Abstract
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 triphos-phate (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 di-phosphate (ADP) are essentially the same in ITP media as in ATP media. How-ever, 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 superprecipi-tation. Such differences are: (a) Calcium without CMB or ADP removes the in-hibition of superprecipitation completely in ITP media but only partially in the ATP media. (b) The magnesium requirement is different from the nucleotide require-ment in ITP media whereas they are the same in ATP media. (c) The pH de-pendence of the ITP effect has been well-known to be different from that of the ATP effect.
