AAT inhibits calpain I activity in vitro. Dose-dependent inhibitory effects of AAT on the enzymatic activity of purified human active calpain I on the specific substrate Suc-LLVY-AMC are shown. (A) The activity of purified calpain I was inhibited by AAT up to 90% in a concentration-dependent manner. Each point represents the mean ± SD of four independent experiments. (B) Active calpain I was incubated at a molar ratio of 1:1.3 with calpastatin or AAT for 5 min, and AMC release was measured upon Suc-LLVY-AMC cleavage with calpain I. The fluorescence value of all samples is corrected by subtracting the value of the blank and then calculating the mean fluorescence value (mean ± SD) for each sample from five independent experiments.