Dissociation of myosin light chains and decreased myosin ATPase activity with acidification of synthetic myosin filaments: Possible clues to the fate of myosin in myocardial ischemia and infarction

Abstract

The effect of mild acidification of synthetic (reconstituted) myosin filaments was studied in order to gain insight into some of the possible effects of ischemia-induced intracellular acidosis on the structure and function of myosin following myocardial infarction and myocardial ischemia. Degradation products of myosin that are soluble (at physiologic ionic strength and pH) would be of potential diagnostic value for myocardial infarction. Acidification of rabbit skeletal synthetic myosin filaments led to a pH dependent partial dissociation of the heaviest (LC 1) and lightest (LC 3) of the 3 light chains. Dissociation was detected from pH 5.0 to 6.5 and was maximal at pH 6.0, at which 30% of LC 1 was dissociated. Acidification of canine cardiac synthetic myosin filaments led to partial dissociation of both light chains; but more LC 1 than LC 2 was dissociated. Light chains reassociated with heavy chains upon return of the pH to 7. Light chains of myosin have recently been reported to appear in the peripheral blood after myocardial infarction but the small amount of free light chains in the heart is insufficient to account for the amount that appears in the blood. Acid-mediated dissociation of light chains in vitro suggests that circulating light chains after myocardial infarction may arise as a result of the intracellular acidosis of ischemic myocytes. The mechanisms responsible for the acidification-induced decrease in myofibrillar actomyosin adenosine triphosphatase (ATPase) activity are unclear. One possibility is that the decreased myofibrillar ATPase activity is due in part to an acid-induced decrease of the myosin ATPase of the myofibril irrespective of the effect of acid on the troponin-tropomyosin regulatory system. This possible mechanism is supported by the observations that acidification of rabbit skeletal and human and canine cardiac synthetic myosin filaments resulted in a reduction of ATPase activity (measured at pH 7.5) of the redissolved myosin which was progressive with greater acidification. The reduction in ATPase activity occurred whether the return of the myosin to pH 7.5 was accomplished in the presence or absence of dissociated light chains.