Nucleosides and nucleotides: natural bioactive substances in milk and colostrum

Nucleotides, nucleosides and nucleobases belong to the non-protein-nitrogen (NPN) fraction of milk. The largest amounts of ribonucleosides and ribonuc leotides - ribose forms only were considered in this review - were measured directly after parturition in bovine milk and other ruminants as well as i n the milk pf humans. Generally, concentrations of most of the nucleos(t)id es tend to decrease gradually with advancing lactation period or nursing ti me. The species-specific pattern of these minor constituents in milk from d ifferent mammals is a remarkable property and confirms, at least, the speci fic physiological impact of these minor compounds in early life. The physio logical capacity of these compounds in milk is given by the total potential ly available nucleosides. The main dietary sources of nucleos(t)ides are nu cleoproteins and nucleic acids which are converted in the course of intesti nal digestion into nuclcosides and nucleobases the preferred forms for abso rption in the intestine. Thus, nucleosides and nucleobases are suggested to be the acting components of dietary and/or supplemented nucleic acid-relat ed compounds in the gut. They are used by the body as exogenous trophochemi cal sources and can be important for optimal metabolic functions. Up to 15% of the total daily need for a breast-fed infant was calculated to come fro m this dietary source. Concerning their biological. role they not only act as metabolites but are also involved as bioactive substances in the regulat ion of body functions. Dietary nucleotides affect immune modulation, e.g. t hey enhance antibody responses of infants as shown by a study with more tha n 300 full-term healthy infants. Dietary nucleos(t)ides are found to contri bute to iron absorption in the gut and to influence desaturation and elonga tion rates in fatty acid synthesis, in particular long-chain polyunsaturate d fatty acids in early stages of life. The in vitro modulation of cell prol iferation and apoptosis has been described by ribonucleosides, in particula r by modified components using human cell culture models. Due to the bio- a nd trophochemical properties of dietary nucleos(t)ides, the European Commis sion has allowed the use of supplementation with specific ribonucleotides i n the manufacture of infant and follow-on formula. From the technochemical point of view, the ribonucleoside pattern is influenced by thermal treatmen t of milk. In addition ribonucleosides are useful indicators for quantifyin g adulterations of milk and milk products.