Secondary biotransformation

One of the most intriguing facets associated with the study of xenobiotic metabolism is the occasional discovery of metabolites whose pathways of formation defy logical explanation. One of the least intriguing is the report by a colleague that a major metabolite was overlooked in one s own study. Each of these situations may arise from what might be termed "hidden" metabolites. These hidden metabolites are common to both primary and secondary biotransformation processes, but this paper deals solely with the latter where the metabolite is referred to as a hidden conjugate. 

Biotechnological processes may be divided into fermentation processes and biotransformations. In a fermentation process, products are formed from components in the fermentation broth, as primary or secondary metabolites, by microorganisms or higher cells. Product examples are amino acids, vitamins, or antibiotics such as penicillin or cephalosporin. In these cases, co-solvents are sometimes used for in situ product extraction. 

In this chapter, by using the examples of -lactams we have briefly examined how microbial cultures may be used to produce sufficient antibiotics to meet market demands. We have also explained how enzymes (or cells) may be used to biotransform, and thereby diversify, antibiotics. By outlining the history of penicillin production, we explained how analysis and manipulation of culture regimes may be used to enhance the yields of antibiotics (and other secondary products). These studies led to die concept of directed biosynthesis by precursor feeding. 

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

The biotransformation of clofexamide (4.33, Fig. 4.4), a compound with anti-inflammatory and antidepressant activities, was investigated in rats [18]. About 15% of the dose administered was found in urine as 2-(4-chlorophe-noxy)acetic acid (4.37). This metabolite was formed via the secondary amine 4.34, the primary amine 4.35, and the acid 4.36 resulting from oxidative deamination. However, direct formation of 2-(4-chlorophenoxy)acetic acid (4.37) from the parent compound (4.33) cannot be excluded. Clofexamide and its metabolite 4.34 also underwent hydroxylation on the aromatic ring, but these hydroxylated metabolites did not appear to be hydrolyzed. 

The cyclic metabolite 11.169 was also a substrate in further biotransformations, being (V-demethylated to the corresponding endocyclic imine, and oxidized to phenolic metabolites. Very little if any of the secondary amine metabolite (11.168) appeared to undergo direct (V-demethylation to the primary amine, in contrast to many other tertiary amines, presumably due to very rapid cyclization of the secondary amine facilitated by steric and electronic factors. The possibility for the iminium cation (11.169 H+) to become deprotonated (a reaction impossible for the iminium 11.166 in Fig. 11.20) should also drive the cyclization reaction. 

The latter part of the book is dedicated to redox biotransformation application, with Chapter 9 disclosing several methods for the synthesis of chiral secondary alcohols using a range of commercially available ketoreductases (alcohol dehydrogenases) which are now being applied regularly on a large scale. 

Herbivores biotransforms xenobiotics from natural or artificial sources to render them harmless. Disarming potentially toxic compounds is termed detoxication, while the term detoxification refers to correcting a state of toxicosis (Johns, 1990). Herbivores degrade secondary plant compounds in a variety of ways, starting in the mouth, and leading to excretion. 

Foley, W. J., McLean, S., and Cork, S. J. (1995). Consequences of biotransformation of plant secondary metabolites on acid-base metabolism in mammals a final common pathway Journal of Chemical Ecology 21,721-743. 

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... 

Kurz, W. G. W. and Constabel, F. 1985. Aspects affecting biosynthesis and biotransformation of secondary metabolities in plant cell cultures. CRC Critical Reviews in Biotechnology, 2(2) 105-118. 

The most significant metabolite of letrozole (3) is its secondary alcohol metabolite (SAM) 23 (Scheme 3.4). Biotransformation of letrozole is the main elimination mechanism, with the glucuronide conjugate of the secondary alcohol metabolite (24) being the prominent species found in urine. However, the total body clearance of letrozole is slow (2.21 L/h). Its elimination half-life is long, at 42 h. Letrozole and its metabolites are excreted mainly via the kidneys. 

Much attention has been paid to the last step of the formation of monoter-penes and sesquiterpenes, which is catalysed by terpenoid synthases. Over 30 complementary DNAs (cDNAs) encoding plant terpenoid synthases involved in the primary and secondary metabolism have been cloned, characterised, and the proteins heterologously expressed [6]. However, because geranyl diphosphate and farnesyl diphosphate are not readily available substrates, their biotransformation by terpenoid synthases is not economically viable. As a result, considerable effort has been put into engineering the total plant terpenoid biosynthetic pathway in recombinant microorganisms. 

Diseases that directly affect hepatic integrity include cirrhosis, viral infections, and collagen vascular diseases. Diseases that indirectly affect function include metabolic disorders (e.g., azotemia secondary to renal insufficiency) and cardiac disease. Although decreased left ventricular output can result in a decrease in hepatic arterial flow, right ventricular failure causes hepatic congestion, reducing the first-pass effect and delaying biotransformation. 

Oxidative reactions at carbon predominate in the biotransformation of cyclic amiiies, and an important consequence of this is often the cleavage of the carbon-nitrogen bond. For example, A-dealkylation of N- alkyl substituted pyrrolidine (or piperidine, morpholine, etc.) involves an initial oxidative attack at the a- alkyl carbon atom to yield an N hydroxyalkyl derivative (carbinolamine), which is then metabolized to a secondary amine and the corresponding aldehyde. The metabolic conversion of nicotine to nornicotine (30 see Scheme 3) probably involves this mechanism, although the iminium ion (31) has also been suggested as an intermediate in the biotransformation (76JMC1168). Carbinolamines are unstable intermediates and have been identified only in a few cases, e.g. A-hydroxymethylcarbazole... 

The biotransformation of (K)-(+)-pulegone was also studied by a Japanese group [116]. The major bioconversion metabolite of this substrate with Botrytis allii was (-)-(1 / )-8-hydroxy-4-p-menthen-3-one. The secondary major product from this biotransformation was isolated and its structure established as piperitenone [117]. It is interesting to note that the same group also investigated the bioconversion of piperitone, the dihydrogenation product of piperitenone a strain of Rhizoctonia solani was found able to hydroxylate the substrate preferentially at the 6-position [118,119]. 

Moreover, biotransformation of secondary arylalkylamines also affords nitrones. Thus, N-oxygenation of a series of 4-substituted iV-benzylanilines in liver microsomal preparations from various animal species has been detected to be a minor pathway of metabolism, usually generating a,7V-diphenylnitrones (34) in a species-dependent manner26. Nitrone formation was most abundant in liver, kidney and lung53. There was a clear sex difference... 

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