Regulation of α-glycerophosphate dehydrogenase activity in human term placental mitochondria

Abstract

1. 1. α-Glycerophosphate dehydrogenase ( sn-glycerol-3-phosphate:(acceptor) oxidoreductase, EC 1.1.99.5) activity in mitochondria isolated from human term placenta was found to be inhibited by ethyleneglycolbis(β-aminoethyl ether)-N,N′- tetraacetic acid (EGTA). Addition of an excess of calcium ions to the incubation medium completely restored the original activity. The concentration of free calcium ion required to activate the α-glycerophosphate dehydrogenase was found to vary between 10 and 100 nM. 2. 2. The pH optimum for α-glycerophosphate dehydrogenase activity varied with substrate concentration. The pH optima were 7.4 and 8.0 in the presence of 2 or 8 mM α-glycerophosphate, respectively. The apparent K m for α-glycerophosphate also varied with pH; the values being 0.4 mM at pH 7.05, 1.5 mM at pH 7.8, and 3.5 mM at pH 8.5. 3. 3. α-Glycerophosphate dehydrogenase activity was inhibited by palmitoylCoA in a competitive manner with an apparent K i value of about 10 μM. This inhibition was less pronounced in the presence of calcium or magnesium ions. 4. 4. The activity of α-glycerophosphate dehydrogenase was inhibited by phospho enolpyruvate, d- and dl-glyceraldehyde 3-phosphate and 3-phosphoglyceric acid, in a competitive manner, the apparent K i values being 0.5, 0.95, 0.12 and 1.5 mM, respectively. 5. 5. α-Glycerophosphate dehydrogenase activity in human placental mitochondria was found to be more sensitive to phospho enolpyruvate, than the activity of the same enzyme in rat skeletal muscle mitochondria, α-Glycerophosphate dehydrogenase activity in rat brown adipose tissue mitochondria was only slightly affected by phosph enolpyruvate under the same conditions. 6. 6. The data obtained suggest that the activity of α-glycerophosphate dehydrogenase in human placental mitochondria may be controlled by changes of the cytosolic level of palmitoyl-CoA, some glycolytic intermediates, and pH.