Abstract 1711 Poster Session III, Monday, 5/3 (poster 88)

Controversy exists as to whether cysteine (CYS), a precursor to the antioxidant glutathione (GSH), is an essential amino acid for neonates, particularly those requiring parenteral nutrition (PN). However, no study has assessed the effect of CYS supplementation of PN on whole body protein kinetics or on erythrocyte glutathione concentration (RBC-GSH), the major source of GSH in blood. Thus, to determine if CYS improves protein metabolism and antioxidant capacity, we measured whole body protein kinetics, nitrogen balance, and RBC-GSH concentrations in neonates who were selected randomly and blindly to receive PN regimens with CYS (*CYS) or without CYS (-CYS). Nine neonates (BW: 1.47 ± 0.25 (SEM)kg; GA: 30.8 ± 1.0 wk) were assigned to +CYS (0.78 ± 0.03 mmol/kg/d of CYS-HCl), and 10 neonates (BW: 1.54 ± 0.16 kg; GA: 29.9 ± 1.4 wk) were assigned to -CYS. The +CYS and -CYS groups received their assigned regimens for 5.9 ± 0.5 and 6.8 ± 0.5 days prior to study, respectively. Protein kinetics were determined using primed, constant infusions of [2H5]phenylalanine and [2H2]tyrosine during continuous PN (+CYS group: 92 ± 4 kcal/kg/d, 2.9 ± 0.1 g/kg/d of amino acids; -CYS group: 99 ± 6 kcal/kg/d, 2.7 ± 0.1 g/kg/d). Nitrogen balance was calculated during the final 48 hours of study. RBC-GSH concentrations were determined by HPLC on the final day of study. Unpaired Student's t-tests were used for statistical analysis. In comparison to the -CYS group, the +CYS group had a lower rate of phenylalanine flux (protein turnover): 116 ± 16 vs. 155 ± 16 µmol/kg/hr, p = 0.10) and a lower rate of endogenous phenylalanine flux (protein breakdown): 81 ± 16 vs. 122 ± 15 µmol/kg/hr, p = 0.08). Nitrogen balance also was greater in the +CYS group (282 ± 35 vs 207 ± 37 mg/kg/d, P = 0.17). However, there was no difference in RBC-GSH between the +CYS and -CYS groups (0.62 ± 0.05 vs. 0.57 ± 0.04 mmol/L, p = 0.47). In conclusion, CYS supplementation modestly improves protein metabolism as demonstrated by lower rates of protein turnover and breakdown and higher rates of nitrogen retention. However, CYS does not appear to improve RBC-GSH concentrations. Whether CYS improves synthetic rates of RBC-GSH remains to be determined.