CYP catalyzed reactions

Mechanism-based CYP inhibition is time dependent, as it is a consequence of metabolic activation. For the same reason, it also depends on substrate concentration. It also, usually, depends on the presence of NAD PH in the incubation system (or an NADPH regeneration system), as most of the CYP-catalyzed reactions require NADPH to occur [5],... 

The membrane-attached cytochrome P450 enzymes are involved in a significant fraction of events associated with drug metabolism. Most of the cytochrome P450 (CYP) catalyzed reactions lead to the detoxification of xenobiotics, by forming hydrophilic metabolites that can be readily excreted from the body. 

Jurva et al. performed comprehensive studies on the comparability of EC-MS and cytochrome P 450 catalyzed reactions [48,49] and found that EC-MS successfully mimicked those CYP reactions that are initiated by a one-electron oxidation, such as /V-dealkylation, 5-oxidation, / -oxidation, alcohol oxidation, and dehydrogenation, whereas the reactions that are initiated via direct hydrogen atom abstraction, for example, O-dealkylation and hydroxylation of unsubstituted aromatic rings, were not mimicked. In the latter cases the oxidation potentials were too high for electrochemical oxidation in aqueous solutions. A further disadvantage is that EC lacks the stereospecifity of the reactions, in contrast to CYP catalyzed reactions [50],... 

CYPs are good at epoxidizing aikenes and alkynes as well, which is probably one reason why more drugs don t contain these groups. Then again, just because a reaction can happen doesn t mean that it will. For carbamazepine (Figure 9.18), epoxidation followed by diol formation is the major metabolic route, but for tamoxifen, metabohc epoxidation of the central double bond hardly happens, and other CYP-catalyzed reactions, including arene epoxidation and N-demethylation (discussed in a later section) are much more in evidence. 

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). 

The predicted metabolites are also the starting point for the phase II metabolic prediction, to find where glucuronidation could occur. All the probable metabolites obtained from CYP metabolism reactions are submitted to a possible phase II reaction catalyzed by UGTs, using the UGT structure(s) as a template. The accessibility component is computed in the UGTcavity to prioritize glucuronic acid transfer. The final metabolite structures are then reported in graphical output or saved to a file. 

Metabolites that are less reactive than suicide inhibitors may impact more distant enzymes, within the same cell, adjacent cells, or even in other tissues and organs, far removed from the original site of primary metabolism. For example, organopho-sphates (OPs), an ingredient in many pesticides, are metabolized by hepatic CYPs to intermediates, which, when transported to the nervous system, inhibit esterases that are critical for neural function. Acetylcholinesterase (AChE) catalyzes the hydrolysis of the ester bond in the neurotransmitter, acetylcholine, allowing choline to be recycled by the presynaptic neurons. If AChE is not effectively hydrolyzed by AChE in this manner, it builds up in the synapse and causes hyperexcitation of the postsynaptic receptors. The metabolites of certain insecticides, such as the phos-phorothionates (e.g., parathion and malathion) inhibit AChE-mediated hydrolysis. Phosphorothionates contain a sulfur atom that is double-bonded to the central phosphorus. However, in a CYP-catalyzed desulfuration reaction, the S atom is... 

Table 4 Comparison of the Apparent Affinity (Km) with Which HLM and Recombinant CYP Enzymes (Bactosomes) Catalyze CYP-Selective Reactions...

Finally, several investigators have examined the reaction of these prolyl isomerases toward substrates of general structure Suc-Ala-Xaa-Pro-Phe-pNA. In a 1989 paper, Fischer investigated CyP-catalyzed isomerization for Xaa = Ala, Gly, Ser, and Val and found values oikJK of 6.6, 2.1, 5.4, and 8.8 sec", respectively. He interpreted these results in... 

The most important reactions of oxidation of sp -carbon atoms are schematized in Figure 32.3. In the simplest cases, a non-activated carbon atom in an alkyl group undergoes CYP-catalyzed hydroxylation (Figure 32.3a). The penultimate position is a preferred site of attack, but... 

The implicit objective of the chapter is to warn medicinal chemists against the danger of over-simplihcation. For too many drug discoverers, biotransformation begins and ends with CYP-catalyzed oxidations. As a result of this narrow view, little is done computationally and experimentally before the development phases to obtain a comprehensive view of the biotransformation of lead and preclinical candidates. One can only wonder about the proportion of metabolism-related side-effects that could have been avoided during clinical phases had medicinal chemists been more attentive to potential toxihcation reactions and drug-drug interactions caused by non-CYP reactions. 

A purely instrumental approach based on using electrochemistry to catalyze reactions and to mimic CYP activity has been proposed as another way to study the formation of metabolites [43,44], The electrochemical cell (EC) is placed in front of the mass... 

CYP oxidation reactions involve a complex series of steps that have been well defined Rose and Hodgson, 2004). The initial step involves the binding of substrate to oxidized CYP, followed by a one-electron reduction catalyzed hy NADPH cytochrome P450 reductase to form a reduced cytochrome-substrate complex. The next several steps involve interaction with molecular oxygen, the acceptance of a second electron from NADPH cytochrome P450 reductase or NADH cytochrome b reductase, followed by the subsequent release of water and the oxygenated product of the reaction. This complicated reaction sequence results in the transfer of one atom of molecular oxygen to the substrate while the other atom is reduced to water. 

He K, Woolf TF, Kindt EK, Fielder AE, Talaat RE (2001) Troglitazone quinone formation catalyzed by human and rat CYP3A An atypical CYP oxidation reaction. Biochem Pahrmacol 62 191-198... 

It s been called nature s most versatile biological catalyst and compared to a molecular blowtorch for its ability to catalyze oxidations that would otherwise require extremely high temperatures. " Altogether, it represents a most extraordinary series of in vivo synthetic reagents, one that will react with almost anything a medicinal chemist can throw at it. When it comes to Phase I metabolism, CYP-catalyzed oxidative reactions usually predo-... 

This method has most often been applied in conjunction to rates of CYP-mediated oxidative metabolism in microsomal systems but can also be applied to conjugation (Lin and Wong, 2002 Miners et al., 2004, 2006 Soars et al., 2002) or FMO-catalyzed reactions (Fisher et al., 2002) or data derived from hepatocytes (Lau et al., 2002) or other in vitro systems (Yamamoto et al., 2005). The scaled clearance values can be compared to the values determined in vivo for a single compound or a set of analogs to give some idea of the predictive capacity of the in vitro systems. 

Cytochrome P450 monooxygenases (CYP) are membrane-bound enzymes that catalyze the addition of oxygen to hydrophobic compounds. CYPs are important enzymes in plant secondary metabolism however, they are also important in selectivity of a large numbor of hmbicides, particularly in cereals (70). CYPs catalyze a variety of reactions, but the most important ones for herbicide detoxification are hydroxylations and dealkylations (Figure 3). Ring hydroxylation reactions frequently render herbicides inactive however, dealkylation reactions may only partially detoxify herbicides. 

Cytochrome P450 (CYP) mono-oxygenases, also called mixed function oxidases, are versatile hemoprotein enzymes that catalyze the cleavage of molecular oxygen to incoiporate one oxygen atom into a substrate molecule and one atom into water [1]. The general stoichiometry of the reaction is as follows (S-H, substrate) ... 

The numerous biotransformations catalyzed by cytochrome P450 enzymes include aromatic and aliphatic hydroxylations, epoxidations of olefinic and aromatic structures, oxidations and oxidative dealkylations of heteroatoms and as well as some reductive reactions. Cytochromes P450 of higher animals may be classified into two broad categories depending on whether their substrates are primarily endogenous or xenobiotic substances. Thus, CYP enzymes of families 1-3 catalyze... 

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