Excretion of Taurine and Sulfate in Rats Fed with a Low Protein Diet

The effects of a low protein diet on the excretion of sulfate and taurine, major metabolites of L-cysteine in mammals, were studied in rats fed with synthetic 10% (group A) and 25% (group B) casein diets. The average excretions of total taurine (taurine plus hypotaurine) and total sulfate (free plus ester sulfate) (μmol/kg of body weight per day after the adaptation to the synthetic diet) in group A were 14.2 ± 13.4 and 122.3 ± 39.6, respectively, which were very low compared with 280.4 ± 93.8 and 943.2 ± 144.8, respectively, in group B. The taurine/ sulfate ratio in group A was 0.12 ± 0.11, which was significantly lower than that (0.30 ± 0.08) in group B. A single intraperitoneal injection of 5 mmol of L-cysteine per kg of body weight in group A resulted in an increase in average taurine and sulfate excretion to 693.4 ± 195.6 and 2440.6 ± 270.0, respectively, and thus the average taurine/sulfate ratio increased to 0.29. These increases were transient and low taurine excretion resumed again 24h after the L-cysteine administration. L-Cysteine injection in group B resulted in a similar increase in taurine and sulfate excretion, but the ratio changed only slightly (0.28). The present results suggest that in vivo production of taurine is reduced preferentially over sulfate production when sulfur amino acid supply is limited.