Endogenous species

In phase 1, the pollutant is converted into a more water-soluble metabolites, by oxidation, hydrolysis, hydration, or reduction. Usually, phase 1 metabolism introduces one or more hydroxyl groups. In phase 2, a water-soluble endogenous species (usually an anion) is attached to the metabolite— very commonly through a hydroxyl group introduced during phase 1. Although this scheme describes the course of most biotransformations of lipophilic xenobiotics, there can be departures from it. 

The anion gap is the concentration of plasma anions not routinely measured by laboratory screening. It is useful in the evaluation of acid-base disorders. The anion gap is greater with increased plasma concentrations of endogenous species (e.g., phosphate, sulfate, lactate, and ketoacids) or exogenous species (e.g., salicylate, penicillin, ethylene glycol, ethanol, and methanol). The formulas for calculating the anion gap are as follows ... 

Tissue also contains some endogenous species that exhibit fluorescence, such as aromatic amino acids present in proteins (phenylalanine, tyrosine, and tryptophan), pyridine nucleotide enzyme cofactors (e.g., oxidized nicotinamide adenine dinucleotide, NADH pyridoxal phosphate flavin adenine dinucleotide, FAD), and cross-links between the collagen and the elastin in extracellular matrix.100 These typically possess excitation maxima in the ultraviolet, short natural lifetimes, and low quantum yields (see Table 10.1 for examples), but their characteristics strongly depend on whether they are bound to proteins. Excitation of these molecules would elicit background emission that would contaminate the emission due to implanted sensors, resulting in baseline offsets or even major spectral shifts in extreme cases therefore, it is necessary to carefully select fluorophores for implants. It is also noteworthy that the lifetimes are fairly short, such that use of longer lifetime emitters in sensors would allow lifetime-resolved measurements to extract sensor emission from overriding tissue fluorescence. 

Endogenous substances in the extracts are more polar than the cannabinoids and elute before them on the reverse phase column. On polar, normal phase columns, strong adsorption of endogenous species requires periodic column clean-up. This problem was not encountered with the reverse phase gradient system. 

In order to cope with the lack of CRMs for endogenous species, the practice of laboratory internal QC materials is developing. A Laboratory Reference Material (LRM) was prepared from Brazil nuts (Bertholletia excelsa) for QC of the selenomethionine determinations. The concentration of selenomethionine was 79.9 p,g g-1. The homogeneity and stability of this candidate CRM passed the relevant tests recommended by the Measurement and Testing Programme [97],... 

There are two critical features of the texaphyrins that led to the consideration that it could function as a radiation sensitizer. First, this class of molecules, like the porphyrins, is known to localize with high selectivity in cancerous tissues. Second, the texaphyrins are known to contain a low-lying LUMO in comparison to porphyrins and most other endogenous species, they are thus very easy to reduce... 

In the SIM mode, the MS is tuned to a particular m/z window (preferably at unit resolution), which corresponds to the ion of interest (i.e., [M + H]+, or a stable adduct such as [M + X]+, where X = Na, K, NFLt, etc.). SIM may require a more elaborate chromatographic separation in order to minimize interference from endogenous species. However, in the SRM approach, higher selectivity and sensitivity are realized. Thus, shorter chromatographic runs (faster injection cycles) and hmited sample pretreatment could be tolerated without significant loss in sensitivity. 

In the study on indinavir in human plasma [21], two sample pretreatment methods, i.e., dilution of urine and LLE with MTBE, two chromatographic systems, i.e., with low and high capacity factor, and the use of ILIS and ANIS were compared. It was concluded that the potential effect of co-eluting unseen endogenous species should be evaluated during method development and validation of bioanalytical LC-MS methods. 

Biofluids contain many endogenous species that can partidpate in chemical exchange processes covering a variety of exchange time scales. These... 

Recent clearance studies use stable isotope dilution methods. MS methods make it possible to measure deuterated and endogenous species simultaneously and specifically. LC-MS methods offer advantages over GC-MS methods, such as streamlined sample preparation (no derivatization necessary), high recovery and sensitivity and superior specificity. 

The advantage of using an internal standard is its simplicity and accuracy resulting from its being processed and analyzed simultaneously with the analyte of interest. However, selection of an appropriate internal standard might be difficult because different systems may need different internal standards and specifically synthesized standards may be necessary to avoid any potential overlap with endogenous species in the analyzed system. Moreover, addition of an appropriate amount of internal standard is not straightforward, but needs some expertise and predetermination (see Section 13.3.2). 

Formed below the surface bFormed mainly via oxidation of endogenous species. 

Phase II reactions which conjugate the dmg or its phase I metabolite with a hydrophilic, endogenous species (conjugation reactions)... 

Catecholamines and indolamines are present in the central nervous system and different physiological and behavioral functions of the brain are based on these chemical messengers. These species are actively involved in the neurotransmission processes of the CNS. Neurosecretion is associated with the metabolic activities which occur in the brain. These metabolic processes stimulate pH fluctuations in the CNS. Thus, secretion of neurotransmitters is accompanied with endogenous species such as protons or ascorbate ions which alter the behavior of redox-active chemical messengers, e.g., catecholamines and indolamines. ... 

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