Metabolic activation, conjugation

Alternative pathways of activation of nitrosamines, including 3-hydroxylation followed by sulfate conjugation and the formation of alkoxydiazen-ium ions are discussed. The formation of alkyldiazo-nium ions from trialkyltriazenes is presented to show that the formation of the putative ultimate carcinogens from nitrosamines can be studied in a system not requiring metabolic activation,... 

Most reactive metabolites produced by CYP metabolic activation are electrophilic in nature, which means that they can react easily with the nucleophiles present in the protein side chains. Several functional groups are recurrent structural features in M Bis. These groups have been reviewed by Fontana et al. [26] and can be summarized as follows terminal (co or co — 1) acetylenes, olefins, furans and thiophenes, epoxides, dichloro- and trichloroethylenes, secondary amines, benzodioxoles (methylenediox-yphenyl, MDP), conjugated structures, hydrazines, isothiocyanates, thioamides, dithiocarbamates and, in general, Michael acceptors (Scheme 11.1). 

These chemical functions are primarily responsible for metabolic activity besides, all active functions of a small hapten should remain accessible in the hapten carrier conjugate to obtain the most exquisitely specific antibody immunoglobulin (IgG) population of which the immune system is capable,... 

Buta-1,3-diene (10.101, Fig. 10.24) is a gaseous chemical used heavily in the rubber and plastics industry, the presence of which in the atmosphere is also a concern. Butadiene is suspected of increasing the risks of hematopoietic cancers, and it is classified as a probable human carcinogen. Butadiene must undergo metabolic activation to become toxic the metabolites butadiene monoepoxide (10.102, a chiral compound) and diepoxybutane (10.103, which exists in two enantiomeric and one meso-form) react with nucleic acids and glutathione [160 - 163], as does a further metabolite, 3,4-epoxybutane-l,2-diol (10.105). Interestingly, butadiene monoepoxide is at least tenfold more reactive than diepoxybutane toward nucleic acids or H20. Conjugation between the C=C bond and the oxirane may account for this enhanced reactivity. 

Figure 2.12 Metabolic activation by the liver of pyrrolizidine alkaloid to the toxic pyrrole (liver bound and highly toxic) and the glutathione conjugate (excretion metabolite).

All of these metabolites possess retinoid activity that is in some in vitro models more than that of the parent isotretinoin. However, the clinical significance of these models is unknown. After multiple oral dose administration of isotretinoin to adult cystic acne patients (18 years of age and older), the exposure of patients to 4-oxo-isotretinoin at steady state under fasted and fed conditions was approximately 3.4 times higher than that of isotretinoin. In vitro studies indicated that the primary P450 isoforms involved in isotretinoin metabolism are 2C8, 2C9, 3A4, and 2B6. Isotretinoin and its metabolites are further metabolized into conjugates, which are then excreted in urine and feces. 

Reichert D, Schutz S, Metzler M. 1985. Excretion pattern and metabolism of hexachlorobutadiene in the rats Evidence for metabolic activation by conjugation reactions. Biochem Pharmacol 34 499-505. 

Conjugation is a major determinant of the metabolic activity of a xenobiotic compound (T, ). One reason is that conjugation significantly Increases the water solubility of the exocon. An exocon that could previously diffuse across cellular membranes does not diffuse as readily once it is conjugated. Thus, the conjugate can be compart-... 

Phase III reactions occur primarily in plants presumably because excretion of Phase II conjugates is Insignificant in plants. Phase III reactions are mechanisms whereby plants can reduce the effective concentration of xenobiotlc compounds in the cytoplasm. Thus, conjugation reactions provide mechanisms for the elimination of xenobiotlc compounds from sites of continuing metabolic activity in all organisms ( ) ... 

Morphine orally is less effective and absorption is very slow. It has variable and high first pass metabolism when given by subcutaneous route, its analgesic effect starts within 10 minutes which persists for 4 to 5 hours and by IV route, it produces immediate action. In plasma, it binds to plasma proteins (approx. 30%). In liver it is metabolized by conjugation to glucuronic acid to form active and inactive products, which are excreted in urine. It is also excreted though bile and in the faeces. 

For compounds that require metabolic activation, avoiding structural features that may provide resonance stabilization of electrophilic metabolites (e.g., conjugated double bonds, or conjugated system/aryl moiety) will decrease the lifetime of the reactive intermediates. 

Another example of a glutathione conjugate responsible for toxicity is the industrial chemical hexachlorobutadiene discussed in chapter 7. The diglutathione conjugate of bromobenzene is believed to be involved in the nephrotoxicity after further metabolic activation (chap. 7, Fig. 7.31). 

Thus, sulfate conjugation and acetylation may be involved in the metabolic activation of N-hydroxy aromatic amines, glutathione conjugation may be important in the nephrotoxicity of compounds, methylation in metal toxicity, glucuronidation in the carcinogenicity of p-naphthylamine and 3, 2 -dimethyl-4-aminobiphenyl. 

Figure 5.32 Metabolic activation of estragole. Conjugation (e.g., with sulfate) can yield a reactive intermediate.

Figure 7.2 The metabolic activation of benzo[a]pyrene by cytochrome P-450 1A1 to a diol epoxide metabolite, a mutagen. This is believed to be the ultimate carcinogenic metabolite. Other routes of metabolism also catalyzed by cytochrome P-450 give rise to the 9,10, and 4,5 oxides and subsequent metabolites namely phenols, diols, and glutathione conjugates. The reactive site (carbon atom) on the metabolite is indicated.

Figure 7.19 Proposed metabolic activation of paracetamol to a toxic, reactive intermediate /V-acetyl-p-benzoquinone imine (NAPQI). This can react with glutathione (GSH) to form a conjugate or with tissue proteins. Alternatively, NAPQI can be reduced back to paracetamol by glutathione, forming oxidized glutathione (GSSG).

Figure 7.29 The metabolic activation of hexachlorobutadiene. The glutathione conjugate is degraded to a cysteine conjugate in two stages involving the action of (1) y-glutamyltransferase and (2) cysteinyl glycinase.

Paracetamol is a widely used analgesic, which causes liver necrosis and sometimes renal failure after overdoses in many species. The half-life is increased after overdoses because of impaired conjugation of the drug. Toxicity is due to metabolic activation and is increased in patients or animals exposed to microsomal enzyme inducers. The reactive metabolite (NAPQI) reacts with GSH, but depletes it after an excessive dose and then binds to liver protein. Cellular target proteins for the reactive metabolite of paracetamol have been detected, some of which are enzymes that are inhibited. Therefore, a number of events occur during which ATP is depleted, Ca levels are deranged, and massive chemical stress switches on the stress response. 

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