An attempt has been made to localize sites of ATPase activity in stretched fibrils from glycerinated rabbit muscle by precipitating the liberated phosphate with lead ions. As a control, the distribution of lead phosphate grains precipitated by adding phosphate solutions to fibrils incubated with lead (without ATP) has been studied. In these conditions, the grains are not uniformly distributed along the fibrils, but show many of the features of the pattern of distribution that is found after the fibres have been incubated in ATP and lead. It is suggested that differential lead binding by the protein filaments, or movement of the grains, may be responsible for this non-uniform distribution, and that consequently no conclusions concerning sites of ATPase activity within the sarcomere can be drawn from the results of methods using lead precipitation.

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