Inhibition of TCA cycle by 3‐isobutyl‐methylxanthine

Abstract

3-isobutyl-methylxanthine (IBMX), an inhibitor of phosphodiesterase, often is utilized in studies of hormone action, cell proliferation and signal transduction pathways. We used IBMX to elucidate the action of agmatine on levels of cAMP, TCA cycle activity and fatty acid oxidation in both isolated mitochondria and a liver perfusion system. When we used 13C-fatty acid as a metabolic tracer, IBMX inhibited formation of 13CO2 but not ketone bodies, suggesting that IBMX might inhibit the incorporation of acetyl-CoA into the TCA cycle. We explored this possibility by incubating isolated liver mitochondria with 13C-labeled pyruvate or octanoate. IBMX did not affect the release of 13CO2 from [1–13C]pyruvate, but it significantly decreased 13CO2 from [3–13C]pyruvate or [13C]octanoate as well as formation of 13C-labeled citrate. Such inhibition occurred in a dose-dependent fashion with an IC50 about 0.9 mM. Taken together, these observations indicate that IBMX inhibits incorporation of acetyl-CoA (formed from fatty acid or pyruvate), into the TCA cycle. These findings may have important implications in interpreting data obtained when IBMX is used as cAMP-elevating agent. Supported by NIH, DK-53761 and CA-79495 (to I.N.)