Ribonucleosides in human milk. Concentration profiles of these minor constituents as a function of the nursing time.

Ribonucleosides are secreted as products of cellular RNA and ribonucleotide metabolism into physiological fluids such as blood, milk and urine. Unmodified and modified ribonucleosides have been detected in the micromolar range as minor constituents in the milk of different mammals. In addition to the common nucleosides adenosine, cytidine, guanosine, inosine and uridine, modified components such as 1-methyladenosine, 1-methylguanosine, 1-methylinosine, N2-methylguanosine, N2-dimethylguanosine, N6-carbamoyl-L-threonyladenosine, pseudouridine and 5-aminoimidazole-4-carboxamide-N-ribofuranoside (AICAR) have been identified and most of them quantified in samples of human and/or bovine and/or goat's milk. From these investigations it is known that nucleosides, in analogy to nucleotides, show a typical species-specific pattern. Longitudinal studies have been carried out to determine the concentration profiles of the individual ribonucleosides in the milk of humans as a function of the nursing time.