Studies on uterine myosin A and actomyosin

Myosin A was extracted from uterine muscle using 1 m LiCl as the extradant. The S₂₀, D₂₀, and [η] of the purified uterine myosin A was 6.4, 1.08, and 1.69, respectively. The molecular weight of this protein was 5.3 × 10⁵ g mole⁻¹ and its axial ratio 56. The optimal ATPase⁴ activity of the Ca²⁺-activated uterine myosin A and actomyosin was found at pH 5.3. In the presence of millimolar concentrations of Mg²⁺ or Ca²⁺, the energy of activation of the uterine myosin A or actomyosin ATPase was 11,000–13,000 cal/mole while in the absence of mm Mg²⁺ or Ca²⁺, the energy of activation was 21,000 cal/mole. The K_m values of uterine myosin A and actomyosin were in the 10⁻⁴m or 10⁻⁵m range. In the presence of 10 mm Ca²⁺ and 0.6 m KCl, the ATPase of uterine myosin A was activated by 0.005 m DNP, while 5 × 10⁻⁶−5 × 10⁻⁵m PCMB inhibited rather than activated the enzyme.If mm Mg²⁺ was added, the V_m of the uterine actomyosin was very low and optimal ATPase activity was found between pH 7.5 and 9.0.In the presence of mm Mg²⁺, the ATPase of the uterine actomyosin enzyme revealed the presence of an initial phosphate burst. In mixtures containing low KCl concentration, the extra phosphate production was 18 times lower in the case of the uterine enzyme than in the case of the skeletal protein. The results are discussed in the light of a proposed correlation between the stability of the enzyme substrate complex (phosphoryl myosin) and the speed of muscular contraction.