PRINCIPLES OF TOXICOLOGY - Biology East Borneo

74 BIOTRANSFORMATION: A BALANCE BETWEEN BIOACTIVATION AND DETOXIFICATIONS-transferases are located predominantly in the cytosol, and hepatic concentrations of the necessarynucleophilic glutathione cosubstrate are high (> 5 mM). The major transferases consist of homo- orheterodimers of a limited number of forms of approximately 25,000-dalton subunits. The differentsubunit combinations confer different but overlapping substrate selectivity and isoelectric points andare expressed differently in different organs within an animal species. The subunits also responddifferently to xenobiotic-inducing agents. In addition to cytosolic enzymes, a glutathione transferaseunrelated to the cytosolic proteins is present in the endoplasmic reticulum.Further metabolic products of glutathione conjugations include mercapturic acids (acetylatedcysteine derivatives), which are the common excretory product. They are formed by sequential removalof glutamate and then glycine from the glutathione portion followed by acetylation of the amino groupof the residual cysteine. Other metabolic products are methylated thiols and sulfones. Episulfoniumions and thioketenes can be formed from glutathione adducts and are reactive enough to form adductswith cell macromolecules and cause toxicity.Phase II; Acetylation, Amino Acid Conjugation, and MethylationThe conjugations, involving acetylation of xenobiotics containing sulfonamide or amine groups,peptide conjugation of xenobiotics containing carboxylic acid groups, and methylation of xenobioticscontaining amine or catechol groups (Figure 3.3), do not contribute much to enhanced excretabilitythrough an increase in water solubility, but serve to mask reactive centers. A problem with some earlysulfonamides was that the acetylated metabolites were sufficiently less water-soluble that, theyprecipitated in the urine, resulting in renal damage. Both acetylations and amino acid conjugationsutilize coenzyme A as a cofactor and require the formation of a thioester with the carboxylic acid group,either of acetate or of the xenobiotic. The thioester then reacts with an amine, either on the xenobiotic(acetylation) or amino acid (amino acid conjugation). In mammals, glycine and glutamate are the aminoacids most commonly employed in xenobiotic conjugation, but taurine and aspartic acid conjugatesare occasionally used, and in birds, ornithine is often used. Methylation reactions require the formationof S-adenosylmethionine (SAM) from ATP and the amino acid, methionine.All the abovementioned conjugates can be deconjugated; deacetylases can remove acetyl groups,cytochrome P450 can remove methyl groups, and peptidases can split amino acid conjugates.Most conjugations occur to varying degrees in tissues other than the liver. Quantitatively they are oftenminor, but can be very important for protection from reactive metabolites generated in extrahepatic tissues.Factors Affecting Drug Metabolizing Capabilities With all that has been documented in this chapterso far, it is easy to overlook the fact that as in most biological systems, xenobiotic metabolism is adynamic situation undergoing constant change. Numerous factors affect the ability to catalyzexenobiotic metabolism. Many are an inherent property of the animal species or strain. In addition,these genetic differences may be further altered by such physiological factors as gender or age.Xenobiotic metabolism in different animal species differs quantitatively and qualitatively from that inhuman. Extrapolation from animals to human and the selection of the most appropriate animal modelis difficult unless the role of species and physiological factors in modulating metabolism is clearlydelineated. The contribution of these various factors is also an important consideration withinexperimental research when there is a need to compare or reproduce findings generated in differentlaboratories.Another factor of major concern is modification of xenobiotic metabolism by temporary stimuli,particularly chemical exposure. Typical human situations of chemical exposure can involve toxicaccidental exposures but originate most often from ingestion of prescribed medications or ingestionof chemicals in the food, either as contaminants or as naturally occurring dietary constituents. Thechanges in xenobiotic metabolizing capability can be in either positive or negative directions, and eachcan occur by more than one mechanism. The response can be generalized over many enzymescatalyzing many different reactions or can be specific for a single isozyme and a single reaction.