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Photogeneration of Superoxide Anion upon Illumination of Bovine Milk Serum Proteins with Fluorescent Light in the Presence of Riboflavin

Department of Food Science, University of Wisconsin, Madison 53706

ABSTRACT

When dissolved in simulated milk ultrafiltrate and illuminated with fluorescent light at pH 6.7 in the presence of 3.2 µM riboflavin, three proteins of bovine milk serum served as oxidizable substrates for photogeneration of superoxide anion. At their concentrations in milk, ß-lactoglobulin, -lactalbumin, and albumin yielded averages of 6.3, 1.6, and 1.6 µM superoxide anion in 1 h. No detectable superoxide anion was produced in the presence of immunoglobulins. When all four proteins were combined at normal concentrations in milk, the amount of superoxide anion photogenerated in simulated milk ultrafiltrate accounted for nearly all of the superoxide anion photogenerated by high molecular weight compounds in bovine milk serum. In .9% sodium chloride solution ß-lactoglobulin photogenerated about half of the amount of superoxide anion formed by the same protein in simulated milk ultrafiltrate while -lactalbumin was four times more effective. Neither immunoglobulins nor albumin yielded measurable amounts of superoxide anion when illuminated in the sodium chloride solution. Heating the four proteins at 63 C for 30 min in simulated milk ultrafiltrate increased the amount of superoxide anion generated upon illumination. However, heating the proteins in .9% sodium chloride solution did not increase superoxide anion. Thus, the major milk serum proteins can be substrates for photogeneration of oxidative superoxide anion; their capacity for this can be modified by changes in their ionic environment or by heat treatments, suggesting that protein conformation may play a role in exposing amino acids subject to photooxidation.