Buffer and pH effects on propranolol binding by human albumin and alpha 1-acid glycoprotein

Propranolol binding to isolated human alpha-1-acid glycoprotein (AGP) and human albumin (HSA) was studied by equilibrium dialysis at 37 degrees C. With AGP (0.067%) and HSA (4%), total propranolol concentration was varied from 0.7 to 93,000 ng mL-1. Over this concentration range the percentage drug bound to HSA declined from 49 to 39% while that to AGP declined from 68 to 4%. Two classes of sites were identified on AGP with n1k1 = 8.50 X 10(4) M-1 and n2k2 = 3.12 X 10(4) M-1. With a pH 7.4 phosphate buffer, propranolol binding to AGP was greatest when the protein was initially dissolved in pH 7.4 water compared with pH 7.2 water or the phosphate buffer. Thus, the method of AGP solution preparation affected propranolol binding by this protein. For both AGP and HSA, greater drug binding was noted with phosphate buffers in comparison with a physiological buffer. With phosphate buffers, decreasing pH from 7.4 to 7.0 decreased propranolol binding by AGP, while decreasing pH from 7.7 to 7.4 had little effect. With HSA, the percent propranolol bound consistently decreased on lowering pH from 7.7 to 7.0.