Regulatory Factor of Smooth Muscle Contraction: Relation of Leiotonin with Myosin Light Chain Kinase

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A new method of preparing leiotonin was developed. It was incidentally found that dimethyl sulfoxide (DMSO) is a very useful agent for protein fractionation, not only for the above purpose but also for general ones. Using DMSO, a fraction exhibiting much larger actomyosin-activating activity, or leiotonin activity, than myosin light chain kinase (MLCK) activity (L/K > 100, taking L/K of 0.1 M extract as 1) was obtained both from chicken gizzard and boving stomach without particular treatment. Using special procedures, a preparation void of MLCK avtivity (L/K > 3,000) could be obtained. The component responsibe for the leiotonin activity was a protein of 155 kDa, indistinguishable from MLCK in its molecular weight.A fragment produced by trypsin under specified conditions exhibited also a definite leiotonin activity without MLCK activity. This may correspond to the old preparation of leiotonin.The primary sequence of the 155 kDa component responsible for leiotonin activity (and/or MLCK activity) has been determined up to 120 kDa from the C- terminal.As a whole the idea that smooth muscle regulation is mediated by leiotonin has further been substantiated.