Fluorescence studies on the lipoprotein complex of the fatty acid synthetase from the insect Ceratitis capitata

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Abstract
The fatty acid synthetase complex from the insect C. capitata forms a stable lipoprotein complex. The intrinsic fluorescence of the complex was studied by observing the emission spectra with different excitation wavelengths, both in the native complex and after treatment with sodium cholate and sodium dodecyl sulfate. The excitation spectrum of the native form also was recorded. The fluorescence behavior of the native enzyme showed 2 families of tryptophan residues. Cholate influenced the fluorescence, suggesting that phospholipids are the conformational support at this level. The 2 families of fluorescing tryptophan residues were similarly accessible to quenching by acrylamide. Thermal changes in the fluorescence characteristics were observed; warming caused a decrease in the quantum yield and a red shift in the emission maximum. The high fluorescence remaining after the thermal transition suggested that the lipid-protein interaction was affected but maintained shielding of the fluorophore by the lipids. Fluorescent probe molecules 1,6-diphenylhexa-1,3,5-triene (DPH) and dansylphosphatidylethanolamine (DPE) also were used. DPH uptake was temperature dependent, with a middle point consistent with the thermal conformation transition, indicating that internal lipids are nonrandomly distributed within the complex. DPE uptake did not reach the saturation of the complex, suggesting that its solubilization sites would be located on the lipoprotein surface.