Binding of catecholamines to alpha-1 acid glycoprotein, albumin and lipoproteins in human serum.

The binding of catecholamines in human serum was determined by equilibrium dialysis at 37 degrees. For serum concentrations of 10-15 nM the bound fractions were 28.8 +/- 2.2%, 25.7 +/- 1.7% and 22.2 +/- 2.2% for (+/-)-isoproterenol (IPR), (+/-)-norepinephrine (NE) and (+/-)-epinephrine (EPI), respectively. At higher serum concentrations saturation occurred. Alpha-1 acid glycoprotein (AAG) possessed one high affinity binding site and approximately 10 low affinity sites. The catecholamines were bound to AAG with the same order of potency for both classes of binding sites: IPR (Kd1: 100 microM Kd2: 2.2 mM) greater than NE (Kd1: 120 microM, Kd2: 6.5 mM) greater than EPI (Kd1: 140 microM, Kd2: 14 mM). Human serum albumin (HSA) and lipoproteins (SLP) interacted with the catecholamines in a non-saturable manner. IPR showed the strongest and EPI the weakest association to both of these serum protein fractions. (-)-Propranolol was able to inhibit the binding of IPR in serum and to isolated AAG, but not to HSA or to SLP. The present results show that AAG is an important catecholamine-binding protein in human serum. AAG, but not HSA or SLP, possesses binding sites shared by adrenergic receptor stimulators and blockers.