Site of metabolism

Figure 5. Glycogen metabolism and glycolysis. Dotted lines indicate sites of metabolic defects involving enzymes l-XI.

Sites of metabolism. When a chemical reaches one of these, it is metabolized. Usually this means detoxication, but sometimes (most importantly) the consequence is activation. The organism acts upon the chemical. 

Sites of storage. When located in one of these, the chemical has no toxic effect, is not metabolized, and is not available for excretion. However, after release from storage, it may travel to sites of action and sites of metabolism. 

A xenobiotic is said to be stored when it is not available to sites of metabolism or action and is not available for excretion. In other words, it is held in an inert position from a toxicological point of view, where it is not able to express toxic action or to be acted upon by enzymes. A xenobiotic is stored when it is located in a fat depot (adipose tissue), bound to an inert protein or other cellular macromolecule, or simply held in a membrane that does not have any toxicological function (i.e., it does not contain or represent a site of toxic action, neither does it contain enzymes that can degrade the xenobiotic). 

MetaSite Site of metabolism prediction for CYP2C9 and CYP3A4 and others www.moldiscovery.com... 

Figure 30-11. Intermediates in i-hydroxyproline catabolism. (a-KA, a-keto acid a-AA, a-amino acid.) Numerals identify sites of metabolic defects in hyperhydroxyprolinemia and type II hyperprolinemia.

Skeletal muscle is the principal site of metabolism of branched-chain amino acids, which are used as an energy source. 

These studies represent the first report of the metabolism of brevetoxins by mammalian systems. PbTx-3 was rapidly cleared from the bloodstream and distributed to the liver, muscle, and gastrointestinal tract. Studies with isolated perfused livers and isolated hepatocytes conflrmed the liver as a site of metabolism and biliary excretion as an important route of toxin elimination. [ H]PbTx-3 was metabolized to several compounds exhibiting increased polarity, one of which appeared to be an epoxide derivative. Whether this compound corresponds to PbTx-6 (the 27,28 epoxide of PbTx-2), to the corresponding epoxide of PbTx-3, or to another structure is unknown. The structures of these metabolites are currently under investigation. 

In terms of other sites of metabolism, we are looking at the metabolism in the dioxole ring and in dealkylation. We have seen some interesting things, but I could not comment on this right now. With respect to the alpha-ethyl, I think that the parent compound is probably the one that is active. 

The question at this point was whether modifications could be made to the oxadiazole molecule to enhance metabolic stability and achieve comparable activity. This approach required knowledge of the site of metabolism and the nature of the metabolic products. This information was obtained from ion mass spectrometry. The identity of these products was determined by comparing the fragmentation pattern of metabolites A and B with the parent compound and the corresponding daughter ions (Fig. 25). 

The compound in the portal blood is transported to the liver, which usually is the major site of metabolism for pharmaceuticals. In the liver there is usually one, or more, of three principal fates for the drug either metabolism excretion into the bile or return to the blood for distribution to the other tissues of the body. These other tissues may also be sites of metabolism or, particularly in the case of the kidney, sites of excretion. 

Assessing the effect of the intestinal metabolism in the Peff as a membrane transport rate parameter is a methodological issue [7, 26, 34, 35, 49]. An evaluation of its influence has to include a study to establish which enzyme(s) is (are) involved and the site of metabolism in relation to the site of the measurements. Intracellular metabolism in the enterocyte, for instance by CYP 3A4 and di- and tri-... 

The incorporation of fluorine into a molecule has been widely used to alter the pharmacokinetic properties and overall drug-like properties of compounds. This includes affecting the metabolism, oral absorption, and brain penetration of these molecules [18]. Metabolism can be affected by addition of fluorine directly at or adjacent to the site of metabolism. In addition, substitution with fluorine can increase the lipophilicity of compounds which has been shown to dramatically affect both oral absorption and brain penetration. Finally, the electron-withdrawing characteristic of fluorine has been exploited to lower the P-gp liability of compounds and modulate the pKa of adjacent groups which resulted in increased brain exposure. In the following section, representative examples will highlight the powerful nature of fluorine to modulate overall drug-like properties. 

However, the reaction of NP with thiols may be a necessary but not sufficient cause for the release of NO from the ion as there are many thiols in frog heart tissue and NP is a vasodilator only under illumination. Furthermore Sogo et al. [50] could not detect NO generation from NP in human plasma containing cysteine, glutathione, homocysteine and reduced cysteine residues. Therefore, there must be a unique component of mammalian tissues which is involved in the release of NO from NP, and this reaction comes after reaction with thiol. Kowaluk et al. [51] report that NP is readily metabolised to NO in subcellular fractions of bovine coronary arterial smooth muscle and that the dominant site of metabolism is in the membrane fraction. This led to the isolation of a small membrane-bound protein or enzyme that can convert NP into NO. The mechanism shown in Scheme 8.2 combines the thiol reaction and that with an enzyme. 

For a commercial database of known metabolic transformations, Borodina et al. [76] extracted all known sites of aromatic hydroxylations. These observed transformations were used to generate all possible transformations for each molecule, giving an estimate of the probability that each transformation would actually occur. The method was 85% accurate in predicting site of aromatic hydroxylation when tested against a second metabolism database containing 1552 molecules. Boyer et al. [77] took a similar approach using reaction center fingerprints to estimate the occurrence ratio of a particular metabolic transformation. The method successfully predicted the three most probable sites of metabolism in 87% of compounds tested. 

Quantum mechanical approaches have been successfully used to predict hydrogen abstraction potentials and likely sites of metabolism of drug molecules [78-81]. AMI, Fukui functions, and density functional theory calculations could identify potential sites of metabolism. Activation energies for hydrogen abstraction were calculated by Olsen et al. [81] to be below 80 kj/mol, suggesting most CH groups can be metabolized which particular one depends on steric accessibility and intrinsic reactivities. 

Factors That Influence the Site of Metabolism Prediction by Cytochrome P450s... 

There are at least two factors that could influence the turnover rate, the site of metabolism (hot spot) and the affinity of a compound toward these enzymes the protein/ligand (substrate or inhibitor) interaction and the chemical reactivity of the compound towards oxidation. Because of the interaction of the protein with the potential ligand, certain atoms of the compound could be exposed to the heme group, and depending on the chemical nature of these moieties the oxidative reaction will take place at different rates, for example celecoxib is metabolized by CYP2C9 at the... 

The theory predicting the stability of the radical formed assumes that the rate-limiting step is the extraction of the hydrogen atom to form a radical, and this hypothesis is in principle valid for aliphatic hydroxylation, but it might not be the case for other reactions. Several examples in the literature show that this method is useful for the prediction of the site of metabolism for compounds undergoing metabolism by CYP3A4 [8]. Nevertheless, there is a lack of a theory that could explain all the different metabolism reactions and mechanisms that may or may not involve radical formation. 

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