Milk protein quantity and quality in low-birth-weight infants. III. Effects on sulfur amino acids in plasma and urine

Well, appropriate-for-gestational age, low-birth-weight infants weighing 2,100 gm or less were divided intothree gestational age groups and assigned randomly within each age group to one of five feeding regimens: pooled human milk; formula 1 (F₁)=1.5 gm/dl protein, 60 parts bovine whey proteins: 40 parts bovine caseins; F₂=3.0 gm/dl, 60:40; F₃=1.5 gm/dl, 18:82; F₄=3.0 gm/dl, 18:82. Plasma and urine concentrations of methionine and of cystathionine were higher in the infants fed F₁ to F₄ than in the infants fed BM. The plasma cystine concentrations of the infants fed F₁ or F₃ were no different from those fed BM. Plasma cystine concentrations of infants fed F₂ (which had a cystine content at least twice that of any of the other formulas) were significantly higher than those of infants fed BM. Plasma taurine concentrations of infants fed F₃ or F₄, which were virtually devoid of taurine, decreased steadily during the course of study becoming lower than those of infants fed BM. Urine taurine concentrations of infants fed F₁, F₃ or F₄ (but not F₂ which had more taurine than F₁, F₃, or F₄) were lower than those of infants fed BM. These results provide further evidence for the limited capacity of the preterm human infant to convert methionine to cystine, owing to delayed maturation of cystathionase, and suggest a limited capacity to convert cystine to taurine. The latter suggestion is consistent with low human hepatic cysteinesulfinic acid decarboxylase activity 0.26 (fetal) and 0.32 (adult) nmoles/mg protein/hour vs 468 in rat liver.