Exchange of adenosine diphosphate bound to actin in superprecipitated actomyosin and contracted myofibrils

Abstract

Up to half of the ADP bound to actin passes into the medium from super-precipitated actomyosin. The presence of ATP or other nucleotide triphosphates is necessary for release to take place. Superprecipitation is also a required condition. If superprecipitation of actomyosin is delayed or inhibited by the omission of magnesium in the presence of the various nucleotide triphosphates or by lowering the temperature, the release is also delayed or inhibited even though ATP or nucleotide triphosphates are present in the medium in great excess. The reaction is an exchange reaction and the actin-bound ADP is replaced by ADP formed from the ATP of the medium. In a system where the actomyosin contains [3H]ADP and the medium [14C]ATP, the loss of [3H]ADP from the actomyosin precipitate is compensated for by the incorporation of [14C]ADP throughout the whole course of the reaction. The same sites can undergo a reaction more than once and a major portion of the newly incorporated [14C]ADP re-exchanges at about the same speed as the unreacted sites containing [3H]ADP. The change on actin leading to the availability of its bound ADP is a transitory one and the ADP newly incorporated into super-precipitated actomyosin cannot serve as cofactor for the creatine kinase-phosphocreatine system. Exchange takes place also on “natural” actomyosin and, though with a slower speed, on washed myofibrils. The exchange reaction with actomyosin starts with a rapid burst, then proceeds at a slower rate. The experiments are interpreted to mean that myosin and ATP in conditions of superprecipitation and contraction impose a change on actin which leads to a loosening of the bond between actin and its bound ADP. The evidence would indicate that repetitive, cyclic reactions occur at the sites of actin and myosin interaction. The rapidly exchanging fraction of ADP may represent the maximum number of actin and myosin sites in contact in an actomyosin precipitate. The slower exchange would be limited by the time required for the completion of the previous cycle and for the unreacted actin sites to come into close proximity with active sites on myosin.†