Effects of heat and β-lactoglobulin on distribution of fluorescently labeled tissue- and urokinase-type plasminogen activators in a model milk system

A fluorescent labeling method was developed to determine the effects of heat and β-lactoglobulin (BLG) on the concentration distribution of added fluorescently labeled tissue-type (tPA-647) and urokinase-type (uPA-546) plasminogen activators (PAs) in a model milk system. Prior to heating, the majority of the added PAs (87% of tPA-647, 72% of uPA-546) became associated with casein micelles and could be dissociated by acidification or NaCl. Addition of 0.5% BLG to the unheated system had no significant effect on the distribution of PAs. Heat-induced binding of uPA-546 to micelles was shown by an increase of uPA-546 from 72% to 88% or 95% in the micelle fraction of systems without or with BLG, respectively. Although heat alone had little effect on the total amount of tPA-647 associated with caseins (88%), addition of BLG to a heated system shifted 32% of the tPA-647 to a non-sedimentable casein fraction. After heating, casein–PA interactions became more complex.