ACTOMYOSIN-LIKE PROTEIN OF ARTERIAL WALL.

A structural protein is extractable from hog carotids in solutions of low ionic strength (0.05 m KCl + 0.02 m histidine buffer). The following procedures cause precipitation of this protein: 1 ) standing 12 hr at 2–4 C, 2 ) dialysis against 0.05 m KCl, or 3 ) addition of 10 mm CaCl 2 . The crude preparation obtained by any of these precipitation procedures fails to show superprecipitation on the addition of ATP, but when dissolved in 0.6 m KCl it does demonstrate a viscosity change on the addition of ATP and is capable of ATPase activity. If this crude preparation is purified by repeated calcium precipitation and dialysis against 0.05 m KCl, it then shows the following characteristics typical of actomyosin: 1 ) superprecipitation, 2 ) reversible viscosity change on addition of ATP, and 3 ) ATPase activity characteristically influenced by calcium, or magnesium, or ionic strength. We conclude that this structural protein from hog carotid is an actomyosin-like protein involved in a standard contractile system, and its initial high solubility at low ionic strength may be due either to a reduction in bound calcium or to the presence of an unusually effective solubilizing factor which, like relaxing factor, can be inhibited by calcium.