Sulfur amino acid metabolism in infants on parenteral nutrition

Abstract

In the infant on parenteral nutrition, cysteine supplementation has been suggesteddue to low levels of hepatic cystathionase activity limiting synthesis from methionine. We have examined the plasma concentrations of sulfur amino acids in four groups of post-surgical infants requiring parenteral nutrition receiving (A) a low methionine + cysteine + taurine formula, (B) a high methionine formula (non-steady state), (C) a high methionine formula (steady state), and (D) a high methionine + cysteine formula. Plasma methionine concentrations were above the normal reference range (2.2–4.9 micromole/dL) of normal breast-fed infants in Groups B (15.9 ± 10.7 micromole/dL) and D (5.7 ± 1.9 micromole/dL) and at the upper limit for Group C (4.9 ± 1.7 micromole/dL). Total cysteine/cystine concentrations (normal reference range, 10.2–20.4 micromole/dL) were highest in Groups A (18.9 ± 3.5 micromole/dL) and D (16.8 ± 5.3 micromole/dL) that received cysteine HCI supplementation, and lowest in Group B (8.6 ± 3.7 micromole/dL) that received no cysteine in non-steady state. All plasma free cystine concentrations were below the normal reference range (3.6–6.8 micromole/dL). Plasma taurine concentrations were not significantly different among the four groups and all were within the normal reference range (0.6–16.2 micromole/dL). The strikingly elevated methionine and low total cysteine/cystine values in Group B suggested the existence of a feedback loop of methionine conversion below the level of homocysteine. Equilibrium of methionine and cysteine/cystine plasma concentrations did occur, in time. Parenteral cysteine administration resulted in a greater proportion of plasma free cysteine concentration, but not cystine. The proportion of free to bound cysteine/cystine, as well as the proportion of free cystine to cysteine, was not normal during parenteral nutrition with or without cysteine HCI supplementation. Little benefit in plasma concentrations was derived from cysteine HCI supplementation to a high methionine formulation.