Phosphorylation of smooth muscle myosin and myosin light chains

The most popular theory to account for the regulation of the contractile activity of smooth muscle, at the contractile protein level, is based on the phosphorylation and dephosphorylation of the myosin molecule. The enzymes involved are a myosin light chain kinase and a phosphatase, respectively. In this communication a method is given for the purification of the kinase. Using the purified kinase in combination with calmodulin, the pH dependence and rates of P transfer were examined. An Arrhenius plot of phosphorylation rates indicated that Q10 is approximately 2. The rates of P transfer to myosin light chains at 25 C and 37 C were about 15 and 34 mumol.min-1.mg-1 kinase, respectively. It is shown also that the rate of phosphorylation of isolated myosin light chains is significantly faster than the rate obtained when whole myosin is used as the phosphate acceptor, the latter being at least an order of magnitude slower. This difference in rates was not due entirely to the difference in physical states of the two substrates since at an increased ionic strength, where myosin was soluble, the rate of phosphorylation of the light chain fraction was still considerably faster than the rate of phosphorylation of whole myosin.