Metabolism of threonine in newborn infants

Threonine kinetics, threonine oxidative pathway, and the relationship between threonine and whole body protein turnover were quantified in 10 healthy term infants during the first 48 h after birth. The kinetic data were obtained 6 h after the last feed (fasting) and in response to formula feeding, using [U-¹³C₄,¹⁵N]threonine, [²H₅]phenylalanine, and [¹⁵N]glycine tracers. The rate of carbon dioxide production (V̇ CO₂) and ¹³C enrichment of the expired CO₂ were measured to quantify the rate of oxidation of threonine. The rate of appearance (R_a) of threonine (136 ± 37 μmol·kg^-1·h^-1) was higher in newborn infants than that reported in adults. Formula feeding resulted in a significant decrease in threonine R_a (P < 0.05). A significant positive correlation was seen between phenylalanine Ra and threonine R_a, both during fasting and after formula feeding (r² = 0.65). In contrast to a 1:1 ratio of threonine and phenylalanine in mixed muscle protein, threonine R_a relative to phenylalanine R_a was 2.2 ± 0.4. The fractional rate of threonine flux oxidized was 20% during fasting and 26% (P < 0.05) in response to nutrient administration. There was a significant correlation between plasma threonine concentration and threonine oxidation (r² = 0.75). No measurable incorporation of threonine in plasma glycine was seen. These data suggest that threonine is exclusively degraded by the glycine-independent serine/threonine dehydratase pathway. A higher flux of threonine relative to phenylalanine indicates higher turnover of threonine enriched proteins.