MODIFICATION OF ACTOMYOSIN ATPase BY PARAMYOSIN : STUDIES ON THE ISOLATED NATIVE MYOFILAMENTS
The native myofilaments were isolated from the three different types of muscles containing different amounts of paramyosin : viz. adductor muscle of the spanish oyster (Atrina japonica), retractor muscle of the horseshoe crab (Tachypleus tridentatus) and psoas muscle of the rabbit. The content of paramyosin (expressed as a weight ratio relative to myosin) estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 0.67 in the adductor muscle of the spanish oyster and 0.46 in the horseshoe crab muscle. Two kinds of enzyme activities, viz. Mg2+- and Ca2+-ATPase activities, and the rates of superprecipitation at various KCI concentrations were compared among the native myofilaments isolated from the three types of muscle. The inhibition of Mg2+-ATPase activity by KCI was reduced in the native myofilaments that contained paramyosin compared with the native myofilaments devoid of paramyosin. The paramyosin-containing myofilaments required higher concentrations of KCI for the maximum activation of the Ca2+-ATPase activity than those devoid of paramyosin. At higher KCI concentrations, superprecipitation took place at faster rates in the native myofilaments containing paramyosin than in the native myofilaments devoid of it These results suggest that paramyosin has an effect on myosin ATPase and in turn on the actin-myosin interaction.