The objective of this study was to investigate the effect of phosphorylation on the sensitivity of mu-calpain to the inhibition induced by calpastatin. Purified mu-calpain was incubated with alkaline phosphatase (AP) or protein kinase A (PKA) to modulate the phosphorylation level of mu-calpain. Accurately 25, 50, 100 and 150 units of AP/PKA-treated mu-calpain were mixed with the same amounts of heat stable proteins and incubated at 4 degrees C. In the calpastatin-free system, AP and PKA-treated mu-calpain had higher proteolytic activity compared to the control. Intact AP-treated mu-calpain deg...
