Regulation of branched-chain ketoacid dehydrogenase flux by extracellular pH and glucocorticoids.

In muscles of rats with metabolic acidosis, branched-chain alpha-ketoacid dehydrogenase (BCKAD) activity is increased. Potential stimulatory signals include acidemia and/or glucocorticoids. It is unclear whether the signal(s) increases BCKAD activity by changing the activation state of the enzyme or by increasing the amount of enzyme. To separate the influences of extracellular pH and glucocorticoids on leucine catabolism, maximal BCKAD flux and the activation state (the ratio of basal to total flux) were measured in two cell types: 1) cells that do not express glucocorticoid receptors and 2) cells stably transfected to express glucocorticoid receptors. Acidification (pH 6.95) increased 1) the activation state from 67.2% at pH 7.4 to 82.8% at pH 6.95, 2) maximal BCKAD flux by 50%, and 3) the BCKAD subunit contents in both cell types (57, 410, and 270% for E2, E1 alpha, and E1 beta, respectively). Dexamethasone increased the BCKAD activation state from 67.2 to 82.3% in cells expressing glucocorticoid receptors, whereas dexamethasone plus acidification increased the activation state to 98%. The time course of stimulation by dexamethasone was slower than that by acidification. These results demonstrate that BCKAD is differentially regulated by extracellular pH and glucocorticoids.