ABSTRACT
We have previously reported our finding that IMP inhibits the Mg2+ -stimulated acto-myosin-ATPase activity of isolated actin and myosin. These experiments were undertaken at 35 degrees C and pH 7.0. It was also shown that the binding of actin to myosin was cooperative and that in the presence of IMP the Hill coefficient was decreased. The experiments shown here were carried out with isolated actin and myosin at three temperatures (25 degrees C, 31 degrees C and 37 degrees C) and three pH values (6, 7 and 8). The results show that: (i) the Mg2+ -stimulated acto-myosin-ATPase activity decreases with decreasing temperature; (ii) the Mg2+ -stimulated acto-myosin-ATPase activity is lower at pH = 6 and 8 compared to pH = 7; (iii) the effect of temperature and pH on the Mg2+ -stimulated acto-myosin-ATPase activity can be explained by a decrease in co-operativity between actin and myosin; (iv) IMP inhibits the Mg2+ -stimulated acto-myosin-ATPase activity at all temperatures and pH values. The greatest inhibition is found at pH = 7; and (v) the inhibition by pH + IMP is about the same for pH = 6 and pH = 7; at pH = 8 this combined inhibition is slightly higher. This leads to the same decrease in Mg2+ -stimulated acto-myosin-ATPase activity. Muscle fatigue can be explained by a combination of non-regulatory factors (for example pH) and regulatory factors (such as IMP) and from our results we conclude that IMP serves as an additional regulatory safety switch to maintain the balance between energy consumption and energy production and thereby preventing an energy crisis during exhaustive exercise of short duration.
