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 o f 1:1. From the effect of temperature on the binding process, the stan dard 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 d escribed 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 ef ficient acceptor for the protein fluorescence. The TNS bound to orosom ucoid presents two fluorescence lifetimes 11 and 4.3 ns. The possible origins of the two lifetimes are discussed.