A high resolution fluorescence decay and depolarization study of human plasma apolipoproteins.

Human plasma apolipoprotein A-I (apoA-I) and apolipoprotein C-I (apoC-I) were investigated by time-resolved fluorescence decay and depolarization. The tryptophyl fluorescence of apoAI undergoes a double-exponential decay with lifetimes of 1.07 and 3.43 ns which remain unchangcd over the range of apoA-I concentration studied. The time-resolved fluorescence of both native and denatured forms of apoC-I exhibits an unusual tryptophyl fluorescence decay that was best fit to a triexponential function with lifetimes at 3.7 'f 0.2. 1.1 +0.1 and 0.1 ns at 2 "C. The native and denatured forms of apoC-I had rotational correlation times of 1.42 and 1.19 ns at 20°C respectively. A shorter rotational correlation time associated with the internal tryptophan motions was not observed or resolved. The decay of tryptophyl fluorescence in apoC-I/DPPC/cholesterol complex at 20°C is also triexponential with lifetimes at 4.94, 1.28 and 0.21 ns, which are longer than those of the uncomplexed forms. Two rotational correlation times of 28.32 and 0.59 ns at 20°C were resolved by fluorescence depolarization measurements. The long rotational time remained constant with temperatures above 30°C. Also, the temperature dependence of the order parameter, S2, resembled a lipid phase transition curve with a transition midpoint at 38°C. The tryptophan and thus apoC-I are found to be affected by the bulk changes in the lipid