Metabolism of [35S]taurine in man

Taurine metabolism in man was defined by an isotope dilution technique during normal taurine intake (five subjects) or increased taurine intake (two subjects). A tracer dose of [35S]taurine was administered intravenously, and the amount and chemical form of radioactivity were determined in blood, urine, bile, and feces. Analysis of plasma specific activity decay curves indicated that taurine metabolism can be described by two exchangeable pools: a small (2 mmoles), rapidly exchanging pool (t1/2 approximately equal to 0.1 hour); and a large (98 mmoles), very slowly exchanging pool (t1/2 approximately equal to 70 hours). A small amount of [35S]isethionic acid was detected in urine, possibly the result of deamination of taurine by tissues; but otherwise no evidence of tissue biotransformation was obtained. Taurine was excreted predominantly (95%) in urine, about 70% as taurine and 25% as sulfate. The sulfate was considered to be formed in the intestine by bacterial degradation of taurine and then absorbed. Supplemental taurine (given orally) was well absorbed, caused a transient increase in plasma taurine levels, was excreted in urine without equilibration with the slowly exchangeable pool, and caused only a modest increase in total body taurine. Thus, taurine resembles other amino acids in having large tissue pools but differs strikingly in being metabolically inert with an extremely slow turnover rate.