A spectroscopic study of the interaction of nigerloxin, a fungal metabolite, with serum albumin.

Nigerloxin [2-amido-3-hydroxy-6-methoxy-5-methyl-4-(prop-1'-enyl) benzoic acid], a fungal metabolite, is an inhibitor of lipoxygenase and aldose reductase with free radical-scavenging properties. The interaction of nigerloxin with bovine serum albumin (BSA) was investigated using fluorescence spectroscopy and circular dichroic measurements. The fluorescence of BSA was quenched following interaction with nigerloxin, and this property was used to generate a binding constant. The estimated association constant was 1.01 +/- 0.2 x 10(6) M(-1). Job's method of continuous variation indicated that nigerloxin formed a 1:1 +/- 0.1 complex with BSA. To understand the nature of the interaction, the variance in the association constant as a function of temperature in the range of 14-45 degrees C was used to calculate the thermodynamic parameters. The thermodynamic parameters at 27 degrees C derived from the mass action plot and van't Hoff's plot were as follows: deltaG = -8.2 +/- 0.1 kcal/mol, deltaH approximately equal to 0 kcal/mol, and deltaS = 27.5 +/- 0.4 cal/mol/K (where deltaG is free energy, deltaH is enthalpy, and deltaS is entropy). Increasing ionic strength did not favor interaction. Circular dichroic measurements revealed that the interaction of nigerloxin with BSA did not lead to changes in the secondary structure of the protein. The reversibility of the interaction verified by the dilution method was found to be reversible. These measurements suggest that partial hydrophobic and partial ionic bonding play a role in the interaction of nigerloxin with BSA.