Studies on the bound nucleotide of muscle fibrils

Abstract

The possibility was investigated that phosphorus uptake from ATP or uptake of ATP molecules by the muscle proteins occurs during contraction and relaxation of rabbit myofibrils. It was found that the phosphorus content was the same in myofibrils contracted with ATP and the control myofibrils although phosphorylation of actin, if it occurred, may not have been detectable. An examination by paper chromatography of the UV-absorbing substances in washed frog and rabbit myofibrils revealed the presence of the adenine nucleotides, IMP, hypoxanthine, and adenine. ADP gave the most intense UV-absorbing spot and was presumably derived from F-actin. An oligo- or polynucleotide, possibly derived from RNA, and a green pigment, were also found. The adenine nucleotides in HCl and HC10 extracts of washed frog myofibrils were estimated. Approximately 2.90 - 3.10 umoles/gm. dry weight of adenine nucleotides were present, most of which was ADP. Considerably more ATP was found in the HCl extracts than was present in the HC1O4 extracts, in which most of the nucleotide was ADP. The concentration of ATP in the HC1 extracts was observed to alter after neutralization in association with a change in the ADP content. It is suggested that a phosphokinase is present in the HC1 extracts, but not in the HC10 extracts, which becomes active after neutralization and synthesises the ATP from ADP and an unidentified substance XP. A higher ATP/ADP ratio was found in the HCl extract from control myofibrils than in the extract from myofibrils contracted with ATP.When a frog muscle homogenate was washed with successive small volumes of water, the myofibrils swelled and structural protein passed into solution, to a much greater extent when conditions favoured relaxation than contraction of the fibrils. The extracted protein was precipitated by Mgions and contained adenine nucleotide. It was concluded to be a form of actomyosin containing F-actin in a low state of polymerisation. A theory of contraction is suggested which is consistent with these observations.