P-450-type hydroxylations

Pitie M, Bernadou J, Meunier B (1995) Oxidation at carbon-1 of DNA deoxyriboses by the Mn-TM-PyP/KEIS05 system results from a cytochrome P-450-type hydroxylation reaction. J Am Chem Soc 117 2935-2936... 

Pitie M, Bemadou J, Meunier B. Oxidation of carbon-T of DNA deoxytiboses by the Mn-TMPYP/KHSOs system results from a cytochrome p-450-type hydroxylation reaction. J clm Chem Soc. 1995 117 2935—2936. 

Ricinoleic acid (Figure 3.8) is the major fatty acid found in castor oil from seeds of the castor oil plant (Ricinus communis Euphorbiaceae), and is the 12-hydroxy derivative of oleic acid. It is formed by direct hydroxylation of oleic acid (usually esterified as part of a phospholipid) by the action of an 02- and NADPH-dependent mixed function oxidase, but this is not of the cytochrome P-450 type. Castor oil has a long history of use as a domestic purgative, but it is now mainly employed as a cream base. Undecenoic acid (A9-undecenoic acid) can be obtained from ricinoleic acid by thermal degradation, and as the zinc salt or in ester form is used in fungistatic preparations. 

Arylhydrocarbon Hydroxylase. Benzo(a)pyrene (BaP) is considered an ideal model compound for monitoring arylhydrocarbon hydroxylase (AAH) MFO levels in rats. Its metabolism has been widely investigated in both rat liver and extrahepatic tissues (40-41,61 -70). Arylhydrocarbon hydroxylase has been compared to biphenyl hydroxylase in the intestines of rats (25,28). Intestinal AAH has been shown to be induced up to 30-fold with cytochrome P-448-type inducers, while being insensitive to P-450-type induction. Therefore, we have used intestinal AAH in conjunction with biphenyl hydroxylation, to evaluate the effects of dietary fibers on intestinal metabolism. 

Although pathway 2 in the oxidation process (Scheme 2) may be considered analogous to mechanisms proposed for carbon hydroxylations catalyzed by cytochrome P-450, abstraction of an electron from the lone pair on nitrogen (pathway 1) would be a more likely first step in these types of reactions. It is reasonable to assume that the nature of substituents R, R2, and R3 would greatly influence the rate and path of reaction. The mechanistic possibilities in Scheme 2 are undoubtedly simplistic in their representation of the active oxygen species of cytochrome P-450 and are by no means comprehensive. However, these pathways do serve to illustrate.the role of radical substrate intermediates in cytochrome P-450-catalyzed reactions. More detailed analyses of mechanistic studies on these and other cytochrome P-450-mediated reactions can be found in recent reviews on the subject 49, 50, 60). 

Enzymatic hydroxylation of biological molecules is often catalyzed by hydroxylases. These types of enzymes are either oxygenases or peroxidases, in which the source of oxygen is O2 or H2O2, respectively. Cytochrome P-450-dependent enzymes represent a common class of enzymes that carry out hydroxylation reactions. L-Carnitine is a metabolite isolated from many organisms and its biosynthesis begins with the enzymatic hydroxylation of trimethyllysine. The intermediate, 3-hydroxyl-e-(A(A(ALtrimethyl)-L-lysine, is further... 

An example of the second type of chiral effect in metabolism is afforded by benzofa]-pyrene, also discussed in more detail in chapter 7. This carcinogenic polycyclic hydrocarbon is metabolized stereos elec lively by a particular cytochrome P-450 isozyme, CYP1A1, to the (+)-7R,8S oxide (chap. 7, Fig. 5.2), which in turn is metabolized by epoxide hydrolase to the (—)-7R,8S dihydrodiol. This metabolite is further metabolized to (- -)-benzo[aIpyrene, 7R,8S dihydrodiol, 9S,10R epoxide in which the hydroxyl group and epoxide are trans and which is more mutagenic than other enantiomers. The (—)-7R,8S dihydrodiol of benzo[aIpyrene is 10 times more tumorigenic than the (+)-7R,8S enantiomer. It was reported that in this case the configuration was more important for tumorigenicity than the chemical reactivity. 

Metalloporphyrins and some related metal complexes are effective catalysts in IOB oxidations, as already discussed for alkenes, and acids (Sections 5.1.1.1 and 5.2.4). Also, sulphides have been oxidized to sulphoxides [58]. Some other substrates of various types underwent such catalysed oxidations, because these systems mimic the natural oxidant cytochrome P-450 [2]. From a synthetic point of view, only a few reactions are of importance alkanes were mainly used which underwent regio- and stereo-specific hydroxylation, for instance the methyl group of a pyrrole derivative was converted into hydroxymethyl, leading to one-pot preparation of dipyrro-methanes [59], The preparation of elaborated catalysts is, however, very demanding and precludes a wider use. 

Formation of the porphyrin iron monoxide (of the Compound I type) as an active oxidizing species in the P-450 system is supported by the successful monoxidation of P 450 by an oxidizing reagent such as iodosobenzene (see Figure 5) (16). This species then effectively hydroxylates an organic substrate. 

Studies with cell-free hydroxylases suggest that the hydroxylation mechanisms are complex. It is assumed that an electron transport system involving an NADPH-dependent flavoprotein, an iron-sulfur protein, and cytochrome P-450 is involved. In the case of the steroid 15/S-hydroxylase system of Bacillus megaierium, these three components have been demonstrated15. The 1 la-hydroxylase of Rhizopus nigricans is also an enzyme of the P-450 monooxygenase type which works with an NADPH-cytochrome P-450 reductase. In this case the enzyme complex is associated with the endoplasmic reticulum of the mycelial cells34. 

The NADPH-cytochrome P-450 system, commonly known as the mixed-function oxygenase (MFO) system, is the most important enzyme system involved in the Phase I oxidation reactions. Cytochrome P-450 system, localized in the smooth endoplasmic reticulum of cells of most mammalian tissues, is particularly abundant in the liver. This system contains a number of isozymes which are versatile in that they catalyze many types of reactions including aliphatic and aromatic hydroxylations and epoxidations,... 

Gut J, Meier UT, Catin T, et al. Mephenytoin-type polymorphism of drug oxidation purification and characterization of a human liver cytochrome P-450 isozyme catalyzing microsomal mephenytoin hydroxylation. Biochim Biophys Acta I986 884(3) 435—47. 

Figure 19.9 Metabolic possibilities for model compounds having representative functionality. Selected phase 1 reactions (1) Hydrolysis of various types of esters, in this case mediated by a carboxylesterase (2) N-dealkylation mediated by certain of the Cytochrome P-450 (CYP) enzymes (3) O-dealkylation mediated by certain of the CYPs and (4) Aromatic hydroxylation also mediated by certain of the CYPs. Depending upon the subtleties of their electronic and steric environments, the relative competitive biotransformation rates for these processes will generally be (1) (2) > (3) (4). Selected phase 2 reactions (5) Formation of a glucuronic acid conjugate (or in some cases a sulfate conjugate) and (6) N-acetylation. In terms of relative biotransformation rates in general (5) >> (6).

Although cytochrome P-450 (hemoprotein P-450) is also a protohemo-protein, it is a hydroxylating enzyme and is not usually regarded as a cytochrome 6. In additon, cytochrome o contains a protoheme but it acts as a terminal oxidase in various microorganisms. Helicorubin and entero-chrome-566 are sometimes included with the type 6 cytochromes. 

Compound 69c was proved to maintain good activity in vitro but had a short half-life in vivo due to hydroxylation of the 4"-side chain by cytochome P-450 followed by conversion to CP-418,001 (69d), which was inactive. To avoid such metabolism, a methyl group was introduced to the a-side chain of 69c to obtain CP-416,890 (69e). It exhibited increased stability to metabolism and good in vivo activity. Finally, synthesized 69f, which bears a smaller side chain, demonstrated superior in vitro and in vivo activity, particularly against S. pneumoniae, including the MLS-i and mef type of resistant strains. Its pharmacokinetic profile is comparable to AZM (19) and telithromycin (TLM) (91i) [87-93]. However, the antibacterial activity of 69f seems to be insufficient against MLS-c resistant strains of S. pneumoniae and H. influenzae compared with 91i and ABT-773 (95) (Schemes 8 and 11). 

Gut, X, T. Catin, P. Dayer, T. Kronbach, U. Zanger, and U.A. Meyer (1986). Debrisoquine/sparteine-type polymorphism of drug oxidation Purification and characterization of two functionally different human liver cytochrome P-450 isozymes involved in impaired hydroxylation of the prototype substrate bufuralol. J. Biol. Chem. 261, 11734-11743. 

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