A human serum protein system affecting muscle contractility: characterization of the five components and their reaction sequence.

A human serum protein system that affects muscle contractility, previously reported to be a calcium transport system (CTS), was fractionated into five protein components, A, B1, Bfi, B3 and C. The system was assayed on frog heart muscle in vitro by its capacity to increase contractile force and at high concentrations to cause contracture. With separated fractions, the reaction sequence was divided into three separate steps: B1 and B3 interacted with the heart in the first step, B2 and B3 in the second, and A and C in the third. Increased contractility occurred only after the third step. Partial purification, with preservation of biological activity, was achieved for B1, B2, A and C. B1 has a M.W. of about 170,000, as determined by its elution from Sephadex G-200. B2, a labile protein of M.W. 220,000, was one of the few serum proteins precipitated by M.W. 5000,000 dextran sulfate at pH 8.0. This was the basis for the preparation of highly purified B2, which was used to produce rabbit anti-B2 antibody. Experiments with this antibody proved that B2 became bound to the heart surface. CTS-A, the smallest protein of the system, had a M.W. of approximately 130,000. The M.W. of C was over 300,000. Purified C antigen induced rabbit antibody that inhibited the action of C without affecting the other components.