ROLE OF CA2- PROTEIN ISOLATE - (STUDIES ON AVAILABLE UTILIZATION OF WHEY PROTEINS .5.)( ON HEAT AGGREGATION OF WHEY)

Our objective was to examine the role of Ca2+ on heat aggregation of w hey protein isolate (WPI). Ca2+ content of WPI was determined with an Atomic Absorption Spectrophotometer. A 10% WPI solution contained 3.01 mM. Ca2+. Dialysis removed 1 mM free Ca2+ from the solution. When 50 mM Ca2+ was added to the WPI solution (10%) followed by dialysis again st distilled water, 10.15 mM Ca2+ bound to WPI. When a 2% WPI solution s was heated at 80 degrees C for 30 min with 5 mM Ca2+, 1.27 mM Ca2+ b ound to the precipitate of WPI after centrifugation at 80 000 X g for 30 min. A time course of turbidity of the WPI solution was measured by heating 3 degrees C/min from 30 to 95 degrees C. Dialysis had no sign ificant effect on heat aggregation of the WPI solution (1%). Aggregati on of WPI was increased slightly by the addition of 1 mM Ca2+. The tem perature of heat aggregation of WPI solution lowered markedly in the p resence of 5 mM Ca2+. Turbidity measurements performed during dialysis experiment of WPI confirm that direct Ca2+ binding to WPI contributes to the heat aggregation of WPI. Addition of 2 mM dithiothreitol (DTT) lowered the temperature of heat aggregation of non-dialyzed and dialy zed WPI solution. These results suggest that both bound and free Ca2in WPI affect the heat aggregation, and the conformational changes of WPI molecules in the presence of DTT enhance the effect of Ca2+ on hea t aggregation of WPI.