A study of the effects of substrate concentration and certain relaxing factors on the magnesium-activated myofibrillar adenosine triphosphatase

Publisher Website
Google Scholar
Abstract
At ionic strength (I) about 0.06 and pH 6.9-7.6 the concentration at which adenosine triphosphate (ATP) became inhibitory for the adenosine triphosphatase (ATPase) of myofibrils, isolated in 0.039 [image] borate buffer (pH 7.1)- 0025 [image] KC1, was dependent on the concentration of MgCl2 added to the system. At 5 mM MgCl2 the concentration of ATP exceeded that of Mg. The substrate inhibition of the Mg-activated ATPase did not occur in the presence of CaCl2 in concentrations 1/10 to 1/20 that of the MgCl2. Myofibrils prepared in 0.1 [image] KC1 from fresh and glycerated muscle exhibited similar substrate inhibition effects at I about 0.16 to those obtained at I about 0.06 with myofibrils isolated in borate-potassium chloride. Inorganic pyrophosphate inhibited the magnesium-activated myofibrillar ATPase when the total concentration of ATP and pyrophosphate exceeded that of MgCl2. Adenosine diphosphate (ADP) was less inhibitory than ATP when it replaced part of the latter; inosine triphosphate had little inhibitory effect. Over a similar range, increasing concentration of uridine triphosphate produced progressively increasing inhibition. No inhibitory action on the Mg-activated myofibrillar ATPase could be demonstrated with myokinase preparations. Ethylenediamine-tetraacetate (EDTA) strongly inhibited the Mg-activated myofibrillar ATPase in concentrations which were only 1/20 of that of the added MgCl2. Calcium activation of the ATPase was much less sensitive to EDTA. These findings are discussed in relation to the relaxation studies by other workers on glycerated muscle fibers.