Résumé
The myofibril is a good model to study the ATPase of the muscle fibre. When myofibrillar ATPase reaction mixtures are quenched in acid, there is a burst of P
i formation, due to AM · ADP · P
i or P
i as shown in the scheme: AM + ATP ↔ A·M·ATP ↔ AM·ADP·Pi ↔ AM·ADP + P
i ↔ AM + ADP. Therefore, in the steady state, either AM·ADP·P
i or AM·ADP or both predominate. To determine which, we studied the reaction using a P
i binding protein (from
E. coli) labeled with a fluorophore such that it is specific and sensitive to free P
i [Brune, M. et al. (1994) Biochemistry 33, 8262–8271]. We show that the P
i bursts with myofibrillar ATPases (calcium-activated or not, or crosslinked) are due entirely to protein bound P
i. Thus, with myofibrillar ATPases the AM·ADP·P
i state predominates.