Letter: Interference with cholesterol determination.

A new method for the simultaneous quantitation of tacrine and the three metabolites, 1-hydroxytacrine (velnacrine, maleate), 2-hydroxytacrine and 4-hydroxytacrine, in human plasma and urine has been developed. The method was based on simple one-step liquid-liquid extraction with ethyl acetate followed by isocratic, reversed-phase high-performance liquid chromatography and fluorescence detection (excitation: 330 nm and emission: 365 nm). The limit of detection in plasma was 0.5 nM for 2-hydroxytacrine and 4-hydroxytacrine, 2 nM for 1-hydroxytacrine and tacrine. In urine it was 60 nM for 2-hydroxytacrine and 4-hydroxytacrine, 30 nM for 1-hydroxytacrine and 80 nM for tacrine. The limit of quantification in plasma was 2.5 nM for 2-hydroxytacrine and 4-hydroxytacrine, 10 nM for 1-hydroxytacrine and 2 nM for tacrine. In urine it was 120 nM for all components. The overall mean recoveries ranged from 84 to 105% in plasma and from 64 to 100% in urine for all four compounds.

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PubMed1998

A sequence within the cytoplasmic tail of GpIIb independently activates platelet aggregation and thromboxane synthesis.

All integrin alpha subunits contain a highly conserved KXGFFKR motif in their cytoplasmic domains that plays a crucial role in the regulation of integrin affinity for their ligands. We show that a lipid-modified peptide corresponding to the cytoplasmic region, 989-995, of the platelet integrin subunit glycoprotein GpIIb (alphaIIb), palmitoyl-KVGFFKR (Ppep; 10 microM), but not a similarly modified scrambled peptide (palmitoyl-FKFVRGK), can specifically induce platelet activation and aggregation equivalent to that of strong agonists such as thrombin. Ppep-induced aggregation is also associated with indices of platelet activation including thromboxane A2 (TXA2) synthesis (EC50 = 45 +/- 5 microM), secretion of alpha-granules detected as enhanced surface expression of P-selectin (EC50 = 52 +/- 8 microM), and conformational changes in GpIIb/IIIa measured by the monoclonal antibody, PAC-1 (EC50 = 3.7 +/- 1 microM). The TXA2 receptor antagonist, SQ29548, PGE1, and the ADP scavenger, apyrase, differentially inhibit the aggregation response and TXA2 synthesis in response to Ppep. Similarly, GpIIb/IIIa antagonists (RO-449883 and integrelin), which inhibit aggregation by greater than 90%, have little effect on peptide-induced TXA2 synthesis, suggesting that this event is independent of fibrinogen binding to GpIIb/IIIa. Alanine-stepping of the Ppep sequence identifies GFFK(991-994) as the critical residues in all peptide-mediated events. We conclude that this peptide can imitate the cytoplasmic domain of GpIIb and initiate parallel but independent signaling pathways, one leading to ligand binding and platelet aggregation and the other to intracellular signaling events such as TXA2 synthesis and secretion.

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