Ca2+--ATPase activity of myosins prepared from hearts with different shortening speeds was measured in order to determine whether an alteration in hydrolitic activity or in the affinity of myosin for its substrate, or both, may be responsible for the species differences in Ca2+--ATPase. The KM values of ATP for cardiac myosins from rat, guinea-pig and rabbit did not differ significantly, whereas the Vmax decreased in the following order: rat greater than guinea pig greater than rabbit. These facts lead us to assume that, in spite of a great similarity of the active centres of the these myosins, the catalytic sites may not be identical.
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