Free metabolites

Adding an excess of free metabolite diat will compete for the bonnd protein dissociates die protein from die chromatographic matrix. The protein pa.s.ses ont of the colnmn complexed with free metabolite. 

Excretion - Absorbed SP and 5-ASA and their metabolites are primarily eliminated in the urine either as free metabolites or as glucuronide conjugates. The majority of 5-ASA stays within the colonic lumen and is excreted as 5-ASA and acetyl-5-ASA with the feces. The calculated clearance of sulfasalazine following IV administration was 1 L/h. Renal clearance was estimated to account for 37% of total clearance. 

The relationship, if any, between the secondary metabolism of L-phenylalanine and carbohydrate degradation during brown-rot wood decay processes has not yet been determined. However, we suspect that the secondary metabolism of this aromatic amino-acid plays an important role in converting monomeric sugars to nitrogen-free metabolites (Shimada, M., and Takahashi, M., In Handbook of Wood and Cellulosic Materials Hon,... 

The residues in edible products of treated animals do not necessarily constitute the pharmaceutical compounds initially administered to the animals. They may consist of various components including the parent compound and/or free metabolites, and metabolites covalently bound to macromolecules. 

In general, safety evaluation is primarily based on the toxicological testing of the parent drug. However, consumers of edible animal products are also exposed to many other products of drug metabolism, including free metabolites of... 

Free metabolites produced by addition, cleavage, oxidation and reduction biotransformations of the parent compound... 

Other, less common amino acids also occur, either as constituents of proteins (through modification of common amino acid residues after protein synthesis) or as free metabolites. 

WA Moats, S A Buckley. Determination of free metabolites of ceftiofur in animal tissues with a liquid chromatographic cleanup. J AOAC Int 81 709-713, 1998. 

All the techniques described in this chapter contribute to improved metabolite characterization. Notably, these techniques will enable the generation of extended time profiles for plasma radioactivity, the assessment of free metabolite concentrations following plasma dialysis, and the characterization of metabolites at very low levels of exposure, for example, following inhaled or dermal routes of administration. Furthermore, the use of high-sensitivity techniques could impact the design of radiolabel studies, for example, by enabling a lower radiolabel dose to be administered to human subjects without compromising the ability to characterize the metabolic fate of the compound of interest. 

LC/MS/MS is an important technique for the analysis of free metabolites and covalent adducts of sulfur mustard in urine and blood. In the case of TDG and TDGO, LC/MS has not yet been able to achieve the LODs obtainable with GC/MS after derivatization. LC/MS/MS has, however, been used successfully to analyze the metabolites (20, 21) derived from an initial reaction of sulfur mustard with glutathione (see Chapter 16). The two metabolites (20), derived from the 3-lyase pathway, can be isolated from urine by SPE on a hydroxylated polystyrene-divinylbenzene polymeric cartridge. Using a sensitive triple sector quadrupole LC/MS/MS system, detection limits of O.lng/ml have been achieved using positive ESI and MRM (56). This provides a useful alternative to GC/MS/MS, which requires reduction of the sulfoxide functions with titanium trichloride. An LC/MS/MS method (detection limit lng/ml) has been developed for the analysis of the hisf N - ace ly I cysteine) metabolite (21) in urine (57). Concentration from acidified urine was achieved on... 

Methods for the Retrospective Detection of Exposure to Toxic Scheduled Chemicals. Part A Analysis of Free Metabolites... 

In addition to urinary metabolites, free metabolites may be detected in blood, provided sampling... 

An HPLC method for the more difficult measurement of plasma free metanephrines was first described in 1993. This method, like those involving measurements of plasma or urinary deconjugated metanephrines, requires a preana-lytical cation-exchange extraction and purification step. The low plasma concentrations of free metanephrines present several technical challenges. In particular, low levels of interfering substances, such as acetaminophen, tend to be more troublesome to measurements of plasma concentrations of the free metabolites than to the higher decon-... 

Bilde DD, Suteri PK, Ryzen E, Haddad JG. Serum protein binding of 1,25-dihydroxyvitamin D A reevaluation by direct measurement of free metabolite levels. I Clin Endocrinol Metab 1985 61 969-75. 

Proper sample preparation and analysis are essential in all studies of xenobiotic metabolism. With known standards of the free metabolites and often with just a radioactive unknown, it is customary to test their stability under conditions of analysis. The same caution is not always possible with conjugates, but much can be done to improve the validity of conjugate identification. Not only will greater caution in sample handling improve the chances of correctly identifying conjugates, it will also reduce the endless hours of time spent by metabolism chemists in inventing bizarre pathways to justify the formation of metabolites which never existed in the first place. 

Hydrolysis of the conjugated PAH metabolite extraction of the free metabolite into n-hexane concentration and methylation with methyl iodide extraction of methylated product... 

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