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

Abstract

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 medications leading to increased morbidity and poor survival. To determine the possible importance of dietary cyst(e)ine availability for whole blood glutathione synthesis in humans, we developed a convenient mass spectrometric method for measurement of the isotopic enrichment of intact GSH and then applied 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 cysteine) free mixture (SAA-free). On day 10, L-[1-(13)C]cysteine was given as a primed, constant i.v. infusion (3 micromol x kg(-1) x h(-1)) for 6 h, and incorporation of label into whole blood GSH determined by GC/MS selected ion monitoring. The fractional synthesis rate (mean +/- SD; day(-1)) of whole blood 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 microM for the adequate and SAA-free periods (P > 0.05), and the absolute rate of GSH synthesis was 747 +/- 216 and 579 +/- 135 micromol x liter(-1) x day(-1), respectively (P < 0.05). Thus, a restricted dietary supply of SAA slows the rate of whole blood GSH synthesis and diminishes turnover, with maintenance of the GSH concentration in healthy subjects.

Figure 1

The assigned nominal structure of the bicyclo-glutarimide parent ion of derivatized GSH, incorporating [1-¹³C]cysteine. The glycyl, glutamyl, and cysteinyl moieties within the molecule are indicated by the arrows.

Figure 2

(A) ¹³C enrichment (atoms percent excess × 10⁻³) of expired carbon dioxide for the adequate (♦) and the sulfur amino acid-free diet (SAA-free) (). (B) Plasma isotopic enrichment (net tracer/tracee ratio) of l-[1-¹³C]cysteine. (C) Change in the whole blood enrichment (net tracer/tracee ratio) of [¹³C]cysteinyl-GSH derived from the l-[1-¹³C] cysteine tracer. Bars are SEM.