Comparison of two calcium-dependent proteinases from bovine heart

We have purified two calcium-dependent proteinases from bovine heart. Each enzyme was a heterodimer. One proteinase (designated CDP-I) contained subunits of 84 and 26 kDa. The other proteinase (designated CDP-II) contained subunits of 80 and 26 kDa. The large subunit of each proteinase accounted for the calcium-dependent proteolytic activity of the respective enzyme and could be isolated from the small subunit by casein-Sepharose affinity chromatography. The large subunits of CDP-I and CDP-II appeared to be distinctly different proteins, based on differences in peptide maps and on the lack of detectable immunologic cross-reactivity. On the other hand, the small subunits of the proteinases appeared to be identical peptides, based on peptide mapping and on two-dimensional gel electrophoresis. The function of the small subunit is unknown. The two calcium-dependent proteinases share many catalytic properties, including the nature of proteinase activity against several myofibrillar proteins. However, the proteinases were distinguished by different calcium-concentration requirements. CDP-II required 300 microM Ca2+ for half-maximal activity and 750 microM Ca2+ for full activity. CDP-I required 30 microM Ca2+ for half-maximal activity and 100 microM for full activity.