Purification of human muscle proteins by 0.5 M KCl extraction, differential solubility and centrifugation produced fractions which reacted with human myasthenic sera. Absorption with either actomyosin or myosin removed all activity shown by the indirect immunofluorescent assay. Quantitative complement fixation showed that active fractions included the initial crude preparations, highly purified actomyosin and myosin. Actin preparations had no antigenic activity. Myosin contained at least 50 times more activity than actomyosin in both the complement fixation and the absorption studies. These data suggest that myosin contains the antigenic sites related to humoral antibody induction in myasthenia gravis.
This study was supported in part by the Russo and Fins Foundation.