Analysis of positional isotope exchange in ATP by cleavage of the beta P-O gamma P bond. Demonstration of negligible positional isotope exchange by myosin.

A method for analysis of positional isotope exchange (PIX) during ATP in equilibrium with HOH oxygen exchange is presented that uses a two-step degradation of ATP resulting in cleavage of the beta P-O gamma P bond. This cleavage yields Pi derived from the gamma-phosphoryl of ATP that contains all four of the gamma oxygens. Both PIX between the beta,gamma-bridge and beta-nonbridge positions and washout of the gamma-nonbridge oxygens can be simultaneously followed by using ATP labeled with 17O at the beta-nonbridge positions and 18O at the beta,gamma-bridge and gamma-nonbridge positions. Application of this method to ATP in equilibrium with HOH exchange during single turnovers of myosin indicates that the bulk of the ATP undergoes rapid washout of gamma-nonbridge oxygens in the virtual absence of PIX. At 25 degrees C with subfragment 1 the scrambling rate is at the limit of detectability of approximately 0.001 s-1, which is 50-fold slower than the steady-state rate. This corresponds to a probability of scrambling for the beta-oxygens of bound ADP of 1 in 10,000 for each cycle of reversible hydrolysis of bound ATP. A fraction of the ATP, however, does not undergo rapid washout. With myosin and stoichiometric ATP at 0 degrees C, this fraction corresponds to 10% of the ATP remaining at 36 s, or 2% of the initial ATP, and an equivalent level of ATP is found that does not bind irreversibly to myosin in a cold chase experiment. A significant level of apparent PIX is observed with subfragment 1 in the fraction that resists washout, and this apparent PIX is shown to be due to contaminant adenylate kinase activity. This apparent PIX due to adenylate kinase provides a possible explanation for the PIX observed by Geeves et al. [Geeves, M. A., Webb, M. R., Midelfort, C. F., & Trentham, D. R. (1980) Biochemistry 19, 4748-4754] with subfragment 1.