Isolation and characterization of myosin in the human term placenta.

The major contractile protein myosin was isolated and characterized from the smooth muscle of human term placentas. Placental myosin originates chiefly in the anchoring villi which bridge the fetal and maternal surfaces of the placenta. The molecular weight of placental myosin is about 460,000; it is composed of two heavy chains of 200,000 molecular weight and two pairs of light chains with 13,500 and 17,500 molecular weights. The adenosine triphosphatase (ATPase) of the myosin is activated by potassium and calcium and it is inhibited by magnesium. Placental actomyosin ATPase is activated by magensium. Contraction and relaxation of the smooth muscle in the anchoring villi are thought to adjust the volume of the intervillous space; thus, actin-myosin interaction is implicated in the regulation of placental hemodynamics.