A rapid procedure for isolating subfragment one (SFl) from myosin was found. SF1 can be isolated specifically from proteolytic digests of myosin in the presence of a millimolar concentration of magnesium chloride. Under such ionic conditions all of the rod portion and undigested myosin is selectively precipitated. A nucleotide trapping experiment indicated how important quick preparation of SFl is for main taining the active site structure. This method can also be utilized in the preparation of heavy meromyosin.

The double-headed portion of the myosin mole cule has been isolated and utilized in studies on muscle contraction and cell motility since the iso lated heads, subfragment one (SFl), carry the ATPase activity and actin binding ability of the parent myosin molecule. Chymotryptic digestion of rabbit skeletal myosin particularly results in a superior SF1 preparation without any serious alter ations of the enzyme function, therefore, chymo tryptic SF1 has been widely used (1-4). Recently the question arose of whether all of the SF1 molecules prepared by the widely used procedure (4) are active and homogeneous or not arose. When we tried to trap MgADP in the active site of SF1 through chemical crosslinking of SH1 and SH2 (5), we never obtained a trapping efficiency of more than 80% of the theoretical maximum value. As a simplest means of check ing we tried to isolate SFl as soon as possible