Control of Non-Respiratory Metabolism by Tissue Oxygen

Abstract

For a number of years the experimental observation that extraction of many intracellular enzymes resulted in their inactivation was considered a pure artifact. The mechanism of inactivation was shown to be the S-thiolation of thiol groups necessary for enzymic activity. How then could these enzymes remain active in situ? In 1970 Brian Hartley suggested that cells may be totally anoxic. On closer examination, this apparently heretical view had real merit. The major oxygen-consuming enzyme, cytochrome a-a₃, can use other electron acceptors, so oxygen might not be the physiological one. Early attempts to measure intracellular oxygen with very small Polarographic electrodes appeared to show complete anoxia (Steele, 1960). The findings with larger electrodes could perhaps be criticized on the grounds that they had torn the cell wall, permitting oxygen to leak in. In general, studies with leuco-dyes could also have alternate explanations in that the color changes could be brought about by other electron acceptors. However, the introduction of dyes whose fluorescence is specifically quenched by oxygen in biological systems showed that the anoxia theory was untenable (Longmuir and Knopp, 1972). How then do thiol enzymes retain their activity? In 1976 Chance showed that perfusion of liver with solutions having a very high partial pressure of oxygen resulted in an efflux of oxidized glutathione (GSSG).