The binding of catecholamines to human serum proteins

The binding of catecholamines at physiological concentrations to human serum and human serum proteins has been studied by equilibrium dialysis and gel nitration chromatography. Serum in equilibrium with a free catecholamine concentration of 100 pg/ml, binds 51 pg (-)-noradrenaline/ml and 28 pg (±)-adrenaline/ml. The binding is rapid and is only partially reversed by prolonged dialysis. Fractionation of the serum proteins by gel nitration chromatography revealed that 63 per cent of the bound (-)-noradrenaline is associated with albumin, 13 per cent with lipoprotein, principally very low density lipoprotein, 7 per cent with IgM and 5 per cent with α₂-macroglobulin. A further 12 per cent is associated with unidentified proteins. The major difference in the binding of (±)-adrenaline is its reduced affinity for IgM and albumin. The constants for the binding of catecholamines to purified IgM, very low density lipoprotein and albumin have been measured. Serum contains a non-dialysable inhibitor of the binding of catecholamines to albumin. The possible physiological consequences of binding are discussed.