Enzyme-mediated hydrolysis

Resistance. Resistance to the cephalosporins may result from the alteration of target pencillin-binding sites (PBPs), decreased permeabdity of the bacterial ced wad and outer membrane, or by inactivation via enzyme mediated hydrolysis of the lactam ring (80,81,138—140). This resistance can be either natural or acquired. Although resistance is often attributed speciftcady to one of these factors, in reaUty it reflects the interplay of several factors. In most instances, however, resistance results from the production of a P-lactamase enzyme, which opens the P-lactam ring as depicted in Figure 2. 

Figure 6.14 Enzymatic side chain cleavage of penicillins. 6-Aminopenicillanic acid, a valuable intermediate for the production of various semi-synthetic penicillins, can be obtained through enzyme-mediated hydrolysis of the phenylacety group of penicillin G or the phenoxyacetyl group of penicillin V. The active site of the enzyme recognises the aromatic side chain and the amide linkage, rather than the penidllin nucleus. Chemical entitles other than penicillins are therefore often good substrates, as long as they contain the aromatic acetamide moiety.

Enzyme mediated hydrolysis of racemic arenesulphinyl alkanoates 279 may also be considered as a method of kinetic resolution. Racemic sulphoxides 279 incubated in the presence of Carynebacterium equi IF 3730 was found to give recovered sulphoxide in optically active form with e.e. higher than 90%338. 

Enzyme-mediated hydrolysis of some racemic co-arenesulfinylalkanoic methyl esters, ArSO(CH2) COOMe, using Corynebacterium equi has led to a kinetic resolution in which the unreacted sulfinyl esters are enriched in one enantiomer at the sulfoxide center49. The enantiomeric purity of unreacted sulfinyl acetates and propionate ranges from 90 to 97%. 

J. P. Patel, A. J. Repta, Enol Esters as Potential Prodrugs. I. Stability and Enzyme-Mediated Hydrolysis of a-Acetoxystyrene , lnt. J. Pharm. 1980, J, 329-333. 

A rule, similar to Prelog s rule, has been proposed for the enzyme-mediated hydrolysis of the esters of secondary alcohols. Esters of the enantiomers 31 usually react faster. This rule correctly predicted the configuration of 14 out of 15 substrates when cholesterol esterase was used, 63 out of 64 substrates with a lipase from Pseudomonas cepacia, and of 51 out of 55 cyclic substrates using a lipase from Candida rugosa24°. 

Resistance. Resistance to the cephalosponns may result from the alteration of target penicillin-binding sites (PBPs), decreased permeability of tire bactenal cell wall and outer membrane, or by inactivation via enzyme-mediated hydrolysis of the lactam nng. This resistance can be either natural or acquired. 

The metabolism of nerve agents is much simpler than that of sulfur mustard. The major pathway for elimination is via enzyme-mediated hydrolysis by esterases, plus some chemical hydrolysis, as shown in Figure 10. In the case of the methylphosphonofluoridates and V agents, the major product is an alkyl methylphosphonic acid (alkyl MPA) (16). A small fraction of the nerve agent binds... 

Scientists at Shell began work onthe enantioselective hydrolysis of racemic amides in the early 1990s 5]. Enzyme mediated hydrolysis of racemic N 1 phenylethylace tamide 2 using whole cells of Arthrobacter sp. enabled the production of enantio merically pure (S) 1 and (f ) 2 (Figure 14.2). The use of whole cells was not optimal and long reaction times were required to obtain pure (R) 2, as the selectivity of the catalyst was not very high. 

In combination with chemical synthesis, glucansucrases (GTF-R from Streptococcus oralis and GTF-A from Lactobacillus reuteri) were used to produce thio-glucosides, which are tolerated by most biological systems but are less sensitive to acid/base or enzyme-mediated hydrolysis than G-glucosides [138]. 

The situation changes rapidly once lipoprotein lipase begins to attack the TG in the lipoprotein core. Enzyme-mediated hydrolysis of the TG leads to a marked decrease in particle volume, whereupon much of the lipoprotein surface becomes superfluous [85]. As a result, UC, PC, and apo C dissociate from the particle and become available for interaction with HDL [86]. Action of LCAT on the HDL then leads to the formation of CE. Some of this CE remains with the HDL, but most of it is transferred by CETP to apo B-containing lipoproteins and perhaps to cells (Fig. 2A). 

The absorption and transport of the majority of drugs across biological membranes occurs by passive diffusion, a process dependent upon physicochemical properties, i.e., lipophilicity, ionization, and molecular size. Since enantiomers have identical physicochemical properties, stereoselectivity would not be expected even though membrane phospholipids are chiral, the significance of lipophilieity appears to outweigh that of compound chirality. In contrast, differences between diastereoisomers may occur as a result of their differential solubility. However, in the case of compounds transported via earrier-mediated meehanisms, e.g., facilitated diffusion or active transport, proeesses involving a direct interaction between a substrate and a carrier maeromoleeule, stereoselectivity is expected. Preferential absorption of the l- eompared to the D-enantiomers of dopa [96] and methotrexate [97,98] have been reported. In the case of the above examples, enantioseleetivity in absorption is observed, whereas in the case of eephalexin, a eephalosporin antibiotic, diastereoselectivity for the L-epimer oeeurs. The L-epimer has shown a greater affinity than, and acted as a competitive inhibitor of o-eephalexin transport [99]. The L-epimer is also more suseeptible to enzyme-mediated hydrolysis, with the result that it cannot be detected in plasma [99]. 

The role of enzyme-mediated hydrolysis (depolymerization) of PL has been investigated (Schakenraad et al., 1990). This study concluded that the m or route of degradation of PL is most likely via simple (non-enzymatic) hydrolysis. However, the results of the study showed that the possibility of some enzyme-mediated hydrolysis could not be ruled out as a minor pathway. The conclusion that the process is not enzyme mediated is supported by another study which showed that the degradation rates measured in-vivo (sheep, dogs, and rats) were essentially the same as measured in-vitro (Leenslag et al, 1987). 

Further work on the preparation of chiral a-amino-acids reported in the past year (see also the section on asymmetric hydrogenation) includes an extension of the utility of anions derived from lactim ethers (228) in the synthesis of such compounds by condensations with aldehydes and ketones chiral inductions are somewhat lower than in the alkylations of (228) reported previously (4, 320). Enzyme-mediated hydrolysis of 5(4H)-oxazolones by chymotrypsin or subtilisin gives a-acylamino-acids with good enantiomeric enrichments, especially if the substrate carries bulky substituents. Schiff s bases of a-amino-esters can be enriched enantiomerically to an extent of up to 70% by sequential deprotonation with a chiral lithium amide and protonation with an optically pure tartaric acid. ... 

In a second approach, the improvement of solubility and metabolic stability based on heterocyclic aglycones was studied. Thereby, triazole (25a-c, entries 10, 12 13), pyrazole (28a,b, entries 15 17), and six-membered heterocyclic moieties (34a-d, entries 19, 21, 23 25 and 39, entry 27) proved highly beneficial to the aqueous solubility but in turn reduced lipophilicity and membrane permeability. Overall, only poor oral absorption could be observed. By contrast, the pyrrolylphenyl mannosides - optimized by the introduction of a chloro or trifluoromethyl substituent on ring A and a methyl group on the heterocycle (42f and 50, entries 37 41) - exhibited sufficient permeability as well as aqueous solubility. Furthermore, incubations with rat liver microsomes, revealed low propensity to enzyme-mediated hydrolysis (ti/2 > 120 min), supporting slow elimination due to expected renal re-absorption of the metabolically stable esters. 

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