The oxidation of L- and D-cysteine to inorganic sulfate and taurine in the rat

Abstract

Oral administration of L-cysteine to rats (8 mmol/kg body wt.) caused a rapid increase of the concentration of cystine in serum, from less than 5 μM in controls to about 200 μM. Concomitantly, the serum concentration of inorganic sulfate increased, reaching a peak 2 h after L-cysteine administration; this level, twice the control level, was maintained for 4 h. Serum sulfate returned to control concentration 23 h after L-cysteine administration. The urinary excretion of inorganic sulfate during the 24 h after administration increased considerably, and 33% of the dose of L-cysteine was recovered as inorganic sulfate in urine. Consumption of comparable amounts of L-cysteine via the food caused the same increase in urinary excretion of sulfate, but did not increase the concentration of sulfate in serum. After oral administration of D-cysteine (8 mmol/kg body wt.), very high cystine levels were reached in serum (mean concentration about 1 500 μM); the sulfate concentration was already maximal 30 min after administration. The increase in urinary excretion of sulfate after D-cysteine was also higher than after L-cysteine administration: 55% of the dose. Possible routes for the rapid degradation of D-cysteine to inorganic sulfate are discussed. The administration of L-cysteine also caused an increase in the serum concentration of taurine, while methionine was not influenced. D-Cysteine did not increase the serum concentration of taurine, indicating that it is probably not or only slowly converted to taurine.