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Effect of ethylene glycol and Ca2+ on the binding of Mg2+ x adenyl-5'-yl imidodiphosphate to rabbit skeletal myofibrils.

The binding of Mg2+ X adenyl-5'-yl imidodiphosphate (Mg2+ X AMP-PNP) to rabbit skeletal myofibrils has been measured in aqueous solution and in 50% ethylene glycol in the presence and absence of Ca2+. In water, the observed binding was weak with less than half the calculated myosin active sites filled even at 1 mM Mg2+ X AMP-PNP. In 50% ethylene glycol, the binding is at least 100-fold tighter and extrapolates to the expected number of binding sites. This is contrasted to the small change seen for Mg2+ X ADP binding between the same sets of conditions. This difference between Mg2+ X AMP-PNP and Mg2+ X ADP is attributed to the strong coupling of Mg2+ X AMP-PNP binding to dissociation of myosin cross-bridges. The Ca2+ sensitivity of Mg2+ X AMP-PNP binding in 50% ethylene glycol is taken as further evidence of the thermodynamic coupling of Mg2+ X AMP-PNP binding to cross-bridge dissociation. In addition, the binding of Mg2+ X AMP-PNP in 50% ethylene glycol is biphasic while Mg2+ X ADP binding under the same conditions is not. The biphasic Mg2+ X AMP-PNP binding could be caused by either the presence of two or more classes of cross-bridges or by negative cooperativity, but the presence of only a single class of Mg2+ X ADP-binding sites implies that if multiple classes of sites are involved, they do not simply differ in steric hindrance or accessibility of the binding site as a whole. The importance of using purified AMP-PNP in the study of actomyosin X AMP-PNP complexes is discussed.