Xenobiotic compounds definition

Phosphatases are numerous and important enzymes (see also Chapt. 2). They are classified as phosphoric monoester hydrolases (phosphatases, EC 3.1.3), phosphoric diester hydrolases (phosphodiesterases, EC 3.1.4), triphosphoric monoester hydrolases (EC 3.1.5), diphosphoric monoester hydrolases (pyrophosphatases, EC 3.1.7), and phosphoric triester hydrolases (EC 3.1.8) [21] [63]. Most of these enzymes have a narrow substrate specificity restricted to endogenous compounds. However, some of these enzymes are active toward xenobiotic organophosphorus compounds, e.g., alkaline phosphatase (EC 3.1.3.1), acid phosphatase (EC 3.1.3.2), aryldialkylphosphatase (para-oxonase (PON1), EC 3.1.8.1) and diisopropyl-fluorophosphatase (tabunase, somanase, EC 3.1.8.2) [64 - 70]. However, such a classification is far from definitive and will evolve with further biochemical findings. Thus, a good correlation has been found in human blood samples between somanase and sarinase activities on the one hand, and paraoxonase (PON1) type Q isozyme concentrations on the other [71]. 

The definition of a poison, or toxicant, also involves a qualitative biological aspect because a compound, toxic to one species or genetic strain, may be relatively harmless to another. For example, carbon tetrachloride, a potent hepatotoxicant in many species, is relatively harmless to the chicken. Certain strains of rabbit can eat Belladonna with impunity while others cannot. Compounds may be toxic under some circumstances but not others or, perhaps, toxic in combination with another compound but nontoxic alone. The methylenedioxyphenyl insecticide synergists, such as piperonyl butoxide, are of low toxicity to both insects and mammals when administered alone but are, by virtue of their ability to inhibit xenobiotic-metabolizing enzymes, capable of causing dramatic increases in the toxicity of other compounds. 

Data shown in Table 3 indicate a definite increase of chlorine content in the products of interaction between HAs and chlorophenoxy compounds, thus suggesting the occurred incorporation of xenobiotic chlorinated units into the humic polymer. 

Table IV lists structures of some uncommon conjugates to which NMR has been applied in characterizations. Both the xenobiotics and the conjugates are of diverse structural types. Ecdysone-type compounds, although not xenobiotics in the strict definition of the term, are included because they represent conjugation with a phosphate group.

Apart from these obviously biogenic compounds only very few definite anthropogenic substances were identified. Within the group of xenobiotics 2,4 - and 4,4 -dichlorophenylmethane, DDT-derived metabolites, were most abundant. A pyrolytic conversion of related DDT metabolites to DDM cannot be excluded. Thus the occurrence of DDM in the pyrogram is only suggestive for DDT group metabolites in general. 

Since almost all of the long-chain PFCs tested are PPAR-a agonists and/or induce xenobiotic-metabolizing enzymes in the liver, PFCs may be expected to induce some or all of the neoplastic effects seen with PFOA in rodents. There is currently insufficient information to definitively classify long-chain PFCs as human carcinogens. More complete characterization of their mechanisms of action in humans versus rodents will be necessary in order to determine whether this class of compounds poses a carcinogenic hazard to humans at current cumulative levels of exposure. 

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