Some aspects of the interaction between the proteins of the myofibril and the adenine nucleotides

Abstract

An investigation of the effect of 2:4-dinitrophenol (DNP) on the ATPase activity of myosin and actomyosin supported an earlier postulate that the protein was phosphorylated during interaction with adenosine triphosphate (ATP). The effect of DNP might be attributed to a breakdown of such an intermediate, or the inhibition of its formation, in a manner similar to that whereby DNP inhibits oxidative phosphorylation in mitochondria. Experiments were designed to detect any phosphate or adenine uptake during enzyme-substrate interaction in the bulk solution phase. No uptake was detected at 0[degrees]C, 19[degrees]C or 37[degrees]C, or when the reaction time was long (5 min.), short (15 sec.), or limited to 2 sec. followed by denaturation, in an attempt to "trap" any possible intermediate.From the work of Hayashi with surface film techniques, it is known that actomyosin, when incompletely spread, retains its enzymic activity, at least partially, and therefore remains to some extent in the native state. A study of the physical changes of the film during enzyme-substrate interaction might be indicative of the chemical changes taking place at the interphase.It has been shown by Cheesman and Sten-Knudsen that the mechanical rigidities of incompletely spread films of myosinand actomyosin on M KCl are reduced when the substrate contains 0.0001 M ATP. This effect was confirmed, but found not to be given by ITP or ADP. It decreases as the ATPase activity of the protein is reduced by heating at 37[degrees]C. DNP inhibits the effect when present in the substrate. Since DNP promotes the ATPase activity, thus not inhibiting the formation of the enzyme-ATP complex, it seems probable that the ATP effect on films is due to the phosphorylation of myosin or actomyosin, unstable in the presence of DNP. The antimonial drug fuadin has been found by Ulbrecht and Ulbrecht to accelerate the exchange of phosphate groups between ATP and ADP by actomyosin. Fuadin reinforces the effect on films of "natural" actomyosin but not on those of L-myosin. This is consistent with the findings of Ulbrecht and Ulbrecht who show that actin is essential to the exchange reaction. The indirect evidence of these authors, together with the results of the present investigation, are strongly suggestive of the phosphorylation theory of muscular contraction, and also offer an explanations of the increased activity of ATPase in the presence of DNP.