alpha 1-acid glycoprotein involvement in high affinity binding of tricyclic antidepressants to human plasma

The binding of the tricyclic antidepressants imipramine (IMI) and desmethylimipramine (DMI) to human plasma and individual proteins was studied by equilibrium dialysis. Both drugs bound extensively to plasma, albumin, and alpha 1-acid glycoprotein, while there was very little binding to the gamma-globulin fraction. The binding of both IMI and DMI to alpha 1-acid glycoprotein was high affinity (association constant K, 9.2 X 10(4)/M and 4.7 X 10(4)/M respectively) and low capacity (number of binding sites, n = 1 for both IMI and DMI), whereas the binding to albumin was low affinity (K for IMI, 2.3 X 10(2)/M and for DMI, 3 X 10(2)/M) and high capacity (n = 7). The binding of IMI to a mixture of human serum albumin and alpha 1-acid glycoprotein revealed two sets of binding sites; a high affinity binding site corresponding to alpha 1-acid glycoprotein and a low affinity binding site corresponding to albumin. The binding affinity and/or number of binding sites for IMI binding to albumin decreased with increasing albumin concentrations. The free fraction in plasma of nineteen normal, male controls was significantly correlated with the concentration of alpha 1-acid glycoprotein (r = 0.601, P less than 0.01), although there was no correlation with albumin or free fatty acid concentrations in plasma.