MODULATION OF 2-OXOGLUTARATE DEHYDROGENASE AND OXIDATIVE-PHOSPHORYLATION BY CA2-CORTEX MITOCHONDRIA( IN PANCREAS AND ADRENAL)

The effect of external free Ca2+ ([Ca2+]ex) on the 2-oxoglutarate dehy drogenase (2-OGDH) activity and the rate of state 3 respiration was st udied in dog pancreas and adrenal cortex mitochondria. A method for th e preparation of mitochondria from pancreas, free of zymogen granules, was developed. The mitochondria showed high respiratory control value s and maintained adequate ADP/O ratios in the course of several hours. Half-maximal stimulation (K-0.5) of 2-OGDH activity and state 3 respi ration by Ca2+ was decreased twofold by spermine, in both pancreas and adrenal cortex mitochondria incubated with Mg2+; Na+ increased the K- 0.5 value for Ca2+ in both kinds of mitochondria. The matrix Ca2+ cont ent was enhanced by spermine at different [Ca2+]ex. The Ca2+ gradient ([Ca2+]m/[Ca2+]ex) determined in Fluo 3-loaded pancreas mitochondria w as found to be below 1, either in the presence of Nai or in the absenc e of spermine. The protonmotive force was increased by 1 mu M Ca2+, in state 3 conditions, by 6-8 mV; lower Ca2+ concentrations kept the Hgradient rather constant. An increase in the spermine/Mg2+ ratio at co nstant external Ca2+ resulted in stimulation of the 2-OGDH activity an d state 3 respiration. The data indicate that matrix Ca2+ may regulate the rate of ATP synthesis in pancreas and adrenal cortex mitochondria , through the modulation of 2-OGDH and the maintenance of a high Hf gr adient. In addition to changes in external Ca2+, the variation in the spermine/Mg2+ ratio may also exert control of oxidative phosphorylatio n. (C) 1995 Academic Press, Inc.