Plasma total cyst(e)ine, glutathione and the disulfide cystine in premature infants during the first month of life. † 1444

Background: Total (T) plasma cyst(e)ine represents the sum of protein-bound cysteine, cystine (disulfide of two molecules of cysteine) and free cysteine. Previous studies have measured cystine and free cysteine. Synthesis of glutathione, the major intracellular antioxidant, is dependant on the presence and availability of cyst(e)ine. Objective: To determine the concentrations of plasma cystine and total cyst(e)ine and their longitudinal relationship with plasma glutathione in premature infants. Design: Plasma samples were obtained on days 3, 7, 14 and 21 from 53 premature infants (mean ± SD: gestation: 27 ± 1.6 weeks; birthweight: 957 ± 176 g). Cystine was measured on a LKB 4151 Alpha Plus amino acid analyzer. The remaining sample was reduced and total cyst(e)ine and glutathione determined by HPLC. Results: see table for cystine and total cyst(e)ine (μmol/L): mean ± se; glutathione (μmol/L): median (range). Plasma glutathione was undetectable (< 0.3 μmol/L) in a significant proportion of infants(36%, 27%, 10%, 22% on days 3, 7, 14 and 21 respectively). In infants where glutathione was measurable, a correlation was found with total cyst(e)ine (r = 0.23, p= 0.01) but not with cystine. Conclusion: Cystine concentrations remain below the normal reference range in premature infants through the first month of life and represent only 20 to 30% of total cyst(e)ine. Total cyst(e)ine, but not cystine, correlates with plasma glutathione. We conclude that total cyst(e)ine should be measured when studying sulfur amino acid metabolism in premature infants.