Cyst(e)ine measurements during total parenteral nutrition

Using a method that measures plasma protein-bound 1/2 cystine, plasma free cyst(e)ine, and urine free cyst(e)ine, we have observed that infants receiving total parenteral nutrition supplemented with cysteine-HCl have significantly higher plasma and urine free cyst(e)ine concentrations (11.3 +/- 3.4 microns/dl and 47.2 +/- 24.0 microns/dl, means +/- SD) than when 1/2 cystine concentrations are measured by automated amino acid analysis (7.4 +/- 2.0 microns/dl, plasma and 25.0 +/- 7.0 microns/dl, urine). These data suggest that both the sulfhydryl and disulfide form of cyst(e)ine must be measured to assess the effect of cysteine-HCl supplemented total parenteral nutrition.