Stereospecific biotransformation

Stereospecific biotransformation has been an often neglected but interesting phenomenon with some toxicological importance. Xenobiotics having a chiral center are usually produced as a 1 1 mixture of the two stereoisomers. The biological activity, rate, and route of the biotransformation of them will be different. 

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Figure 2.3 An example of a stereospecific biotransformation resolution of bicyclic lactams. A and Ai are an enantiomer pair as are B and Bi.

This enantiomeric specificity has been of interest in other contexts, and stereospecific biotransformation has been observed. Examples include the enantiomeric oxidation of sulfides to sulfoxides (Chapter 11, Part 2) and steroid and triterpene hydroxylation (Chapter 7, Part 2). 

Stereospecific Biotransformation of (/ )-Linaiooi by Corynespora cassiicola DSM 62475 into Linalool Oxides... 

Figure 12.6 Stereospecific biotransformation of (R,S)-linalool by C. cassiicola via the (6S)-configured epoxylinalools as postulated intermediates...

Zegers BN, Mets A, Van Bommel R, Minkenberg C, Hamers T, Kamstra J, Pierce GJ, Boon JP (2005) Levels of HBCD in harbom porpoises and common dolphin from western European seas with evidence for stereospecific biotransformation by cytochrome P450. Environ Sci Technol 39 2095-2100... 

Figure 2.3 An example of a stereospecific biotransformation resolution of bicyclic lactams.

Stereospecific biotransformation is frequently observed. Bauveria sulfure-scens stereospecifically hydroxylated an azabrendane at the quaternary carbon atom (Archelas et al. 1988 Chapter 6, Section 6.1.2), while steroid and terpenoid hydroxylations are discussed in Chapter 6, Section 6.11.2. 

Kim M, Kim S-I, Han J, Wang X-L, Song D-G, Kim S-U (2009) Stereospecific biotransformation of (fibydrodaidzein into (3S)-equol by the human intestinal bacterium Eggerthella strain Julmig 732. Appl Environ Microbiol 75 3062-3068... 

The biotransformation of (/f,5)-Iinalool by fungi is a useful method for the preparation of natural linalool oxides. The stereospecific conversion of (J ,5)-linalool by Corynespora cassiicola DSM 62475 led to 5/f-configured furanoid linalool oxides and 55-configured pyranoid linalool oxides, both via bS -configured epoxylinalool as postulated intermediate (Figure 12.6). The biotransformation protocol affords an almost total conversion of the substrate with high enantioselectivities and a molar conversion yield close to 100% (Table 12.4). Pure linalool oxides are of interest for lavender notes in perfumery. ... 

More recently the biotransformation of limonene by another Pseudomonad strain, P. gladioli was reported [76,77]. P. gladioli was isolated by an enrichment culture technique from pine bark and sap using a mineral salts broth with limonene as the sole source of carbon. Fermentations were performed during 4-10 days in shake flasks at 25°C using a pH 6.5 mineral salts medium and 1.0% (+)-limonene. Major conversion products were identified as (+)-a-terpineol and (+)-perillic acid. This was the first time that the microbial conversion of limonene to (+)-a-terpineol was reported, see pathway 4. The conversion of limonene to a-terpineol was achieved with an enzyme, a-terpineol dehydratase (a TD), by the same group [78]. The enzyme, purified more than tenfold after cell-disruption of Pseudomonas gladioli, stereospecifically converted (4 )-(+)-limonene to (4/ )-(+)-a-terpineol or (4S)-(+)-limonene to (4S)-(+)-a-terpineol. a-Terpineol is widely distributed in nature and is one of the most commonly used perfume chemicals [27]. 

Pyrrolizidine Alkaloids.—The necic acid component of senecionine (8) derives from two molecules of isoleucine, radioactivity from precursor amino-acid being equally incorporated into both halves of the necic acid fragment, as shown in Scheme 2 (c/. Vol. 9, p. 4). It has now been shown that biotransformation of isoleucine into the necic acid involves loss of half of a tritium label from C-4 in each of the two amino-acid fragments.6 Removal of a proton is, therefore, stereospecific, and oxidation at C-4 does not proceed beyond the two-electron level i.e., a higher intermediate oxidation level, corresponding to a ketone, is excluded. Further results indicate that for each molecule of isoleucine it is the 4-pro-S proton [see (14)] which is lost. 

To summarize these studies, oxidation and epimerization of the 3 3-hydroxyl and 5p-hydroxylation and glucosylation of the 3-hydroxyl appear to be quite common reactions, whereas other stereospecific hydroxylations as well as conjugation with deoxysugars are probably more species specific. The combination of the biosynthetic potential of unrelated plant species and the formation of novel cardenolides by biotransformation was achieved by Kawaguchi et at (1990) who administered digitoxigenin to hairy root cultures... 

Oxidation of divalent sulfur atoms in thioethers is a common biotransformation of sulfur-containing compounds [Eq. (9)]. Oxidation proceeds in two stages, first to the sulfoxide and then to the sulfone. Sulfoxides have increased polarity and are often observed as excreted metabolites, but they can also be reduced back to the sulfide. Formation of the sulfoxide creates an asymmetric center, and stereospecific oxidation can occur. Sulfones tend to be terminal metabolites no evidence for their reduction exists. 

Some biotransformations introduce an asymmetric center into a drug and these often proceed stereospeci-fically. The most common examples are hydroxylation of a secondary carbon and the reduction of ketones to secondary alcohols. Ibuprofen undergoes both co and co-l oxidation of the isobutyl side chain, and formation of the resulting carboxylic acid metabolite introduces a second asymmetric center into the molecule. Both ibuprofen enantiomers have been shown to undergo stereospecific oxidation to give a metabolite with the same configuration at the new asymmetric center. 

Because many drugs contain either chiral centers, prechiral centers, or both, interest in stereochemical substrate-enzyme interactions, the stereospecificity of biotransformations, and species (and strain) differences in these parameters is increasing. Since enzymes themselves contain chiral centers, differential interaction of R and S isomers of drugs with drug metabolizing enzymes is the rule rather than the exception. Beckett reported stereoselectivity in the N-dealkylation, deamination, and formation of the nitrone and secondary hydroxylamine metabolites (+) -and (-) - N-benzylamphetamine ( ) in rabbits. Stereoselectivity has also been observed in the dealkylation of d-, 1-, and d,1-fenfluramine (22), an anorexiogenic agent. 

In practice biotransformations are carried out by microorganisms. They offer efficient transformation of substrates both as to yields, and often, more importantly, stereospecificity. Today, the selection of enzymes available through microbial use is vast. 

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