Quinidine and propranolol binding to very low and low density lipoproteins of human plasma

The interaction of quinidine (Q) and propranolol (P) with human very low density lipoproteins (VLDL) and low density lipoproteins (LDL) was determined by ultrafiltration in phosphate buffer (pH 7.4) for Q and in phosphate and Tris buffers (pH 7.4-7.5) for P, respectively. The Scatchard plots of Q are curved and were best described by a model assuming two independent classes of binding sites on lipoproteins. When working with P, the Scatchard plots were also nonlinear, but positive cooperativity was observed for the VLDL fraction in phosphate buffer and apparently also for the LDL fraction in Tris buffer. These nonlinear curves were described by the model used for Q, excluding any data falling in the cooperating region. The binding parameters, primary (K1) and secondary (K2) affinity constants and the number of sites in each class of independent binding sites, were generated by a computer program. The results of this in vitro study suggest that drug binding (and possibly distribution) to lipoproteins may be affected by the nature and concentration of some ions present in serum.