Metabolites, definition

Tea flavonoids, or tea extracts, have been linked to benefits in reducing the risk of certain cancers and cardiovascular diseases in experimental animals. However, epidemiological studies have produced inconsistent evidence in the relationship between tea drinking and cancer (Blot et a/., 1997 Goldbohm etal, 1996 Hertog eta/., 1997 Yang eta/., 1996). Therefore, further research is needed before definitive conclusions on the impact of tea consumption upon the cancer risk in humans can be reached. The metabolites of catechins and flavonols after consumption of tea infusions have scarcely been investigated, and thus more research is needed as to the role of those compounds in the reported health benefits of tea consumption. 

If the relevant residue cannot be properly determined using a routine multi-method, an alternative method must be proposed. In the case of residues consisting of a variety of structurally related compounds, a common moiety method may be acceptable in order to avoid the use of an excessive number of methods for individual substances. For example, the relevant residue of isoproturon in plant material is defined as the sum of isoproturon and all metabolites containing the 4-isopropylaniline group. Therefore, residues are determined following hydrolysis as 4-isopropylaniline and are expressed as 4-isopropylaniline equivalents. It is not necessary to validate the method individually for all possible metabolites which are covered by the residue definition (e.g., all metabolites which contain the 4-isopropylaniline group), provided that it is demonstrated that in the first step, the conversion to the common moiety is complete. However, common moiety methods often lack sufficient specificity, and should therefore be avoided if possible. If need be, their use must be justified. 

The method must be capable of determining all components (a.i. and relevant/major metabolites) that are included in the residue definitions used in the assessment of risk to nontarget organisms. For ground (drinking) water and air, the risk to con-sumers/operators or bystanders must also be considered. 

The activities of enforcement laboratories should not be focused on irrelevant problems. Therefore, a clear definition of the relevant residue is needed. In the crops and food sector, procedures are well established to derive the two residue definitions, one for risk assessment and one for monitoring, from metabolism studies. As far as environmental samples are concerned, there is much potential for improvement. There are no clear criteria as to which metabolites should be included in monitoring and control programs. Additionally, the development of criteria for nonpriority pesticides, e.g., naturally occurring compounds or low-risk products, which can be excluded from monitoring exercises would be helpful for laboratories and evaluators. 

The toxic effects of selected plant analytes will be assessed by comparison with the toxicides of similar metabolites found in animal metabolism studies. The amount of the analytes reported in the plant metabolism study is one of the important factors used to establish the residue definition. 

Prior to registration, an agreed commitment to the residue component(s) which should be analyzed does not exist. This is contrary to the situation with residue methods, which are developed after MRL setting. Therefore, to establish an acceptable residue definition is the first step necessary prior to any method development. This residue definition for enforcement methods is based on the results of metabolism studies and may cause serious difficulties, especially if the metabolic pathways of the parent compound are very complex, generating a large number of metabolites. 

Owing to the potential for low levels of residues (parent plus metabolites) in crop tissues, the definition of dinitroaniline residues in crop samples is expressed as the parent molecule only. 

Sulfonylurea herbicides are generally applied to crops as an early post-emergent herbicide. Crops that are tolerant to these herbicides quickly metabolize them to innocuous compounds. At maturity, residues of the parent compound in food and feed commodities are nondetectable. Metabolites are not considered to be of concern, and their levels are usually nondetectable also. For this reason, the residue definition only includes the parent compound. Tolerances [or maximum residue limits (MRLs)] are based on the LOQ of the method submitted for enforcement purposes and usually range from 0.01 to 0.05 mg kg (ppm) for food items and up to O.lmgkg" for feed items. There is no practical need for residue methods for animal tissues or animal-derived products such as milk, meat, and eggs. Sulfonylurea herbicides are not found in animal feed items, as mentioned above. Furthermore, sulfonylurea herbicides intentionally dosed to rats and goats are mostly excreted in the urine and feces, and the traces that are absorbed are rapidly metabolized to nontoxic compounds. For this reason, no descriptions of methods for animal-derived matrices are given here. 

The crop residue definition includes parent flucarbazone-sodium and the A/-desmethyl flucarba-zone metabolite (I). The soil and water residue definitions include parent flucarbazone-sodium and the sulfonic acid, sulfonamide and A/,0-dimethyl triazoli-none (NODT) metabolites... 

Regulatory position The definition of residues includes orbencarb and its metabolites, methyl 2-chlorobenzylsulfone (1) and 2-chlorobenzoic acid (II)... 

Reguiatory position The residue definition includes terbacil and its three primary metabolites. Metabolite A, Metabolite B and Metabolite C... 

The definition of crop residues for thiamethoxam and thiacloprid includes the parent and its metabolite fV-(2-chlorothiazol-5-ylmethyl)-fV -methyl-A"-nitroguanidine (designated the guanidine compound) for thiamethoxam, and 3-(6-chloro-3-pyridylmethyl)-l,3-thiazolidin-2-ylideneaminocarboxamide (designated the amide compound) for thiacloprid (Figure 3). 

The polar character of neonicotinoids makes them, in general, potentially mobile in soil. Acetamiprid and nitenpyram have short soil persistence. Imidacloprid and thi-amethoxam, however, are sufficiently persistent in soil to be used for soil treatment. The definition of soil residues for the various neonicotinoid compounds except for imidacloprid are the parent compound and it metabolites. The metabolites of acetamiprid are lM-1-2, lM-1-4 and lC-0 (Figure 6). The metabolites of nitenpyram are 2-[N-(6-chloro-3-pyridyl-methyl)-A-ethyl]amino-2-methyliminoacetic acid (CPMA) and A-(6-chloro-3-pyridylmethyl)-Ai-ethyl-A -methylformamidine] (CPMF). 

The definition of water residue for the neonicotinoid insecticides except for niten-pyram and thiacloprid is the parent molecule. For nitenpytram and thiacloprid both the parent and its metabolites are determined. These metabolites are 2-[A-(6-chloro-3-pyridylmethyl)-A-ethyl]amino-2-methyliminoacetic acid (CPMA) and... 

Oxime carbamates are generally applied either directly to the tilled soil or sprayed on crops. One of the advantages of oxime carbamates is their short persistence on plants. They are readily degraded into their metabolites shortly after application. However, some of these metabolites have insecticidal properties even more potent than those of the parent compound. For example, the oxidative product of aldicarb is aldicarb sulfoxide, which is observed to be 10-20 times more active as a cholinesterase inhibitor than aldicarb. Other oxime carbamates (e.g., methomyl) have degradates which show no insecticidal activity, have low to negligible ecotoxicity and mammalian toxicity relative to the parent, and are normally nondetectable in crops. Therefore, the residue definition may include the parent oxime carbamate (e.g., methomyl) or parent and metabolites (e.g., aldicarb and its sulfoxide and sulfone metabolites). The tolerance or maximum residue limit (MRL) of pesticides on any food commodity is based on the highest residue concentration detected on mature crops at harvest or the LOQ of the method submitted for enforcement purposes if no detectable residues are found. For example, the tolerances of methomyl in US food commodities range from 0.1 to 6 mg kg for food items and up to 40 mg kg for feed items. ... 

Regulatory position The residue definition is for both imibenconazole and its primary metabolite (imibenconazole-debenzyl)... 

The residue definition include mepanipyrim and its hy-droxylated metabolite 1-(2-anilino-6-methylpyrimidin-4-ynyl)-2-propanol (abbreviated as propanol form)... 

The residue definition includes alanycarb and its metabolite, methomyl (determined as methomyl oxime)... 

The residue definition includes benfuracarb and two of its metabolites, carbofuran and 3-hydroxycarbofuran... 

Studies with rats and chickens given oral doses of TOCP and tn-/ ara-cresyl phosphate provide more definitive evidence that, following absorption, organophosphate esters in hydraulic fluids (or their metabolites) may be widely distributed among tissues with a preferential distribution to fatty tissues, the liver, and the kidneys (Abou-Donia et al. 1990a, 1990b Kurebayashi et al. 1985 Somkuti and Abou-Donia 1990 Suwita and Abou-Donia 1990). 

In the case of the methylated xanthines, particularly theophylline, theobromine and caffeine, the preponderance of data on the metabolism of these compounds in man suggests that a methylated uric acid is the principal product. However, the data presented earlier proposes at best a 77 per cent accounting of the methylated xanthine administered. The question can be raised as to whether the final products observed upon electrochemical oxidation of these compounds aids these studies. Very recently studies of metabolism of caffeine have revealed that 3,6,8-trimethylallantoin is a metabolite of caffeine 48>. This methylated allantoin is, of course, a major product observed electrochemically. The mechanism developed for the electrochemical oxidation seems to nicely rationalize the observed products and electrochemical behavior. The mechanism of biological oxidation could well be very similar, although insufficient work has yet been performed to come to any definite conclusions. There is however, one major difference between the electrochemical and biological reactions which is concerned with the fact that in the former situation no demethylation occurs whereas in the latter systems considerable demethylation appears to take place. 

Another refinement, that avoids the necessity of developing suitable fecal extraction and chromatographic methods, is to dose the radiolabeled compound by both the i.v. and p.o. routes in two separate studies. Knowing that, by definition, the whole of the i.v. dose must have been bioavailable, a comparison of the proportion of the dose in the urine after the two different routes allows estimation of the percent absorbed. An analogous approach can be used without the use of a radiolabel, when the urine from the two studies is analyzed either for the parent compound or, more usually, for a major common metabolite. Assuming quantitatively identical clearance after both the i.v. and p.o. doses, the ratio of the amounts of analyte in the two experiments gives the absorption. 

Growth and diet are subject to seasonal changes. The presence (or absence) and relative concentrations of secondary metabolites in the extracts of organisms are factors which chemists should document along with observations made during collection. Often, in preliminary experiments guided by bioassay, mixtures, rather than individual compounds, are tested. Association of a certain activity with structurally related compounds may serve as a lead for further tests, but should not be considered definitive. 

Biogenetic considerations [71], and a correlation of all polypropionates from pulmonate molluscs led to revision of the relative stereochemistry of pectinatone (20) and nor-pectinatone (21), metabolites of the pulmonate Siphonaria pectinata [72, 73] the point had already been questioned by Oppolzer s synthesis [74] and was definitively confirmed by two independent X-ray diffraction studies [71, 75] of pectinatone (20). 

Another example of a nudibranch, which probably modifies dietary metabolites to obtain more effective allomones, is seen in Aldisa cooperi (= A. sanguinea cooperi) [155]. It elaborates two fish antifeedant bile acids (104,105) that are absent in its prey, the sponge Anthoarcuata graceae, where the main steroid is 4-cholesten-3-one (106). Biosynthetic experiments starting from both labelled mevalonic acid and labelled 4-eholesten-3-one would definitely clarify, whether, the two allomones (104-105) are biosynthetized de novo by the mollusc, or if they are derived from a food source. 

These are the same quinones that are formed when 6-hydroxy-BP is oxidized by air or microsomes (19). However, there is no definitive evidence that 6-hydroxy-BP is an intermediate in their formation by PGH synthase. Among all of the stable metabolites of BP, the quinones are distinctive because, unlike phenols and dihydrodiols, they are not derived from arene oxides. Thus, arene oxides do not appear to be products of BP oxidation by PGH synthase (19,20). 

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