Bioactive Proteins in Bovine Milk - Studies on Glutathione Peroxidase, Lactoferrin and Immunoglobulins

Abstract

Many proteins in bovine milk exhibit specific biological activity in addition to their established nutritional value as source of protein. Examples of such bioactive proteins are extracellular glutathione peroxidase, lactoferrin and immunoglobulins. Extracellular glutathione peroxidase (eGSHPx) fulfils an antioxidative function, immunoglobulins are antimicrobial, and it has been suggested that lactoferrin exhibits both of these properties. These functions may be of importance in the quality of milk and other dairy products, the influence of milk on human health and in the use of milk as a source of bioactive components in food or pharmaceutical products. Therefore, a good under-standing of the relation between thermal stability and bioactivity is important for the optimal use of bioactive proteins. In this thesis, antioxidative factors in milk are reviewed, both enzymatic and non-enzymatic factors, and a relationship between the protein antioxidants is proposed. For one of the antioxidant enzymes, eGSHPx, a purification strategy for its isolation from bovine plasma is described for the first time. The purified eGSHPx was used to develop a new immunological assay of it and, moreover, a method of measuring its activity in milk and whey was optimised. The glutathione peroxidase (GSHPx) activity in bovine milk was found to be similar to that in human milk, 25 to 50 U mL-1. The effect of storage and heating on GSHPx in milk and on pure eGSHPx and cellular glutathione peroxidase (cGSHPx) was also studied. An important finding was that the activity of GSHPx in milk and whey persisted after heat treatment at 72°C for 2 min, indicating that some industrially used pasteurisation processes will not affect the enzyme activity. Furthermore, studies on the thermal stability of immunoglobulins showed them to have a higher unfolding temperature, 80°C, than eGSHPx (69°C), but both of these unfolding temperatures were in the same range as those of other whey proteins. Finally, the influence of the antimicrobial activity of immunoglobulins and lactoferrin on the growth of starter cultures was evaluated in milk with a somatic cell count (SCC) below about 400 000. The concentration of the immunoglobulins IgA, IgG2, IgM and lactoferrin, as well as the prolonged fermentation time, could be used as markers of processability in addition to SCC. The addition of zinc to cows’ feed significantly increased the concentration of IgA, IgG2 and lactoferrin, while no effect was observed on SCC. The delay in time before growth commenced of starter cultures was also extended indicating that the immune response was affected by zinc supplementation