Taurine kinetics assessed using [1,2-13C2]taurine in healthy adult humans

To assess the dynamics of taurine metabolism in vivo, two sets of studies were carried out in healthy volunteers. First, pilot studies were carried in a single human subject to determine the time course of plasma and whole blood isotope enrichment over the course of an 8-h, unprimed continuous infusion of [1,2-13C2]taurine. Second, five healthy adult males received two tracer infusions on separate days and in randomized order: 1) a 6-h continuous infusion of [1,2-13C2]taurine (3.1 ± 0.2 μmol·kg−1·h−1) and 2) a bolus injection of [13C2]taurine (3.0 ± 0.1 μmol/kg). Isotope enrichments in plasma and whole blood taurine were determined by gas chromatography-mass spectrometry. The pilot experiments allowed us to establish that steady-state isotope enrichment was reached in plasma and whole blood by the 5th h of tracer infusion. The plateau enrichment reached in whole blood was lower than that obtained in plasma taurine ( P < 0.02). In the second set of studies, the appearance rate (Ra) of plasma taurine, determined from continuous infusion studies was 31.8 ± 3.1 μmol·kg−1·h−1. After a bolus injection of tracer, the enrichment decay over the subsequent 2 h was best fitted by a two-exponential curve. Taurine Ra was ≈85% higher when determined using the bolus injection technique compared with continuous infusion of tracer. We conclude that 1) taurine Ra into plasma is very low in healthy postabsorptive humans, and, due to taurine compartmentation between the extra- and intracellular milieus, may represent only interorgan taurine transfer and merely a small fraction of whole body taurine turnover; and 2) the bolus injection technique may overestimate taurine appearance into plasma. Further studies are warranted to determine whether alterations in bile taurine dynamics affect taurine Ra.