Investigation of Apparent Isotope Effect Between exp 13 CO sub 2 and exp 14 CO sub 2 Trioctanoin Breath Tests

摘要

A decrease in the recovery of exp 13 CO sub 2 relative to exp 14 CO sub 2 has been previously reported at low oxidation rates of 1-C- labeled trioctanoin. To determine whether or not this phenomenon is due to an isotope effect, and if so, to identify the metabolic step at which it occurs, the in vivo oxidation of exp 13 C and exp 14 C acetate, octanoate and trioctanoin were compared. The exp 13 C and exp 14 C substrates were administered simultaneously to rats whose expired CO sub 2 was collected in 1N NaOH, and subsequently released by acidification, and cryogenically purified in vacuum. Total CO sub 2 was measured manometrically, and the exp 14 C and exp 13 C recoveries were determined by scintillation counting and mass spectrometry, respectively. The trioctanoin breath tests did not reproduce the previously reported isotopic discrepancy. Further investigation revealed that the emulsified corn oil used as a pancreatic stimulant significantly depressed the endogenous exp 13 CO sub 2 abundance in breath and caused the discrepancy. These studies did reveal a small time delay in the appearance of exp 14 CO sub 2 relative to exp 13 CO sub 2 during the trioctanoin breath test, which is probably due to a kinetic isotope effect. This difference in oxidation rates, however, is too small to affect the results of clinical breath tests. (ERA citation 04:020411)