Serum albumin and lipoproteins as the quinidine binding molecules in normal human sera

Abstract

To substantiate the binding of quinidine in human sera and predict variations of binding dissociation constants and number of binding sites were determined for separate serum proteins. Human sera were fractionated by gel filtration and ultracentrifugation, and binding was evaluated by equilibrium dialysis at pH 7·30 at 20° and 37° in a Krebs-Ringer phosphate buffer. Quinidine was bound to all serum lipoproteins and to serum albumin. The binding was influenced by the buffer composition. In sodium phosphate buffer there were two separate binding sites for quinidine on LDL and HDL, while there was only one detectable binding site on VLDL and HDL in a Krebs-Ringer phosphate buffer. On LDL also there appeared to be one binding site but it exhibited a positive cooperative binding effect at lower concentrations of quinidine. This effect was assumed to be caused by inorganic ions of the Krebs-Ringer phosphate buffer. At a therapeutic level of quinidine in normal human serum the concentration of quinidine bound to serum proteins was 1·062 × 10 −5 M. Calculated from the evaluated binding parameters VLDL contributed with 0·101 × 10 −5 M of this binding, LDL with 0·143 × 10 −5 M, HDL with 0·083 × 10 −5 M and albumin with 0·699 × 10 −5 M.