The effect of storage and heat treatment on glutathione peroxidase in bovine milk and whey

The stability of glutathione peroxidase (GSHPx) activity in milk and whey, and of pure GSHPx enzymes was monitored following storage and heat treatment. After heat treatment of whey at 72 degreesC for 2 min the GSHPx activity was maintained at pH 6.7 but at pH 4.6 20% was lost. Among individual enzymes, the activity of extracellular GSHPx (eGSHPx), which is the major form in milk, was more stable to heat treatment compared to that of cellular GSHPx (cGSHPx), but both were less stable than the GSHPx activity in whey. In contrast to results from activity measurements, differential scanning calorimetry at pH 6.7 revealed that cGSHPx was more thermally stable than eGSHPx, the unfolding temperature, T-max, being 75 degreesC compared with 69 degreesC for eGSHPx and cGSHPx also had a higher transition enthalpy. The thermal unfolding of cGSHPx started at a lower temperature, however, which may explain the results from the activity measurements. In general, pasteurisation and storage at 8 degreesC had only minor effects on the enzymatic and conformational properties of eGSHPx. (C) 2001 Elsevier Science Ltd. All rights reserved.