Virtual cooperativity in myoglobin oxygen saturation curve in skeletal muscle in vivo

BACKGROUND: Myoglobin (Mb) is the simplest monomeric hemoprotein and its physicochemical properties including reversible oxygen (O(2))binding in aqueous solution are well known. Unexpectedly, however, its physiological role in intact muscle has not yet been established in spite of the fact that the role of the more complex tetrameric hemoprotein, hemoglobin (Hb), in red cells is well established. Here, I report my new findings on an overlooked property of skeletal Mb. METHODS: I directly observed the oxygenation of Mb in perfused rat skeletal muscle under various states of tissue respiration. A computer-controlled rapid scanning spectrophotometer was used to measure the oxygenation of Mb in the transmission mode. The light beam was focused on the thigh (quadriceps) through a 5-mm-diameter light guide. The transmitted light was conducted to the spectrophotometer through another 5-mm-diameter light guide. Visible difference spectra in the range of 500–650 nm were recorded when O(2 )uptake in the hindlimb muscle reached a constant value after every stepwise change in the O(2 )concentration of the buffer. RESULTS: The O(2 )dissociation curve (ODC) of Mb, when the effluent buffer O(2 )pressure was used as the abscissa, was of a sigmoid shape under normal and increased respiratory conditions whereas it was of rectangular hyperbolic shape under a suppressed respiratory condition. The dissociation curve was shifted toward the right and became more sigmoid with an increase in tissue respiration activity. These observations indicate that an increase in O(2 )demand in tissues makes the O(2 )saturation of Mb more sensitive to O(2 )pressure change in the capillaries and enhances the Mb-mediated O(2 )transfer from Hb to cytochrome oxidase (Cyt. aa(3)), especially under heavy O(2 )demands. CONCLUSION: The virtual cooperativity and O(2 )demand-dependent shifts of the ODC may provide a basis for explaining why Mb has been preserved as monomer during molecular evolution.