Changes in structures of milk proteins upon photo-oxidation.

Changes in protein structures as a result of riboflavin-induced photo-oxidation were studied for six milk proteins: alpha-casein, beta-casein, kappa-casein, lactoferrin, alpha-lactalbumin, and beta-lactoglobulin. The milk proteins showed significant variability in sensitivity to photo-oxidation. After photo-oxidation, an increase in carbonyl content because of oxidation of tryptophan, histidine, and methionine, as well as formation of dityrosine, was observed for all proteins studied, although at very different levels. Generally, the increment was highest for alpha- and beta-casein and was lowest for lactoferrin. Loss of tryptophan because of photo-oxidation was well-correlated with the formation of the tryptophan oxidation products, N-formylkynurenine and kynurenine. Changes at the tertiary protein structure level were observed after photo-oxidation of the globular proteins, where tryptophan fluorescence emission indicated unfolding of alpha-lactalbumin and beta-lactoglobulin, whereas lactoferrin achieved a more compact tertiary structure. Changes in secondary structure were observed for alpha-lactalbumin and beta-lactoglobulin, whereas the secondary structure of lactoferrin did not change. Polymerization of alpha- and beta-casein and of lactoferrin was observed, whereas kappa-casein, alpha-lactalbumin, and beta-lactoglobulin showed little tendency to polymerize after photo-oxidation. Lability toward photo-oxidation is discussed according to the structural stabilities of the globular proteins.