Incorporation of orally applied 13C-galactose into milk lactose and oligosaccharides

Abstract

Biosynthesis and functions of human milk oligosaccharides (HMO) are not well known. A typical housekeeping enzyme, β1,4-galactosyltransferase, links galactose to glucose to form lactose which is then used as backbone for the assembly of HMO. We investigated whether milk lactose and HMO may be labeled in vivo by an orally given ¹³C-galactose bolus. Eleven exclusively breastfeeding mothers were given a ¹³C-galactose bolus at the end of their breakfast. Milk and urine samples from each nursing up to 36 h were analyzed for carbohydrate composition by high-performance thin-layer chromatography, high-pH anion-exchange chromatography, and fast atom bombardment mass spectrometry. ¹³C enrichment of milk fractions, urinary carbohydrates, lactose, and oligosaccharides as well as of breath CO₂ was determined by isotope ratio mass spectrometry. Up to 10% of the orally given galactose bolus was directly transported to the mammary gland and incorporated into milk components. Characteristic for most milk samples was the appearance of two ¹³C-peaks, the first immediately after the ¹³C-bolus was taken and the second on the next morning. The highest ¹³C enrichment was found in lactose followed by neutral and acidic oligosaccharides. In breath samples, the ¹³C-excretion followed the same pattern as in milk. ¹³C nuclear magnetic resonance of isolated lactose revealed ¹³C only at C₁-atom of galactose and C₁-atom of glucose. This label was without any exception at the same position as the ¹³C-label of the orally applied galactose. Neutral and acidic HMO can easily be ¹³C-labeled in vivo which facilitates investigations of their metabolic fate in infants.