Methionine Oxidation Enhances κ-Casein Amyloid Fibril Formation

The effects of protein oxidation, for example of methionine residues, are linked to many diseases, including those of protein misfolding, such as Alzheimer's disease. Protein misfolding diseases are characterized by the accumulation of insoluble proteinaceous aggregates comprised mainly of amyloid fibrils. Amyloid-containing bodies known as corpora amylacea (CA) are also found in mammary secretory tissue, where their presence slows milk flow. The major milk protein kappa-casein readily forms amyloid fibrils under physiological conditions. Milk exists in an extracellular oxidizing environment. Accordingly, the two methionine residues in kappa-casein (Met(95) and Met(106)) were selectively oxidized and the effects on the fibril-forming propensity, cellular toxicity, chaperone ability, and structure of kappa-casein were determined. Oxidation resulted in an increase in the rate of fibril formation and a greater level of cellular toxicity. beta-Casein, which inhibits kappa-casein fibril formation in vitro, was less effective at suppressing fibril formation of oxidized kappa-casein. The ability of kappa-casein to prevent the amorphous aggregation of target proteins was slightly enhanced upon methionine oxidation, which may arise from the protein's greater exposed surface hydrophobicity. No significant changes to kappa-casein's intrinsically disordered structure occurred upon oxidation. The enhanced rate of fibril formation of oxidized kappa-casein, coupled with the reduced chaperone ability of beta-casein to prevent this aggregation, may affect casein-casein interaction within the casein micelle and thereby promote kappa-casein aggregation and contribute to the formation of CA.