14A-6 |
The effects of native and denatured whey proteins on plasmin, plasminogen and plasminogen activators |
K. M. RIPPEL1, S. S. Nielsen2, and K. D. Hayes1. (1) Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Dr., West Lafayette, IN 47907-2009, (2) Dept. of Food Science, Purdue University, 745 Agricultural Mall Dr., West Lafayette, IN 47907
The plasmin system native to bovine milk consists of the caseinolytic serine proteinase plasmin (PL); its inactive zymogen, plasminogen (PG); plasminogen activators (PAs); and inhibitors. PL activity is desirable in dairy products such as cheese. Because whey proteins are incorporated into cheese curd to increase yield, their effects on components of the plasmin system must be elucidated. Evidence in the literature indicates that whey proteins inhibit PL, but there is no mention of their effect on PG or PAs. The objective of this research was to determine the effect of β-lactoglobulin (BLG), α-lactalbumin (ALA) and bovine serum albumin (BSA) on PAs. PL activity was measured using the chromogenic substrate Spectrozyme® PL. Indirect PA activity was assayed in a coupled reaction involving simultaneous conversion of PG to PL using urokinase-type PA (uPA) and measurement of resulting PL activity with Spectrozyme® PL. Direct PA activity was measured using the chromogenic substrate Spectrozyme® UK. The whey proteins, in their native or denatured states, were added individually and in mixtures to both reactions at levels consistent with milk and curd. None of the whey proteins directly stimulated or inhibited PL activity. Native and denatured ALA added separately to the coupled reaction significantly (P < 0.0001) stimulated PG activation by averages of 69% and 96%, respectively. Direct PA activity measurement in the presence of native and denatured ALA showed neither stimulation nor inhibition. This indicates that the stimulation effect may be on PG, and not on uPA. Native and denatured BLG or BSA, and mixtures of native and denatured ALA and BLG, resulted in much less stimulation of PG activation. These results are significant for cheesemaking, as whey proteins (specifically ALA) stimulate the conversion of PG to PL, potentially resulting in higher levels of PL in curd.
Session 14A, Dairy Foods: General developments in dairy technology I
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