18 O-exchange properties of enzymically active proteolytic fragment of myosin molecule--subfragment 1 were studied. It was shown that 18O-exchange activity of subfragment 1 is similar to that of parental myosin and its other proteolytic fragment--heavy meromyosin. The experimental data suggest that the 18O-exchange activity is an integral property of myosin and is not determined by its contaminations.

Glycerol-extracted rabbit psoas muscle fibres were treated with a solution containing the head subunits of myosin. Interaction of the isolated myosin heads with actin filaments in situ was indicated by an increase in the density of the I-band and by an increased diameter of actin filaments alongside their whole length. We conclude that actin filaments are able to interact with a considerably larger number of myosin molecules than that available in the myofibril under in vivo conditions.