Possible Role of Myosin Light-Chain Phosphatease in the Relaxation of Chicken Gizzard Muscle

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Abstract
Isolation of myosin light-chain phosphatase (MLCP) from chicken gizzard muscle was at-temped, using three different methods of fractionation successively, in the following order: ammonium sulfate salting-out, DEAE-cellulose chromatography, and Sephadex G-200 gel filtration. Three different preparations of MLCP were obtained with increasing degrees of purity: the ammonium sulfate fraction, the chromatography fraction, and the gel filtration fraction. The following findings were obtained with these preparations. 1. The ATPase reaction catalyzed by skeletal acto-gizzard phosphorylated myosin proceeded at the same rate in the presence or absence of calcium ions, and it was inhibited by addition of the MLCP preparations. 2. Superprecipitation of acto-phosphorylated myosin was induced by adding ATP, and it was reversed by a subsequent addition of the MLCP preparations. 3. Urea-gel electrophoresis showed that inhibition of the ATPase reaction or reversal of the superprecipitation was always accompanied by the dephosphorylation of phosphorylated light chains. 4. The concentration of MLCP preparations required to produce the same extent (at a limited incubation time) of dephosphorylation of phosphorylated light chains decreased as the fractionation progressed: the ammonium sulfate fraction the chromatography fraction the gel filtration fraction. The same order was found for inducing the same rate of reversal of superprecipitation. It is thus strongly suggested that dephosphorylation of light chains by MLCP is essential for the relaxation of gizzard muscle.

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