Biotransformation reactions methylation

Biotransformation reactions can be classified as phase 1 and phase 11. In phase 1 reactions, dmgs are converted to product by processes of functionalization, including oxidation, reduction, dealkylation, and hydrolysis. Phase 11 or synthetic reactions involve coupling the dmg or its polar metaboHte to endogenous substrates and include methylation, acetylation, and glucuronidation (Table 1). 

Biotransformation reactions of the original xenobiotic may result in compounds with fundamentally different physical properties and toxicities. The apparently ubiquitous distribution of haloge-nated anisoles (Wittlinger and Ballschmiter 1990 Fiihrer and Ballschmiter 1998) attests to the importance of O-methylation reactions that are discussed in Section 6.11.4. 

The O-methylation of halogenated phenols has been briefly noted in Chapter 4 as an example of a biotransformation reaction, and they have attracted attention in view of the unacceptable flavor that chloroanisoles impart to broiler chickens (Gee and Peel 1974), freshwater fish (Paasivirta et al. 1987), and wine corks (Buser et al. 1982). A few additional comments may usefully be added to place these observations in a wider perspective. 

In the liver two phases of biotransformation reactions can take place phase I transformations that introduce polar groups (hydroxyls) in the molecule, and phase II reactions which include conjugations with glucuronic acid, sulphate, or glycine to yield water-soluble metabolites which are excreted in urine. 0-Methylations, to inactivate catechol moieties in these molecules, are also phase II reactions. Phase I reactions are not important in natural phenolic metabolites since they generally contain several polar hydroxyl groups). Phase II reactions increase the molecular weight of the phenolics and promote their secretion into bile. 

In principle, numerous reports have detailed the possibility to modify an enzyme to carry out a different type of reaction than that of its attributed function, and the possibility to modify the cofactor of the enzyme has been well explored [8,10]. Recently, the possibility to directly observe reactions, normally not catalyzed by an enzyme when choosing a modified substrate, has been reported under the concept of catalytic promiscuity [9], a phenomenon that is believed to be involved in the appearance of new enzyme functions during the course of evolution [23]. A recent example of catalytic promiscuity of possible interest for novel biotransformations concerns the discovery that mutation of the nucleophilic serine residue in the active site of Candida antarctica lipase B produces a mutant (SerlOSAla) capable of efficiently catalyzing the Michael addition of acetyl acetone to methyl vinyl ketone [24]. The oxyanion hole is believed to be complex and activate the carbonyl group of the electrophile, while the histidine nucleophile takes care of generating the acetyl acetonate anion by deprotonation of the carbon (Figure 3.5). 

The same method described in Procedure 2 (Section 12.1.2) was used for preparative-scale biotransformation of 150 mg of 2-methyl-1,4-naphthoquinone, except that reactions were incubated for only 72 h before being combined, centrifuged, extracted and chromatographically purified to give 50 % yield (92 mg) of product. 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

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