INTERACTION BETWEEN HUMAN ALPHA(1)-ACID GLYCOPROTEIN (OROSOMUCOID) AND 2-P-TOLUIDINYLNAPHTHALENE-6-SULFONATE

The interaction between human alpha(1)-acid glycoprotein (orosomucoid) and the fluorescent probe, 2-p-toluidinylnaphthalene-6-sulfonate (TNS) has been studied. An association constant of 16.7 (+/-3) x 10(3) M(-1) was obtained for the complex at 20 degrees C with a stoichiometry of 1:1. From the effect of temperature on the binding process, the standard enthalpy change for the binding is calculated to be Delta H-0 = -18 +/- 3 kJ mol(-1) and the standard entropy change Delta S-0 = 19 +/- 12 J K-1 mol(-1). The tryptophan fluorescence of the protein can be described by a sum of three exponentials. Upon TNS binding, the average fluorescence lifetime of the protein in the complex changes much less than the fluorescence intensity. The bound TNS is therefore a very efficient acceptor for the protein fluorescence. The TNS bound to orosomucoid presents two fluorescence lifetimes 11 and 4.3 ns. The possible origins of the two lifetimes are discussed.