Infused and ingested labeled lysines: appearance in human-milk proteins

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Abstract
Incorporation of two labeled forms of lysine into human-milk proteins was studied in fasted lactating subjects to determine whether highly labeled proteins could be produced for subsequent nutritional studies and whether the kinetics of milk synthesis could be studied in humans with stable isotope techniques. Five subjects, maintained on formula diets, received L-[13C1]lysine (27 mumol/kg) as an IV bolus and L-[15N2]lysine (27 mumol/kg) as an oral bolus 4 h postprandially. Milk samples were collected at 30, 45, 90, 150, 240, and 360 min. Tracer lysine levels in the hydrolysate of unfractionated milk protein were determined by gas chromatography-mass spectrometry isotope ratiometry. After a delay of at least 45 min, significant labeling of milk protein was detected and reached a maximum at 150 min with cumulative percent does recovery over 6 h of 0.5%. Human-milk proteins can be labeled for nutritional investigations and in vivo kinetics of milk protein synthesis can be studied with stable isotope techniques.