Fujisaki H, Asai H
J Biochem. 1982 Nov;92(5):1577-83. doi: 10.1093/oxfordjournals.jbchem.a134082.
Enzymatic characteristics of heavy meromyosin (HMM) were investigated by measuring beta-naphthyl triphosphate (beta-NapP3) hydrolysis in the presence and absence of F-actin. beta-NapP3 hydrolysis by HMM was inactivated by F-actin in the presence of Mg ions; in the presence of sufficient F-actin, the activity was about one-half of that in the absence of F-actin. In the presence of Ca ions the activity disappeared almost completely on addition of sufficient F-actin. Two different values of the Michaelis constant (Km) were obtained from the data for beta-NapP3 hydrolysis by HMM in the presence of Mg ions; one of them vanished in the presence of sufficient F-actin, and only one Km value was obtained in the presence of Ca ions. These results suggest the existence of two distinct enzymatic sites in HMM, one inactivated by F-actin in the presence of Mg or Ca ions, the other inactive in the presence of Ca ions but active in the presence of Mg ions and not influenced by F-actin. Use of subfragment-1(A1) (S-1(A1)) and S-1(A2) instead of HMM confirmed that these enzymatic characteristics are not due to a difference in the alkali light chains on myosin heads.
通过在有和没有F-肌动蛋白的情况下测量β-萘基三磷酸（β-NapP3）的水解，研究了重酶解肌球蛋白（HMM）的酶学特性。在镁离子存在的情况下，F-肌动蛋白使HMM催化的β-NapP3水解失活；在有足够F-肌动蛋白存在时，活性约为没有F-肌动蛋白时的一半。在钙离子存在时，加入足够的F-肌动蛋白后活性几乎完全消失。从HMM在镁离子存在下催化β-NapP3水解的数据中得到了两个不同的米氏常数（Km）值；其中一个在有足够F-肌动蛋白存在时消失，在钙离子存在时只得到一个Km值。这些结果表明HMM中存在两个不同的酶位点，一个在镁离子或钙离子存在时被F-肌动蛋白失活，另一个在钙离子存在时无活性，但在镁离子存在时具有活性且不受F-肌动蛋白影响。使用亚片段-1（A1）（S-1（A1））和S-1（A2）代替HMM证实，这些酶学特性不是由于肌球蛋白头部碱性轻链的差异所致。
