Effect of the sialylation state of alpha 1-acid glycoprotein on propranolol binding.

alpha 1-Acid glycoprotein (AAG) exists in a variety of sialylated states which can be influenced by certain disease states. The purpose of the present study was to establish the effect of the sialylation state of AAG on the binding of a model cationic drug (propranolol). Extensive in vitro enzymatic desialylation of isolated human AAG was achieved utilizing neuraminidase bonded to agarose beads. Propranolol binding to native and desialylated AAG was determined by equilibrium dialysis. Desialylated AAG exhibited a modest increase in propranolol free fraction due to a decrease in the affinity constant (Ka) compared to untreated AAG (3.3 X 10(5) M-1 versus 4.0 X 10(5) M-1, respectively). A modest 15% increase was predicted for the free fraction of propranolol in serum containing desialylated AAG at therapeutic propranolol concentrations (less than 0.1 microgram/ml). Therefore, the clinical significance of desialylated AAG appears to be minor with respect to propranolol binding.