The Non-Protein Nitrogen Components in Human Milk: Biochemistry and Potential Functional Role

doi:10.1201/9780367812805-9

Chapter

The Non-Protein Nitrogen Components in Human Milk: Biochemistry and Potential Functional Role

ABSTRACT

The non-protein nitrogen (NPN) fraction of milk comprises 20 to 25% of the total nitrogen in human milk ^1–6 compared to only 3 to 5% in cow’s milk. ⁷ Classically, the non-protein nitrogen of milk has been identified as the acid-soluble nitrogen remaining in the supernatant following protein precipitation (usually with trichloroacetic acid [TCA], ⁴ ^, ⁶ ^, ⁸ or the dialyzable nitrogen following dialysis of whole milk using membranes with molecular weight cutoffs of about 12,000 D. ⁸ ^, ⁹ Characterization of this acid-soluble/dialyzable nitrogen has led to the quantification of more than ten classes of compounds: peptides, ¹⁰ urea, ¹ ^, ² ^, ⁶ ^, ⁹ ^, ¹² ^, ¹⁷ free amino acids, ⁶ ^, ^13–16 creatine and creatinine, ¹ ^, ² ^, ¹² ^, ¹⁷ uric acid, ¹ ^, ² ^, ⁶ ^, ⁹ ^, ¹² ammonia, ⁶ ^, ⁷ nucleic acids and nucleotides, ^18–21 polyamines, ²² ^, ²³ carnitine, ^24–26 choline, ²⁷ ^, ²⁸ amino alcohols of phospholipids, ²⁸ amino sugars, ⁹ ^, ¹³ ^, ^29–31 low molecular weight peptide hormones (reviewed in Reference 32), and other biologically active compounds such as growth factors (reviewed in Reference 33).