抄録
It has long been known that very high concentrations of magnesium are required for contraction of vertebrate smooth muscle (1-6). We thought of three possible reasons to account for the requirement of high concentrations of magnesium; 1) A high concentration of magnesium may be required for activation of myosin lightchain kinase. 2) It may be required for actin-activation of myosin-ATPase. 3) It may be required for formation of thick filaments.
The three possible reasons were examined by studying superprecipitation of chicken gizzard actomyosin and its ATPase reaction. The following results were thus obtained. a) A MgCl2 concentration higher than 1mM (in the presence of 1mM ATP) was required for activity of myosin light-chain kinase. b) However, high concentrations of magnesium (>ca. 3mM MgCl2 in the presence of 1mM ATP) were also required for superprecipitation of acto-phosphorylated myosin. c) In the presence of 1mM MgCl2 (and 1mM ATP), superprecipitation of acto-phosphorylated myosin did not occur but actin-activation of phosphorylated myosin-ATPase did occur to a full extent. d) The ATPase activity of unphosphorylated myosin was about 1 nmol P1/min/mg of myosin at all the concentrations of magnesium we tested (1-20mM MgCl2), but that of phosphorylated myosin increased from 3 to 9 nmol P1/min/mg of myosin when magnesium concentration increased to higher than approximately 3mM. e) Judging from the turbidity of the myosin suspensions, a magnesium concentration higher than approximately 3 mM (in the presence of 1mM ATP) was also required for formation of thick filaments even when myosin was phosphorylated. It is therefore suggested that formation of thick filaments is responsible for the increase in the ATPase activity of phosphorylated myosin from 3 to 9 nmol P1/min/mg of myosin.
