ATPase activity of requiem shark myosin.

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Effects of KCI, substrate and actin concentrations, along with pH, on ATPase activity of requiem shark myosin were examined.With increasing KCI concentration, the Ca2+-ATPase activity decreased almost linearly, while the K+ (EDTA)-ATPase activity increased significantly. The Ca2+-ATPase activity increased with increasing ATP concentration up to 0.6mM beyond which the activity remained at a roughly constant level. Kinetic constants, Km and Vmax were calculated to be 0.53mM and 0.40μmol Pi/min⋅mg, respectively. The myosin exhibited the highest K+ (EDTA)-ATPase activity at around 0.7mM ATP. The Km and Vmax calculated were 0.34mM and 0.27μmol Pi/min⋅mg, respecspectively.The Ca2+-ATPase of requiem shark myosin showed the pH optima at 6.5 and around 9.5, whereas the K+ (EDTA)-ATPase showed a single activity peak at around pH8.5. Actin significantly enhanced the Mg2+-ATPase activity of this myosin, the activity reaching a plateau at the actin to myosin weight ratio of 0.3-0.4.