Effects of human serum albumin concentration on binding of [3H]amphetamine, [3H]atropine, [3H]epinephrine, and [14C]histamine

Abstract

The binding of [3H]amphetamine, [3H]atropine, [3H]epinephrine, and [14C]histamine to human serum albumin was examined with a constant ligand concentration and varying albumin concentrations. Two concentrations of each ligand were studied. Scatchard plots of the binding data were linear and had positive slopes; the slopes were greater at the higher ligand concentration. The lowest point was for the highest albumin level and increased in sequence to the lowest albumin concentration. The β (fraction bound) value increased with an increasing amount of albumin in the system. The change in β per change in albumin concentration [d(β)/d(albumin)] when plotted against albumin concentration yielded curves with negative slopes; the slope was greatest at the lower albumin levels and was much less at the higher albumin levels (equivalent to those encountered in human blood). In agreement with literature data for other systems, the nK value (number of binding sites times the association constant) decreased with increasing albumin concentration. In previous studies of the same ligands and protein, when the albumin concentration was kept constant and the ligand concentration was varied, the Scatchard plots had positive slopes. Positive slopes also were found in this study with constant ligand concentrations and varying albumin levels. These results suggest cooperativity in the binding of these ligands to human serum albumin.