Indication for an allosteric effect of adenosine diphosphate in actomyosin gels from insect fibrillar flight muscle

Abstract

Actomyosin can be extracted as the intact contractile complex from insect fibrillar flight muscle at high ionic strength. The ATPase activity is activated by calcium concentrations above 10 −8 m and reaches an initial maximum at 7.5 × 10 −7 m Ca ++. This Ca ++-activated actomyosin ATPase is under cooperative inhibition by ADP. In presence of inhibitory levels of ADP, the Ca ++-activated enzyme also exhibits cooperative effects with respect to the substrate. A structural transition can be observed in presence of ADP. Both the cooperative effects of ADP on the enzyme activity and the structural changes induced by ADP are lost as a result of various desensitizing treatments, although the enzyme activity either increases or remains unaffected. An allosteric model and its possible implication for the oscillatory contractile activity of insect flight muscle is discussed.