Inhibition of myosin ATPase by vanadate ion.

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Abstract
Inhibition of the myosin ATPase by vanadate ion (Vi) was studied in 90 mM NaCl/5 mM MgCl2/20 mM Tris .cntdot. HCl, pH 8.5, at 25.degree. C. Although the onset of inhibition during the assay is slow and dependent upon Vi concentration (kapp .apprxeq. 0.3 M-1 s-1), the final level of inhibition approaches 100%, provided the Vi concentration is in slight excess over the concentration of ATPase sites. Inhibition is not reversible by dialysis or the addition of reducing agents. The source of this irreversible inhibition consists of the formation of a stable inactive complex with the composition M .cntdot. ADP .cntdot. Vi (where M represents a single myosin active site). The complex was isolated and its mechanism of formation from M, ADP and Vi was studied. Omission of ATP increases the rate of formation by about 35-fold (kapp .apprxeq. 11 M-1 s-1), yet this rate is still low in comparison with the rates of simple protein-ligand association reactions. This slowness is interpreted in terms of a rate-limiting isomerization step that follows the association of M, ADP, and Vi: M .cntdot. ADP .cntdot. Vi .fwdarw. .**GRAPHIC**. .cntdot. ADP .cntdot. Vi .**GRAPHIC**. indicates the inactive product of the isomerization). The properties of .**GRAPHIC**. .cntdot. ADP .cntdot. Vi are compared with those of the ATPase intermediate M .cntdot. ADP .cntdot. Pi, and the possible role of Vi as an analog of Pi is discussed.