CONTINUOUS MEASUREMENT OF (CO2)-C-13-O-16 PRODUCTION FROM [C-13]PYRUVATE BY INTACT LIVER-MITOCHONDRIA - EFFECT OF HCO3-

We have measured continuously the production of mass 45 CO2 ((CO2)-C-1 2-O-16) from C-13-labeled pyruvate in a guinea pig liver mitochondrial suspension and simultaneously the O-2 consumption at 37 degrees C and pH 7.4. The reactions took place in a closed 3-ml volume, stirred, th ermoregulated chamber separated from the ion source of a mass spectrom eter by a gas-permeable membrane that permitted recording the mass pea ks of any gas dissolved in the reaction mixture with a response time a s fast as 3 s. If the pyruvate was labeled on C-2, no (CO2)-C-13-O-16 was formed, even after 1 h, indicating that C-2 and C-3 were not metab olized in the citric acid cycle. We found that production of (CO2)-C-1 3-O-16 was five times greater in the presence of 25 mM HCO, than in it s absence. A probable mechanism of this CO2/HCO3- effect is carboxylat ion of pyruvate to oxaloacetate, which would react with acetyl CoA to form citrate and with NADH to form malate, thus removing two major inh ibitors of pyruvate dehydrogenase. We conclude that CO2/HCO3- has a po tent and hitherto unappreciated regulatory effect on liver pyruvate de hydrogenase.