Properties of the calcium-sensitive components of bovine arterial actomyosin.

A calcium-sensitive actomyosin was prepared from bovine aortic muscularis. Calcium binding to this arterial actomyosin was measured using a centrifugation method. The amount of bound calcium necessary for full activation of the arterial actomyosin adenosine triphosphatase was approximately 36 μmol of Ca²⁺/kg of aorta. Calcium stimulation of the actomyosin ATPase could be prevented by lowering the free magnesium from 7 to 1 mm. However, calcium binding to the actomyosin increased slightly with a reduction of free magnesium levels. Positive cooperativity was evident in the sequence of reactions beginning with the binding of calcium and ending with the hydrolysis of ATP. However, there was no evidence for the cooperativity occurring at the initial calcium-binding step.