Oxidative biotransformations

Technical-grade endosulfan contains at least 94% a-endosulfan and (3-endosulfan. The a- and (3-isomers are present in the ratio of 7 3, respectively. The majority of the studies discussed below used technical-grade endosulfan. However, a few examined the effects of the pure a- and (3-isomers. Endosulfan sulfate is a reaction product found in technical-grade endosulfan as a result of oxidation, biotransformation, or photolysis. There is very little difference in toxicity between endosulfan and its metabolite, endosulfan sulfate. However, the a-isomer has been shown to be about three times as toxic as the P-isomer of endosulfan. 

In some cases it is more attractive to use whole microbial cells, rather than isolated enzymes, as biocatalysts. This is the case in many oxidative biotransformations where cofactor regeneration is required and/or the enzyme has low stability outside the cell. By performing the reaction as a fermentation, i.e. with growing microbial cells, the cofactor is continuously regenerated from the energy source, e.g. glucose. 

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. 

Recently, an oxidative biotransformation of secondary amines into nitrones applying cyclohexanone monooxygenase, an enzyme isolated from Acinetobacter... 

Although UGTs catalyze only glucuronic acid conjugation, CYPs catalyze a variety of oxidative reactions. Oxidative biotransformations include aromatic and side chain hydroxylation, N-, O-, S-dealkylation, N-oxidation, sulfoxidation, N-hydroxylation, deamination, dehalogenation and desulfation. The majority of these reactions require the formation of radical species this is usually the rate-determining step for the reactivity process [28]. Hence, reactivity contributions are computed for CYPs, but a different computation is performed with the UGT enzyme (as described in Section 12.4.2). 

Accordingly, very little is known so far about the metabolic fate of the parent ring system and of simple derivatives thereof. In experiments using rats, it has been found that pyridazine undergoes extensive oxidative biotransformation to afford monohydroxy derivatives along with 4,5-dihydroxy- and 4,5-dihydro-4,5-dihydroxypyridazine. 3-Methylpyridazine was found to be metabolized under these conditions in a similar manner yielding additionally 3-hydroxymethylpyridazine [442]. 

Oxidative biotransformation provides a commonly reported means of removing alkyl groups from substituted oxygen and nitrogen atoms. These reactions are believed to proceed via monooxygenase-mediated a-hydroxylation of the alkyl group to an unstable hemiacetal or hemiaminal, as outlined in Fig. 4 for an 0-alkyl substituted compound. 

The second type of oxidative biotransformation comprises dealkylations. In the case of primary or secondary amines, dealkylation of an alkyl group starts at the carbon adjacent to the nitrogen in the case of tertiary amines, with hydroxylation of the nitrogen (e.g lidocaine). The intermediary products are labile and break up into the dealkylated amine and aldehyde of the alkyl group removed. 0-dealkylation... 

Acute pyridine treatment (single intraperitoneal dose of 200 mg/kg bw) increased the metabolism of 2-butanol twofold in Sprague-Dawley rat liver microsomes and threefold in rabbit (New Zealand White) liver microsomes (Page Carlson, 1993). In liver microsomes from pyridine-treated (one intraperitoneal injection of 100 mg/kg bw, daily for four days) male Sprague-Dawley rats, increased oxidative biotransformation of the chlorofluorocarbon l,2-dichloro-l,l,2-trifluoroethane was found the day after the last injection (Dekant et al, 1995). 

Most psychotropics undergo extensive oxidative biotransformation leading to the formation of more polar metabolites, which are then excreted in the urine. The necessary biotransformation steps may involve one or several of the following steps ... 

Oxidative biotransformation results in the formation of metabolites whose pharmacological effects may be similar or dissimilar to the parent compound. Either way, active metabolites contribute to the final overall clinical effects. For example, norfluoxetine has essentially the same activity as fluoxetine in terms of both serotonin uptake blockade and inhibition of several CYP enzymes, but is cleared more slowly (11, 36, 37). As a result, norfluoxetine accumulates extensively in the body following chronic administration of fluoxetine, making it, rather than the parent compound, the principal determinant of the clinical effect. 

Drinking on a regular basis for several weeks to months can induce CYP enzymes, resulting in a lower TCA plasma concentration. Thus, subacute and subchronic alcohol consumption induces liver enzymes and causes lower plasma levels of drugs that undergo oxidative biotransformation as a necessary step in their elimination. 

Among the most popular oxidative biotransformations, Baeyer-Villiger monooxygenases (BVMOs) belong to the main fields of research. Nowadays, manifold enzymes catalyzing the Baeyer-Villiger oxidation are expressed in common recombinant organisms, such as E. coli or S. cerevisiae. The mechanism of the enzymatic... 

Li Z, van Beilen JB et al (2002) Oxidative biotransformations using oxygenases. Curr Opin Chem Biol 6 136-144... 

Alkaloids of this group are susceptible to oxidative biotransformations by many microorganisms resulting in N-demethylation, C-hydroxylation, or ring-closure reactions (11). Many of the observed biotransformations parallel or are closely related to processes thought to occur in the normal biosynthesis of the ergot alkaloids and may indeed involve the same or similar enzyme systems to those responsible for the normal production of the alkaloids themselves (11, 64, 65). 

For most drugs, oxidative biotransformation is performed primarily by the mixed-function oxidase enzyme system, which is present predominantly in the smooth endoplasmic reticulum of the liver. This system comprises (1) the enzyme NADPH cytochrome P450 reductase (2) cytochrome P450, a family of heme-containing proteins that catalyze a variety of oxidative and reductive reactions and (3) a phospholipid bilayer that facilitates interaction between the two proteins. Important exceptions to this rule are ethyl alcohol and caffeine, which are oxidatively metabolized by enzymes primarily present in the soluble, cytosolic fraction of the liver. 

At this time I was interested in the natural tolerance of houseflies to structural analogues of dieldrin and, with Harrison, I soon showed that whereas tolerance to cyclodienes was often related to oxidative detoxication and could be reduced or eliminated by benzodioxole synergists, dieldrin-resistance in houseflies did not respond to synergism and was apparently not a consequence of oxidative detoxication (33) Several laboratories (for their subsequent reviews see 34-36) confirmed the importance of oxidative biotransformations in insects and in 1964-5, at Slough, J. W Ray showed that microsomal preparations from houseflies and other insects contained cytochrome P450 (37) Thus, the links between insect and mammalian biochemical pharmacology were finally and firmly established. 

Glimepiride is metabolized in vivo through oxidative biotransformation and producing two major metabolites, cyclohexyl hydroxy methyl derivative (Ml) and the carboxyl derivative (M2). Ml is the active metabolite... 

In some cases. Phase I metabolites may not be detected, owing to their instability or high chemical reactivity. The latter type are often electrophilic substances, called reactive intermediates, which frequently react non-enzymically as well as enzymically with conjugating nucleophiles to produce a Phase II metabolite. A common example of this type is the oxidative biotransformation of an aromatic ring and conjugation of the resulting arene oxide (epoxide) with the tripeptide glutathione. Detection of metabolites derived from this pathway often points to the formation of precursor reactive electrophilic Phase I metabolites, whose existence is nonetheless only inferred. 

Of the various pha.se I reactions that are considered in this chapter, oxidative biotransformation processes are, by far. the most common and important in drug metabolism. The general stoichiometry that describes the oxidation of many xenobiutics (R-H) to their corresponding oxidized metabolites (R-OH) is given by the following equation ... 

Species variation has been observed in many oxidative biotransformation reactions. For example, metabolism of amphetamine occurs by two main pathways oxidative deamination or aromatic hydroxylation. In the human, rabbit, and guinea pig. oxidative deamination appears to be the predominant pathway in the rat. aromatic hydroxylation appears to be the more important route. Phenytoin is another drug that shows markeii species differences in metabolism. In the human, phenytoin undergoes aromatic oxidation to yield primarily (5K-)-/r-hydioxyphenytoin in the dog. oxidation occurs to give mainly (If)(-1-)-iM-hydroxyphenyt-oin. There is a dramatic difference not only in the pasition (i.e.. meta or para) of aromatic hydroxylation but also in which of the two phenyl rings (at C-S of phenytoin) undergoes aromatic oxidation. 

Figure 7 Oxidative biotransformation of /V- 3,5-dichlorophenyl) succinimide (NDPS).

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