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Toxicological Chemistry
Published in Stanley E. Manahan, Environmental Chemistry, 2022
A Phase II reaction occurs when an endogenous species is attached by enzyme action to a polar functional group that often, though not always, is the result of a Phase I reaction on a xenobiotic species. Phase II reactions are called conjugation reactions in which enzymes attach conjugating agents to xenobiotics, their Phase I reaction products, and nonxenobiotic compounds (Figure 22.7). The conjugation product of such a reaction is usually less toxic than the original xenobiotic compound, less lipid-soluble, more water-soluble, and more readily eliminated from the body. The major conjugating agents and the enzymes that catalyze their Phase II reactions are glucuronide (UDP glucuronosyltransferase enzyme), glutathione (glutathione S-transferase enzyme), sulfate (sulfotransferase enzyme), acetyl (acetylation by arylamine N-acetyltransferase enzymes), and methyl groups (methyltransferases).4 The most abundant conjugation products are glucuronides. A glucuronide conjugate is illustrated in Figure 22.8, where –X–R represents a xenobiotic species conjugated to glucuronide and R is an organic moiety. For example, if the xenobiotic compound conjugated is phenol, HXR is HOC6H5, X is the O atom, and R represents the phenyl group –C6H5.
Streptomyces Host-Vector Systems
Published in Yoshikatsu Murooka, Tadayuki Imanaka, Recombinant Microbes for Industrial and Agricultural Applications, 2020
Puromycin is an aminoacyl-adenosine antibiotic that is produced by S. alboniger. The antibiotic, a functional analogue of aminoacyl tRNA, inhibits protein synthesis by substituting for the incoming coded aminoacyl tRNA, and serving as acceptor for the nascent peptide chain of ribosome-bound peptidyl tRNA [48]. An enzyme O-demethylpuromycin-O-methyltransferase is present, which converts O-demethylpuromycin to puromycin in extracts of S. alboniger. We and other groups found that the producer organism possesses a puromycin-inactivating enzyme (puromycin 2'-N-acetyltransferase) that may have a role in self-resistance [49-51]. The genes coding for the enzyme and O-demethylpuromycin-O-methyltransferase have been cloned and are present on the same 2.4-kb DNA fragment [52].
Biological and biological-effect monitoring
Published in Sue Reed, Dino Pisaniello, Geza Benke, Kerrie Burton, Principles of Occupational Health & Hygiene, 2020
Biological monitoring techniques may be used to identify susceptible individuals. For example, the ratio of the activities of two blood enzymes, paraoxonase and arylesterase, may be used to determine potential susceptibility to the toxic effects of organophosphorus insecticides. A high paraoxonase/arylesterase ratio suggests more rapid metabolism of OP compounds and greater resistance to adverse effects. A low ratio suggests a greater risk of adverse effects following exposure. Similarly, low activity of the enzyme N-acetyltransferase is implicated in the development of bladder cancer in workers exposed to aromatic amines. The development of these indicators, and their use in pre-employment screening and workplace exclusion practices, is beyond the scope of this book. For further details, see Australasian Faculty of Occupational Medicine (2003).
Xenobiotic metabolism and transport in Caenorhabditis elegans
Published in Journal of Toxicology and Environmental Health, Part B, 2021
Jessica H. Hartman, Samuel J. Widmayer, Christina M. Bergemann, Dillon E. King, Katherine S. Morton, Riccardo F. Romersi, Laura E. Jameson, Maxwell C. K. Leung, Erik C. Andersen, Stefan Taubert, Joel N. Meyer
Humans have two N-acetyltransferase enzymes that are xenobiotic-metabolizing, NAT1 and NAT2. To date, no N-acetyltransferase genes in C. elegans that are homologous to the human xenobiotic-metabolizing NATs have been identified. In addition to the 4 major families of phase II enzymes described here, it is also possible to conjugate xenobiotics to other cellular substrates including amino acids, but these reactions are less studied and less common.