MODIFICATION OF BETA-LACTOGLOBULIN BY ALIPHATIC-ALDEHYDES IN AQUEOUS-SOLUTION

Each of the secondary lipid oxidation products pentanal, hexanal and heptanal was found to react with beta-lactoglobulin (beta-lg) in a two-phase model system (aqueous phosphate buffer-1-octanol) yielding fluorescent condensation products (emission maximum, 410 nm; excitation maximum, 350 nm). Protein polymers were detected by size-exclusion HPLC, and the rate of reaction paralleled the formation of fluorescent products, with the reactivity being pentanal > hexanal > heptanal. Simultaneously, the reaction also changed the intrinsic fluorescence of beta-lg, and in particular pentanal reduced the intensity of tryptophan fluorescence (emission maximum, 332 nm; excitation maximum, 288 nm) by 30%. These findings are discussed with reference to the effect of peroxidizing lipids on the physical properties of whey proteins and the use of protein fluorescence (induced by the reaction with aldehydes) as marker for the oxidative status of milk and whey protein products.