Relaxation of Glycerinated Muscle Fibers and Clearing Response of Myosin B in Magnesium-Inosine Triphosphate Medium

In the presence of a calcium chelator, namely ethylene glycol-bis-(β-aminoethylether)-N-N'-tetraacetic acid, glycerinated muscle fibers can undergo both contraction and relaxation as effectively in inosine triphosphate (ITP) media as in adenosine triphosphate (ATP) media. Like the ATP-induced superprecipitation, the ITP-induced superprecipitation requires only magnesium in the absence of relaxing protein, and requires both magnesium and calcium in the presence of relaxing protein. As in ATP media, a phenomenon called the burst hydrolysis is also observed in ITP media. Even the effects of p-chloromercuribenzoate (CMB) and of adenosine diphosphate (ADP) are essentially the same in ITP media as in ATP media. However, there still remain significant differences between ITP- and ATP-induced super-precipitations. Some of these differences can be explained by assuming that the presence in the nucleotide purine ring of an OH group (ITP) or NH2 group (ATP) makes a great difference in the nucleotide binding with myosin A as well as in a subsequent process of a conformational change in myosin A. On the other hand, some other differences suggest that the molecular events involved in the ITP-induced superprecipitation are different from those involved in the ATP-induced superprecipitation. Such differences are: (a) Calcium without CMB or ADP removes the inhibition of superprecipitation completely in ITP media but only partially in the ATP media, (b) The magnesium requirement is different from the nucleotide requirement in ITP media whereas they are the same in ATP media. (c) The pH dependence of the ITP effect has been well-known to be different from that of the ATP effect.