Biotransformation reactions glucuronidation

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). 

The concept of microbial models of mammalian metabolism was elaborated by Smith and Rosazza for just such a purpose (27-32). In principle, this concept recognizes the fact that microorganisms catalyze the same types of metabolic reactions as do mammals (32), and they accomplish these by using essentially the same type of enzymes (29). Useful biotransformation reactions common to microbial and mammalian systems include all of the known Phase I and Phase II metabolic reactions implied, including aromatic hydroxylation (accompanied by the NIH shift), N- and O-dealkylations, and glucuronide and sulfate conjugations of phenol to name but a few (27-34). All of these reactions have value in studies with the alkaloids. 

Another therapeutic class to be briefly discussed is that of the lipid-lowering agents known as fibrates, e.g., clofibrate and fenofibrate (8.5). Here also, the acidic metabolite is the active form clofibrate (an ethyl ester) is rapidly hydrolyzed to clofibric acid by liver carboxylesterases and blood esterases [11], Human metabolic studies of fenofibrate (8.5), the isopropyl ester of fenofibric acid, showed incomplete absorption after oral administration, while hydrolysis of the absorbed fraction was quantitative [12], This was followed by other reactions of biotransformation, mainly glucuronidation of the carboxylic acid group. 

The quantitatively most significant second-phase reaction of hepatic biotransformation is glucuronidation that takes place in the ER. The transfer of glucuronosyl group from... 

The level and activity of specific enzymes involved in biotransformation can differ depending on the species, strain, age, and sex of the test animal. For example, cats cannot carry out glucuronidation reactions, newborn rats have relatively low cytochrome P450 activity, and male rats are more sensitive to carbon tetrachloride toxicity than female rats. These differences are important to consider when interpreting the results from toxicological studies. The observation that age, sex, and genetics can significantly influence biotransformation reactions in animals raises the question of whether these characteristics also affect the biotransformation capacity of humans. 

TNT is readily absorbed through skin, especially when skin is moist. It is excreted in urine more than in feces some is found in bile. The major biotransformation reaction is nitroreduction and, to a lesser extent, oxidation. The main metabolite formed by nitroreduction seems to be 4-amino-2,6-dinitrotoluene (4-ADNT). Other metabolites include 2-amino-4,6-dinitrotoluene (2-ADNT), 2,4-diamino-6-nitrotoluene, and 2,6-diamino-4-nitrotoluene. The metabolites are excreted in the urine as glucuronide conjugates and in the free form. Ring oxidation products of TNT such as trinitrobenzylalcohol, trinitrobenzoic acid, and simultaneous oxidation and reduction metabolites such as 2,6-dinitro-4-amino-benzylalcohol and 2,6-dinitro-4-amino-m-cresol are of less importance. Untransformed TNT is also excreted in the urine. ADNT and TNT concentrations were found in workers in explosives factories. 4-ADNT excretion was reported to be complete within 3M days after exposure. However, another study reported detectable urine concentration of ADNT in explosives workers even after 17 days away from the workplace. 

It should be emphasized that investigations of embryonic enzymatic catalysis of the four retinoid biotransforming reactions (2ill-trans/9-cis isomerization, retinol and retinal dehydrogenation, glucuronidation and monooxygenation) have been numerous [6,7,10-13,15,18-22, 77-85]. 

Most drugs used in anaesthesia are metabolised in the liver by phase I reactions, mediated by cytochrome P-450 enzymes. These are susceptible to destruction by cirrhosis, so that the biotransformation of drugs, such as opioids (except morphine), benzodiazepines, barbiturates, and inhalational agents, may be markedly altered in severe liver disease. These enzymes are found in the centrilobular areas, which are more prone to hypoxia. In contrast, the enzymes responsible for phase II reactions, found predominantly in the peripheral areas, often function normally even in advanced disease. The disposition of benzodiazepines that are eliminated primarily by glucuronidation, e.g. lorazepam and oxazepam, are unaffected by chronic liver disease. For drugs with low hepatic extraction, advanced hepatocytic dysfunction decreases phase I and II biotransformation with a reduced clearance and prolongation of the elimination half-life. This is often partially offset by an increased free fraction due to decreased protein binding. 

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