha-cypermethrin and deltamethrin) by pure human CEs (hCE-1 and hCE-2), a rabbit CE (rCE), and two rat CEs (Hydrolases A and B). Hydrolysis rates were based on the formation of 3-phenoxybenzyl alcohol (PBAlc) (CAS no. 13826-36-2) for the cis and trans isomers of permethrin. For bioresmethrin, hydrolysis was based on the production of the trans-chrysanthemic acid (CPCA) (CAS no. 10453-89-1). For alpha-cypermethrin and deltamethrin, hydrolysis was based on the formation of c/s-permethrinic acid (DCCA) (CAS no. 57112-16-0) and 3-phenoxybenzyl aldehyde (PBAld CAS no. 39515-51-0), respectively. Human CE-1 and hCE-2 hydrolyzed trans-permethrin 8- and 28-fold more efficiently (based on kcat/Km values) than did c/s-permethrin, respectively. The kinetic parameters (Fmax> for the hydrolysis of trans- and c/s-permethrin, bioresmethrin and alpha-cypermethrin by rat, mouse, and human hepatic microsomes are given in Table 7. The trans- isomer of permethrin is more readily hydrolyzed by rat, mouse and human hepatic microsomal carboxylesterase than c/s-permethrin (13.4, 85.5 and 56.0 times, respectively). However, the lower for hydrolysis of cis-permethrin in human microsomes suggests that there are both stereoisomer and species-specific differences in metabolism kinetics. 

The relative levels of hCE-1 protein were measured in human microsomes by immunoblotting to determine if variation in hydrolytic metabolism was related to the abundance of hCE-1 in the microsomes. Human CE-1 levels did not correlate (r = 0.294) well with F ,ax values for the hydrolysis of Irans-permethrin. Esterases other than hCE-1 are believed to be responsible for the different values. CDMB (1,2-ethanedione, l-(2-chlorophenyl)-2-(3,4-dimethoxyphenyl)-(CAS no. 56159-70-7)) was used to inhibit the activity of hCE-2 (Wadkins et al. 2005). 

The results suggested that a significant portion of the microsomal activity was catalyzed by hCE-2. An antibody was not available for detecting or measuring hCE-2. Turnover numbers ( cat/ m> mM min ) indicate that trani-permethrin is as efficiently hydrolyzed by hCE-1 as it is for hCE-2. Bioresmethrin is not hydrolyzed by hCE-2 suggesting that metabolism is primarily mediated by hCE-1. The rates of hydrolytic metabolism by rabbit carboxylesterase followed the order bioresmethrin > frans-permethrin > cA-permethrin > deltamethrin > alpha-cypermethrin. Ross et al. (2006) did not extrapolate hydrolytic in vitro V ,ax values (nmol min mg ofmicrosomalprotein)intoin vivo values (p,mol h kg of body weight (bwt)) for use in PBPK models. 

TEPP (CAS no. 107-49-3) (200 pM) was used to inhibit hydrolytic metabolism Source Table 2, Godin et al. 2006. Published with permission... 

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