The Effects of Ionic Conditions, Temperature, and Chemical Modification on the Fluorescence of Myosin during the Steady State of ATP Hydrolysis A COMPARISON OF THE FLUORESCENCE AND ELECTRON SPIN RESONANCE SPECTRA OF THE SPIN-LABELED ENZYME*

The ATP-induced enhancement of the intrinsic fluorescence of myosin and heavy meromyosin (HMM) that persists during the steady state of hydrolysis has been investigated. To compare the substrateinduced changes in fluorescence with those in the electron spin resonance spectrum of the spin-labeled enzyme, we studied the influence of temperature, pH, and ionic strength, as well as the effect of chemical modification (spin labeling) of the SH-I sulfhydryl groups. Changing the pH between 6 and 9 does not affect the enhancement of fluorescence of myosin or HMM; changing the ionic strength, which could be studied only with HMM, also has no effect; and decreasing the temperature from 20 to 5” slightly diminishes the enhancement with both myosin and HMM. Chemical modification with N-(loxyl-2,2,6,6-tetramethyl-4-piperidinyl) iodoacetamide, which blocks the SH-1 thiol groups, reduces the enhancement of fluorescence, induces a strong dependence on ionic strength and pH, and substantially increases the dependence on temperature. The enhancement with labeled myosin or labeled HMM increases with increasing pH, ionic strength, and temperature, closely paralleling the effects of these parameters on the electron spin resonance spectrum of spin-labeled myosin (SEIDEL, J. C. AND GERGELY, J. (1973) Arch. Biochem. Biophys. 158, 853), suggesting that the same molecular change, induced by ATP and associated with formation of the M**ADP.P, complex, underlies both the change in fluorescence and the change in ESR spectrum. Those analogues of ATP that produce the maximal enhancement of fluorescence (WERBER, M., SZENT-GY~RGYI, A. G., AND FASMAN, G. (1972) Biochemistry 11, 2872) also produce the maximal change in the ESR spectra. Both an amino group at position 6 of the substrate and an unmodified triphosphate chain are required for maximal change in either fluorescence or ESR spectra. The smaller enhancement of fluorescence produced by spin labeling the SH-1 groups persists after the nitroxide has been chemically changed to a diamagnetic species. Thus the smaller enhancement cannot be attributed to paramagnetic quenching of tryptophan fluorescence by the spin label. An initial burst of phosphate liberation accompanies the hydrolysis of ATP, cytidine 5’-triphosphate, uridine 5’-triphosphate, guanosine 5’.triphosphate, inosine 5’triphosphate, 2’.deoxyadenosine 5’.triphosphate, adenosine 5’.tetraphosphate, and tripolyphosphate. The presence or absence of the burst does not correlate with the extent of the spectral change.