Arginine kinase has been purified from body-wall muscle of a marine polychaetous annelid, Sabella pavonina. The preparation is homogeneous to analytical ultracentrifugation and disc electrophoresis on polyaorylamide gel. The enzyme is labile and undergoes a rapid decrease of its specific activity, even in the presence of protecting agents. The molecular weight of the native enzyme is 150000 f 5000, the sedimentation coefficient si,,, being found to be equal to 7.5.Unlike the enzyme from lobster muscle (mol. wt -43000) and that from Sipunculus nudus muscle (mol. w t -83000), the phosphokinase from Sabella pavonina muscle is not strictly specific towards L-arginine. The enzyme shows no optical isomer specificity and is slightly active on a number of analogues of L-arginine, the integrity of the guanidine and carboxylic groups of which is indispensable to their activity. The enzyme is strictly specific for the nucleotidic substrate ATP.Partial isotopic exchange and kinetic studies suggest that the reaction mechanism is somewhat different from that described for sea-water crustacean muscle arpinine kinase and more similar to that reported for other phosphagen phosphokinases.
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