INHIBITION OF ATP BINDING TO MYOFIBRILS AND ACTO-MYOSIN SUBFRAGMENT-1 BY CAGED-ATP

The inhibitory effect of P-3-[1-(2-nitrophenyl)ethyl] adenosine 5'-triphosphate (caged ATP) on the binding of Mg2+-ATP to myofibrils was investigated. The most sensitive method was found to be the monitoring of single turnovers of [gamma_P-32] ATP hydrolysis using the quench flow technique. The method was tested using ADP, which was found to have an inhibition constant of 145 mu M, in agreement with previous reports. Caged ATP behaved as a simple competitive inhibitor of ATP binding with an inhibition constant of 1.6 mM. The inhibitory effect of these ligands on the binding of ATP to acto-myosin subfragement 1 was investigated using the same method. The inhibition constants of caged ATP and ADP were found to be 0.35 mM and 50 mu M, respectively. This inhibitory effect of caged ATP on ATP binding accounts for the lower rate of ATP binding to fibers, deduced from caged ATP [(0.5-1) X 10(6) M(-1) s(-1)], than that reported for acto-S1 (3.5 x 10(6) M(-1) s(-1)) [Goldman, Y. E., Hibberd, M. G., and Trentham, D. R. (1984) J. Physiol. (London) 354, 577].