Influence of milk proteins on lipid oxidation in aqueous emulsion: II. Lactoperoxidase, lactoferrin, superoxide dismutase and xanthine oxidase

Abstract

The milk-related model system described by Allen & Wrieden (1982) was used to assess the effects of a number of purified milk metalloproteins, at concentrations relevant to those in milk and dairy products, on triglyceride oxidation in emulsions. Oxidations were monitored by the rate of O2 utilization (ROU) and by the thiobarbituricacid (TBA) assay. The proteins lactoferrin, lactoperoxidase, superoxide dismutase and xanthine oxidase were used either on their own or in conjunction with 10 μM-Cu2+ or Fe2+. Lactoferrin alone had little effect on oxidation, although both the iron-free and iron-saturated protein were able to protect the lipid to some extent from Fe2+-catalysed oxidation. Cu2+-catalysed oxidation was slightly promoted. Lactoperoxidase was pro-oxidative in the absence and presence of added Cu2+ or Fe2+. Heat treatment at 72°C/20 s had little effect, but oxidation was greatly reduced by treatment at 80°C/20 s. Superoxide dismutase strongly reduced the rate of lipid oxidation in the absence of added metals, but had no effect in the presence of 10 μM-Cu2+. xanthine oxidase had little effect on lipid oxidation in the absence of added metals, but was strongly pro-oxidative in the presence of 10 μ-Cu2+. Similar effects were observed down to 1 μM-Cu2+. The enzymic activity of xanthine oxidase was rapidly eliminated by 10 μM-Cu2+, but not by 10 μM-Fe2+. A moderate pro-oxidative effect was observed in the presence of 10 μM-Fe2+. Heat treatment of xanthine oxidase at 80°C/20 s reduced its lipid pro-oxidative effect in the presence of Cu2+ more effectively than treatment at 72°C/20 s.