UNCOUPLING OF ACTOMYOSIN ADENOSINE-TRIPHOSPHATASE BY HEPARIN AND ITS FRAGMENTS

Heparin and its enzymatic fragments, prepared by degradation of hepari n with heparinase from Flavo-bacterium heparinum, were capable of inhi biting the actomyosin-ATPase activity obtained from striated and smoot h vascular muscles. Heparin did not inhibit the myosin-ATPase activity in absence of actin. The results show that heparin changes the step o f ATP hydrolysis of the complex actomyosin-ATPase by uncoupling the co nformational transition on the myosin-head induced by actin upon the n ucleotide-binding site. This mechanism is cooperative and dependent on conformational states of actomyosin complex which in turn is regulate d by ATP and calcium levels. It was observed that in the presence of A TP, actin does not compete with heparin for binding to myosin showing that heparin and actin have different binding sites on myosin. The bin ding of heparin and ATP is cooperative suggesting that the nucleotide binding leads to an exposition of a second heparin-binding site. Howev er, in the absence of ATP, actin competes with heparin for a binding s ite on the myosin. These results strongly suggest that in the weakly b inding state of actin to myosin, the binding of heparin is powerful an d in the rigor state its binding is decreased.