Fractionation and characterization of two molecular variants of myosin from adult human atrium.

Monoclonal antibodies (MAb) to myosin heavy chains were prepared from one adult human ventricular myocardium. Several of these MAb reacted by indirect immunofluorescence in a heterogenous way on cryostat transverse sections of fibers from human atrial myocardium, suggesting the presence of different forms of myosin within the human atrium and prompting the further use of the MAb to attempt to fractionate preparations of native atrial myosins. Two molecular variants of human atrial myosins or myosin fragments were thus separated by immunoaffinity chromatography performed with one antiventricular myosin MAb. Seven MAb located at different positions along the myosin heavy chains, as deduced from blotting and immuno-electron microscopy experiments, were used to characterize the structural relationships between the separated human atrial isomyosins and between each of them and the main human ventricular myosin. As deduced from competitive radioimmunoassay measurements, the primary structures of the two atrial myosins differ in at least five antigenic determinants and share at least two of them; similarly located structural differences were observed between one of the atrial myosins and the ventricular myosin. Conversely, the primary structures of the other atrial myosin and of the ventricular myosin differ in at least two antigenic determinants and share at least five of them. Differences in the primary structures of the human cardiac myosins were confirmed by analysis of the peptides produced by limited enzymatic digestion of the heavy chains; a few peptide differences were consistently found. To summarize the two separated forms of atrial myosin have different heavy chains, but they have similar if not identical in vitro ATPase activities and the same light chains. One of the atrial myosins is immunologically close to the ventricular myosin, but they each differ with respect to their heavy chains, light chains, and enzymatic activities.