Human plasma alpha 1- and alpha 2-thiol proteinase inhibitors strongly inhibit Ca-activated neutral protease from muscle.

Human plasma alpha 1- and alpha 2-thiol proteinase inhibitors (alpha 1,2TPIs) inhibited purified Ca-activated neutral protease (CANP) most strongly among a number of thiol proteinases tested. When CANP was added to plasma, it was also inhibited by alpha 2-macroglobulin (alpha 2M). At low CANP concentrations, CANP was bound mainly to alpha 1,2TPIs; and after saturation of alpha 1,2TPIs the additional CANP was bound to alpha 2M. These data suggested that a probable role of alpha 1,2TPIs is to neutralize the proteolytic activity of the CANP derived from the tissues in collaboration with alpha 2M.