Myocardial calpain 2 is inhibited by monoclonal antibodies specific for the small, noncatalytic subunit

Calpains (EC 3.4.22.17) are nonlysosomal intracellular proteinases which require calcium ion for activity. The calpains are heterodimers composed of a large catalytic subunit and a small subunit which may have a regulatory function during the catalytic cycle. However, whether calpains remain in the dimeric form or dissociate upon exposure to calcium is controversial. To resolve this issue, two monoclonal antibodies which specifically recognize the small calpain subunit were prepared using bovine calpain 2 heterodimer as the antigen. Both antibodies, designated P-1 and P-2, were capable of inhibiting bovine or canine calpain 2, and partially purified human erythrocyte calpain 1. However, neither could produce full inhibition. Further studies with P-1 and bovine calpain 2 indicated that the antibody decreased the calcium requirement for the proteinase. The Km for casein was increased and the Vmax was decreased. The addition of P-1 to the assay mixture several minutes after initiation of proteolytic activity resulted in a rapid inhibition. The P-1 antibody was also capable of decreasing the ability of the protein inhibitor of calpains (calpastatin) to inhibit bovine calpain 2. These studies indicate that the small subunit remains bound to the large subunit during catalysis and may influence its activity.