PT - JOURNAL ARTICLE AU - Glyn B. Steventon TI - Diurnal Variation in the Metabolism of<em>S</em>-Carboxymethyl-<span class="sc">l</span>-Cysteine in Humans DP - 1999 Sep 01 TA - Drug Metabolism and Disposition PG - 1092--1097 VI - 27 IP - 9 4099 - http://dmd.aspetjournals.org/content/27/9/1092.short 4100 - http://dmd.aspetjournals.org/content/27/9/1092.full SO - Drug Metab Dispos1999 Sep 01; 27 AB - The routes of metabolism ofS-carboxymethyl-l-cysteine in humans are dependent on the time of dosing. Administration of 750 mg ofS-carboxymethyl-l-cysteine (Day 1) during the day at 8:00 AM followed by a 8:00 AM to 4:00 PM urine collection revealed thatS-carboxymethyl-l-cysteineS-oxide was the major urinary metabolite produced. The 4:00 PM to midnight urine collection resulted inS-(carboxymethylthio)-l-cysteine being identified as the major urinary metabolite. However, the administration of 750 mg of S-carboxymethyl-l-cysteine (day 15) during the night at midnight and analysis of the midnight to 8:00 AM urine collection found that thiodiglycolic acid was the major urinary metabolite, whereas thiodiglycolic S-oxide was identified as the major urinary metabolite in the 8:00 AM to 4:00 PM urine collection. A diurnal variation in the metabolism ofS-carboxymethyl-l-cysteine was seen and, in particular, the timing ofS-carboxymethyl-l-cysteine administration had a profound effect on the identity of urinary S-oxide metabolites produced. After administration at 8:00 AM the urinaryS-oxides produced wereS-carboxymethyl-l-cysteineS-oxide andS-methyl-l-cysteine S-oxide but at midnight the major urinary S-oxide metabolite produced was thiodiglycolic acid S-oxide. The American Society for Pharmacology and Experimental Therapeutics