Biotransformation processes hydroxylation

Achromobacter xylosoxydans has been used to cany out the selective hydroxylation in high yield, using the enzyme which catalyses the first step in nicotinic acid degradation. The whole-cell biotransformation process has been scaled-up to 12 m, which is sufficient to produce high purity 6-hydroxynicotinic acid for the subsequent chemical reactions. The hydroxylation is oxygen requiring, so that oxygen transfer rate-limits the reaction. 

Benzodiazepines undergo extensive and complex metabolism. They are excreted mainly in the urine, largely in the form of several metabolites. Biotransformation processes include mainly hydroxylation and A-dealkylation reactions, whereas the end-products include both free and conjugated compounds (116). Chlordiazepoxide, for example, is metabolized to oxazepam and other metabolites and, depending on its dosage, urine may contain significant concentrations of oxazepam (117). 

The mass shifts of some fragment peaks in the MS-MS spectra of KR-60436 and its metabolites are summarized in Table 10.4 [14], From the interpretation of the data, four biotransformation processes can be recognized, each related to a specific site in the stracture (see the structirre in Table 10.4) (A) hydroxylation (mass shift of +16 Da), (B) double-bond formation (-2 Da), (C) loss of the hydroxyethyl side chain (-44 Da), and (D) demethylation (-14 Da). Combination of these processes takes place as well, e.g., in M2, where both double-bond formation and hydroxylation takes place. From the MS-MS data, it can be concluded that hydroxylation at (A) blocks the fragmentation of the pyridine ring, i.e., no loss of 84 Da is observed, and that double-bond formation at (B) blocks the formation of the fragment at m/z 150. The identification of the metabolites was as follows Ml and M2 contained a hydroxy-group in (A). M2, M4, M6, and M7 contained a donble bond in (B). M3 and M4 were demethylated at D. In M5 and M7, the hydroxyethyl group was lost [14]. 

The biotransformations of 6)8-eudesmanolides functionalized at C(3), obtained from santonin, with Curvularia lunata and Rhizopus nigricans cultures have been also studied (Schemes 17 and 18) [27]. Rhizopus nigricans was more active in the biotransformation processes against these substrates. It is noteworthy that incubation of compound 109 with Rhizopus nigricans produced epimerization at C(4) and, in decreasing order, hydroxylation at C(8), C(l), or C(4). The authors attributed this epimerization to the participation of the hydroxyl group at C(3), and noticed that microbial functionalization at C(8) could provide access to the synthesis of 8,12-eudesmanolides. 

Other relevant metabolic pathways result in detoxified substances, such as biotransformation processes in the liver — conjugation wifli glycine, glucuronic acid and sulphuric acid (e.g., via hydroxylation of toluene) or biotransformation by hydrolysis, oxidation and conjugation (e.g., glycol ethers). 

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). 

Biotransformation The process by which a drug is converted to more polar substances (i.e., metabolites), which are then eliminated from the body either in the urine or in the stool (e.g., demethylated and hydroxylated metabolites of tricyclic antidepressants the three metabolites of bupropion). 

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). 

As mentioned above, the process of biotransformation of persistent CACs has been substantiated by the finding of metabolites in laboratory and wildlife species. In particular for PCBs, the presence of both the hydroxylated congeners in blood as well as the lipophilic methylsulfonyl-PCBs in adipose, liver and lung tissue of different species has proved that metabolism proceeds according to the mechanisms outlined above. Similarly, hydroxy metabolites of PCDFs have been found in rodents.73... 

In recent years biotransformations have also shown their potential when applied to nucleoside chemistry [7]. This chapter will give several examples that cover the different possibiUties using biocatalysts, especially lipases, in order to synthesize new nucleoside analogs. The chapter will demonstrate some applications of enzymatic acylations and alkoxycarbonylations for the synthesis of new analogs. The utQity of these biocatalytic reactions for selective transformations in nucleosides is noteworthy. In addition, some of these biocatalytic processes can be used not only for protection or activation of hydroxyl groups, but also for enzymatic resolution of racemic mixtures of nucleosides. Moreover, some possibilities with other biocatalysts that can modify bases, such as deaminases [8] or enzymes that catalyze the synthesis of new nucleoside analogs via transglycosylation [9] are also discussed. 

This first section wiU study different regjoselective processes of several types of nucleosides depending on the lipase used. Application of biotransformations over these compounds has acquired great importance in order to prepare new derivatives with interesting pharmacological activities. Two lipases, namely, Candida antarctica type B (CALB) and Pseudomonas cepacea, free (PSL) or Pseudomonas cepacea, immobilized (PSLC), are selective towards one of the two hydroxyl groups of different 2 -deoxynucleosides. Thus, it is possible to prepare the acylated compounds in S -position with CALB [10], whereas PSL is selective towards the secondary hydroxyl group [11]. Vinyl or oxime esters can be used as acyl donors. 

---