Effect of albumin on the kinetics of ascorbate oxidation.

The fluorescence intensity of the fluorophore in dansyl piperidine-nitroxide is intramolecularly quenched by the nitroxyl fragment. Therefore, the oxidation of ascorbic acid by the fluorophore-nitroxide (FN) probe can be monitored by two independent methods: steady-state fluorescence and electron paramagnetic resonance. Bovine serum albumin (BSA) affects the rate of this reaction. The influence of BSA on the rate is attributed to the adsorption of both ascorbate and the probe to BSA. Adsorption of ascorbate to BSA is confirmed by NMR relaxation experiments. The spatial distribution of the molecules on the BSA surface changes the availability of ascorbate and FN to each other. The results also point out that, in the presence of BSA, the autoxidation of ascorbate is significantly slowed down. The effect is studied at different pH values and explained in terms of the electrostatic interaction between the ascorbate anion and the BSA molecule.