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Not norepinephrine but its oxidation products bind specifically to plasma proteins

F Boomsma, AJ Man in 't Veld and MA Schalekamp

Department of Internal Medicine I, University Hospital Dijkzigt, Erasmus University, Rotterdam, The Netherlands.

Specific binding (up to 50%) of catecholamines to the plasma proteins albumin, transthyretin and alpha-1 acid glycoprotein has been reported repeatedly, but data are conflicting. In binding studies of tritiated norepinephrine (NE) to human serum albumin, we found specific binding to be an artifact, due to the presence of one or more oxidation products of NE which are rapidly formed at pH greater than 6.5. In the presence of large amounts of antioxidants NE is stable, and no specific binding occurs. The decrease in binding of tritiated NE with increasing amounts of nontritiated NE, previously thought to represent displacement, is actually the result of the inherent greater stability of NE at higher concentrations. A similar decrease in binding, this time representing real displacement, was obtained with increasing amounts of oxidation products, obtained by treating NE first with sodium hydroxide. Results with transthyretin, alpha-1 acid glycoprotein and with plasma were in full agreement with these conclusions. These observations emphasize the necessity for careful investigation of the purity and stability of the labeled ligand, not only in binding assays with catecholamines, but in binding assays in general. Our findings may also have bearings on the currently hotly pursued topic of catecholamine release in in vivo and in vitro experiments.

Volume 259, Issue 2, pp. 551-557, 11/01/1991
Copyright © 1991 by American Society for Pharmacology and Experimental Therapeutics




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Copyright © 1991 by the American Society for Pharmacology and Experimental Therapeutics.