Blood glutathione synthesis rates in healthy adults receiving a sulfur amino acid-free diet

The availability of cysteine is thought to be the rate limiting factor for synthesis of the tripeptide glutathione (GSH), based on studies in rodents. GSH status is compromised in various disease states and by certain medicat ions leading to increased morbidity and poor survival. To determine the pos sible importance of dietary cyst(e)ine availability for whole blood glutath ione synthesis in humans, we developed a convenient mass spectrometric meth od for measurement of the isotopic enrichment of intact GSH and then applie d it in a controlled metabolic study, Seven healthy male subjects received during two separate 10-day periods an L-amino acid based diet supplying an adequate amino acid intake or a sulfur amino acid (SAA) (methionine and cys teine) free mixture (SAA-free). On day 10, L-[1-C-13]cysteine was given as a primed, constant i.v. infusion (3 mu mol.kg(-1).h(-1)) for 6 h, and incor poration of label into whole blood GSH determined by GC/MS selected ion mon itoring. The fractional synthesis rate (mean +/- SD; day(-1)) of whole bloo d GSH was 0.65 +/- 0.13 for the adequate diet and 0.49 +/- 0.13 for the SAA -free diet (P < 0.01). Whole blood GSH was 1,142 +/- 243 and 1,216 +/- 162 mu M for the adequate and SAA-free periods (P > 0.05), and the absolute rat e of GSH synthesis was 747 +/- 216 and 579 +/- 135 mu mol.liter(-1).day(-1) , respectively (P < 0.05). Thus, a restricted dietary supply of SAA slows t he rate of whole blood GSH synthesis and diminishes turnover, with maintena nce of the GSH concentration in healthy subjects.