BINDING OF CATECHOLAMINES TO HUMAN SERUM ALBUMIN

¹ University of Pittsburgh School of Medicine, Department of Medicine, Section of Clinical Pharmacology, Pittsburgh, Pennsylvania

The binding of ¹⁴C-labeled norepinephrine and epinephrine to human serum albumin was studied at pH 7.4 and room temperature. Of the three methods used to determine the binding constants of this interaction, the equilibrium gel filtration of Hummel and Dreyer and the direct untrafiltration method yielded comparable results, whereas continuous ultrafiltration resulted in falsely high binding parameters. High-affinity binding was observed with an association constant for norepinephrine or approximately 10⁷ liters/mol. However, there was a low binding capacity (in the order of 10^-7 M). Epinephrine was bound to human serum albumin to the same extent as norepinephrine. This high-affinity, low-capacity protein represented only 0.03% of the total albumin by weight. Thus, the extremely low binding capacity suggested that the active species might not be albumin but rather a second protein. However, it was not possible to separate this high-affinity, low capacity binding protein from the main albumin species by gel permeation chromatography. There was no binding stereoselectivity for the l or d forms of norepinephrine. There was a specific binding affinity for catechols, which decreased as the hydroxyl groups were substituted or omitted. On the other hand, neither the nature of the alkyl side chain nor even its presence affected the binding affinity. From the data it is estimated that about 50% of a physiologic concentration of norepinephrine would be bound to protein in the plasma.