Hepatic UDP-glucose ¹³C isotopomers from [U-¹³C] glucose: A simple analysis by ¹³C NMR of urinary menthol glucuronide

Menthol glucuronide was isolated from the urine of a healthy 70-kg female subject following ingestion of 400 mg of peppermint oil and 6 g of 99% [U- ¹³C]glucose. Glucuronide ¹³C-excess enrichment levels were 4-6% and thus provided high signal-to-noise ratios (SNRs) for confident assignment of ¹³C-¹³C spin-coupled multiplet components within each ¹³C resonance by ¹³C NMR. The [U- ¹³C]glucuronide isotopomer derived via direct pathway conversion of [U-¹³C]glucose to [U-¹³C]UDP-glucose was resolved from [1,2,3-¹³C₃]- and [1,2-¹³C₂] glucuronide isotopomers derived via Cori cycle or indirect pathway metabolism of [U-¹³C]glucose. In a second study, a group of four overnight-fasted patients (63 ± 10 kg) with severe heart failure were given peppermint oil and infused with [U-¹³C]glucose for 4 hr (14 mg/kg prime, 0.12 mg/kg/min constant infusion) resulting in a steady-state plasma [U- ¹³C]glucose enrichment of 4.6% ± 0.6%. Menthol glucuronide was harvested and glucuronide ¹³C-isotopomers were analyzed by ¹³C NMR. [U-¹³C]glucuronide enrichment was 0.6% ± 0.1%, and the sum of [1,2,3-¹³C₃] and [1,2- ¹³C₂]glucuronide enrichments was 0.9% ± 0.2%. From these data, flux of plasma glucose to hepatic UDPG was estimated to be 15% ± 4% that of endogenous glucose production (EGP), and the Cori cycle accounted for at least 32% ± 10% of GP.