Endogenous reference compounds

Brunner M, Joukhadar C, Schmid R, Erovic B, Eichler HG, Muller M. Validation of urea as an endogenous reference compound for the in vivo calibration of microdialysis probes. Life Sciences 2000, 67, 977-984. 

Trace amines are a family of endogenous monoamine compounds including (3-phenylethylamine (PEA), p-tyramine (TYR), tryptamine (TRP) and octopamine (OCT). The trace amines share close structural similarity with the well known classical monoamine neurotransmitters such as dopamine (DA), norepinephrine (NE) and serotonin (5-HT). As their name suggests, trace amines occur in comparably much lower abundance than monoamine neurotransmitters. For historical reasons, other endogenous amine compounds which might share some structural similarities with PEA, TYR, TRP or OCT are not referred to as trace amines. 

When the identify of the inhibitory activity present in a sample is unknown (as is frequently the case when RRAs are used to screen for novel substances), the absolute amount of the substance cannot be determined from an RRA. Nevertheless, RRAs are useful to detect the presence of unidenhfied substances m tissue extracts, to monitor the success of purification techniques used to concentrate or isolate an unknown agent, or to detect the dilution or loss of the inhibitory activity during separation or purification attempts Even in instances when the identity of the inhibitory activity present in a sample is known, as is the case for the RRA for endogenous opiates (Childers et al., 1977, Simantov et al., 1977), the concentration of the substance may remain indeterminant, since this assay does not discriminate between the various opioid peptides that occur in brain. In either of these latter situations, the difficulty m determining the absolute amounts of the inhibitory activity present in the initial sample is circumvented by expressing the displacement of specifically bound ligand in arbitary units of equivalence relative to a reference compound. 

The identification of GA3 in vegetative shoots of maize and the establishment of its biogenesis from GA20 by [ C]-labelling and GC-MS, firmly establishes that GA3 is an endogenous plant gibberellin. Previously identifications of GA3 have been uncertain because of the possibility of contamination of plant extracts by the ubiquitous use of GA3 as a reference compound. Previously the formation of [ H]GA3 from [ H]GAs has been reported on the basis of GC-RC [6] or GC-SIM [3, 4,16,17] in several plant tissues in which either GA5 and GA3 were known to occur [6, 16] or only GA5 had been identified [3, 4, 17]. 

Hablitz JJ (1984) Picrotoxin-induced epileptiform activity in hippocampus role of endogenous versus synaptic factors. J Neurophysiol 51 1011-1027 Htmmel HM (2008) Safety pharmacology assessment of central nervous system function in juvenile and adult rats effects of pharmacological reference compounds. J Pharmacol Toxicol Methods 58 129-156... 

Since the last edition of this handbook, HPLC has become more common and many compounds are now routinely analyzed with HPLC. However, GC is still very practical for the analysis of many compounds of interest to neurochemists. Methods published since 1982 are the primary focus of this chapter, and representative examples for many compounds, both endogenous and exogenous, are included here. For a review of information prior to 1982, readers are referred to Coutts and Baker (1982). 

The use of GC with capillary columns and sensitive and selective detectors permits the analysis of a wide variety of compounds, both endogenous and exogenous, that are of interest to neurochemists. The detectors covered in this chapter do not provide the same degree of selectivity and sensitivity that MS does however, when used with reference samples for verification they are more than satisfactory for routine analysis. 

One of the early steps in an allergic reaction consists in the release of a series of endogenous compounds referred to as mediators from sensitized cells. The finding in the early 1960s that cromolyn sodium, still the only approved drug of this class, blunts this reaction has led to an intense search for additional examples. It is of interest that a relatively simple anthranilic acid derivative has shown mediator-release inhibiting activity. Reaction of 3,4-dimethoxybenzalde-hyde (167) with isatoic anhydride 168 gives the condensation product 169, which, upon hydrolysis, affords tranilast (170) (43]. 

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