Saks 2001 Biochem J
|Saks VA, Kaambre T, Sikk P, Eimre M, Orlova E, Paju K, Piirsoo A, Appaix F, Kay L, Regitz-Zagrosek V, Fleck E, Seppet E (2001) Intracellular energetic units in red muscle cells. Biochem J 356:643-57.|
Abstract: The kinetics of regulation of mitochondrial respiration by endogenous and exogenous ADP in muscle cells in situ was studied in skinned cardiac and skeletal muscle fibres. Endogenous ADP production was initiated by addition of MgATP; under these conditions the respiration rate and ADP concentration in the medium were dependent on the calcium concentration, and 70±80% of maximal rate of respiration was achieved at ADP concentration below 20 µM in the medium. In contrast, when exogenous ADP was added, maximal respiration rate was observed only at millimolar concentrations. An exogenous ADPconsuming system consisting of pyruvate kinase (PK; 20± 40 units}ml) and phosphoenolpyruvate (PEP; 5 mM), totally suppressed respiration activated by exogenous ADP, but the respiration maintained by endogenous ADP was not suppressed by more than 20±40%. Creatine (20 mM) further activated respiration in the presence of ATP and PKPEP. Short treatment with trypsin (50±500 nM for 5 min) decreased the apparent Km for exogenous ADP from 300±350 µM to 50±60 µM, increased inhibition of respiration by PKPEP system up to 70±80%,with no changes in MgATPase activity and maximal respiration rates. Electron-microscopic observations showed detachment of mitochondria and disordering of the regular structure of the sarcomere after trypsin treatment. Two-dimensional electrophoresis revealed a group of at least seven low-molecular-mass proteins in cardiac skinned fibres which were very sensitive to trypsin and not present in glycolytic fibres, which have low apparentKm for exogenous ADP. It is concluded that, in oxidative muscle cells, mitochondria are incorporated into functional complexes (`intracellular energetic units') with adjacent ADP producing systems in myofibrils and in sarcoplasmic reticulum, probably due to specific interaction with cytoskeletal elements responsible for mitochondrial distribution in the cell. It is suggested that these complexes represent the basic pattern of organization of muscle-cell energy metabolism.
• Keywords: ADP, ATPases, Compartmentation, Mitochondria, Myocytes
Tissue;cell: Heart, Skeletal muscle Preparation: Permeabilized tissue, Isolated mitochondria