Nucleosides and nucleotides: natural bioactive substances in milk and colostrum

Prolactin, cortisol, and insulin regulation of nucleoside uptake into mouse mammary gland explants

Nucleosides are essential components of milk that are used for the nourishment of newborns. Effects of the three primary lactogenic hormones, including prolactin (PRL), insulin (I), and cortisol (H), on nucleoside uptake and incorporation into cultured mammary tissues taken from 12- to 14-day pregnant mice were determined; most experiments focused on the regulation of uridine uptake. Insulin alone, as well as PRL in the presence of insulin and cortisol, was shown to stimulate uridine uptake and incorporation into RNA in mammary explants taken from 12- to 14-day pregnant mice. The PRL effects were expressed at concentrations of 25 ng/ml and above, which are physiological plasma concentrations. In the absence of sodium, uridine uptake and incorporation were diminished, suggesting the presence of a sodium-dependent uridine transporter. In kinetic studies the apparent Km for uridine uptake was calculated to be 312 microM, and the Vmax 2.90 micromol/hr/L cell water; PRL had no effect on the Km but increased the Vmax to 5.88 micromol/hr/L cell water. When assessing uridine uptake in the presence of the other nucleosides at 0.1 mM, only cytidine competed with uridine uptake. The fact that distribution ratios of greater than 15:1 were achieved with uridine indicates that uridine uptake may be via an active transporter. These studies show that PRL enhances uridine update in mammary tissues by stimulating the activity, and probably synthesis, of a sodium-dependent, active uridine and cytosine transporter.

Innovation in infant formula development: a reassessment of ribonucleotides in 2002

Nucleotides play a significant role in many physiologic functions, ranging from the encoding of genetic information to signal transduction. Accumulating evidence from recent animal and clinical studies supports the original notion that nucleotides are semi-essential dietary nutrients. The development of a new technique, total potentially available nucleosides, to accurately quantify the nucleotide content in various biological fluids has allowed investigators to properly assess the importance of nucleotide function and availability in various conditions. Data from animal studies indicate that exogenous nucleotides produce beneficial gastrointestinal and immunologic effects, especially during times of rapid growth, when nucleotide availability may be low. Infant studies confirm these findings and are presented in this review. Regulatory agencies are currently using this information collectively to support the rationale for the supplementation of infant formulas with higher amounts of nucleotides.

Immunonutrients and neonates

The gastrointestinal tract is the largest surface area of the body and the primary site for microorganisms, foreign antigens and toxins to gain entry to the host's internal milieu. The use of enteral feedings enriched with immune-enhancing ingredients is attracting considerable interest because there is increasing application of enteral feeding and appreciation of the role of the gut in the development of infection and of multiple organ failure in critically ill patients.

CONCLUSION

in this review, we will discuss nutrients, such as glutamine, arginine, omega 3 fatty acids, nucleotides, probiotics, and lactoferrin, and how they might be used as immunonutrients in neonatal clinics.

Bioactive components in milk

Milk is the only biomaterial that evolved under the Darwinian selective pressure to nourish growing mammals. The purpose of this article is to review the scientific research that is using new techniques of integrating biological sequence, structure and function, to understand the innovative biology underlying the products of that evolutionary pressure. As it emerges that milk is actively communicating between the maternal mammary epithelia and the infant's gastrointestinal system, actively directing and educating the immune, metabolic and microflora systems within the infant, enhancing nutrient absorption and delivery, and conferring multiple means of protection, nutritionists are gaining a host of new molecular targets towards which to build scientific strategies for future foods and clinical applications. As new components and functions are being discovered in milk by using traditional methods and modern genomic tools, the complexities of demonstrating, and particularly in humans, the functional mechanisms behind milk's newly observed physiological benefits are becoming the next challenge of this rapidly growing field.

Ribonucleic acid hydrolysis by intestinal explants of neonatal piglets

BACKGROUND

Healthy human adults are able to hydrolyze dietary nucleic acids and nucleotides in the gastrointestinal tract, thus facilitating absorption of the resulting nucleosides. However, little is directly known of the ability of infants to hydrolyze nucleic acids early in life.

METHODS

Purified RNA (50, 100, 250, and 500 mg/L) in a suitable medium was exposed to jejunal explants of weaning piglets to determine if enzymes expressed by the explants were sufficient to hydrolyze the nucleic acid. Aliquots from the media were taken at time intervals, from 0 to 6 hours, and assayed for ribonucleoside content by high-performance liquid chromatography.

RESULTS

The investigators found a significant increase of free ribonucleosides during the study period and for all RNA concentrations tested, thus suggesting that intestinal explants are able to hydrolyze RNA. The kinetics of nucleoside concentrations varied for each nucleoside. For example, inosine increased over the entire 6-hour period and adenosine increased for the first 2 hours, decreasing afterward.

CONCLUSIONS

It is concluded that cells from the intestinal epithelium are capable of hydrolyzing RNA. These results suggest that RNA present in human milk is hydrolyzed in the intestinal tract of the breast-fed infant, thus providing an additional source of nucleosides. The results indicate that current supplementation of infant formulas with nucleotides should be reconsidered to take into account the contribution of RNA present in human milk to the pool of bioavailable nucleotides.

Nucleosides are efficiently absorbed by Na(+)-dependent transport across the intestinal brush border membrane in veal calves

In previous work, a comparatively high capacity for Na(+)-dependent transport of nucleosides across the intestinal brush border membrane (BBM) was observed in dairy cows, which might be related to digestion of the large amount of nucleic acids present in ruminal microorganisms in the ruminant small intestine. If this were the case, the capacity for Na(+)-dependent intestinal nucleoside transport should be much lower in veal calves, in which only small amounts of nucleic acids, nucleotides, and nucleosides reach the small intestine via the milk replacer. To test this hypothesis, we investigated Na(+)-dependent transport of 3H-labeled thymidine and guanosine across the BBM using BBM vesicles (BBMV) isolated from the small intestine of veal calves. In the presence of a transmembrane Na+ gradient both substrates were transported against a concentration gradient. Inhibitory studies showed that thymidine and guanosine are transported by two different transporters with overlapping substrate specificity, one accepting predominantly pyrimidine nucleosides (N2) and one accepting particularly purine nucleosides (N1). Nucleoside transport was inhibited by glucose along the whole small intestine. Maximal transport rates similar to those in dairy cows were obtained for the proximal, mid-, and distal small intestine. These findings suggest that the high absorptive capacity for nucleosides is a genetically fixed property in the bovine small intestine, which is already present in the preruminant state of veal calves. It may contribute to the high digestibility of nucleic acids observed by others in veal calves receiving milk replacer supplemented with RNA. Its main function may be the efficient absorption of nucleosides resulting from the digestion of nucleic acids associated with desquamated enterocytes. Due to the limited de novo synthesis of nucleotides in enterocytes intracellular uptake of nucleosides across the BBM may contribute to nucleic acid synthesis in enterocytes and thus may have a trophic effect on the intestinal epithelium.

Evolution of the mammary gland defense system and the ontogeny of the immune system

A decisive event in the evolution of mammals from synapsid reptiles was the modification of ventral thoracic-abdominal epidermal glands to form the mammary gland. The natural selection events that drove the process may have been the provision of certain immunological agents in dermal secretions of those nascent mammals. This is mirrored by similar innate immune factors in mammalian sebum and in protherian and eutherian milks. On the basis of studies of existing mammalian orders, it is evident that immune agents in milk such as immunoglobulins, iron-binding proteins, lysozyme, oligosaccharides, and leukocytes compensate for developmental delays in early postnatal production of antimicrobial factors. At least in human milk, anti-inflammatory and immunomodulating agents also evolved to provide different types of protection for the offspring. In addition, investigations reveal that the types or concentrations of immunological agents in milk vary depending upon the type of placenta, lactation pattern, and environment of the species.

Growth hormone and prolactin--molecular and functional evolution

Growth hormone, prolactin, the fish hormone, somatolactin, and related mammalian placental hormones, including placental lactogen, form a family of polypeptide hormones that share a common tertiary structure. They produce their biological effects by interacting with and dimerizing specific single transmembrane-domain receptors. The receptors belong to a superfamily of cytokine receptors with no intrinsic tyrosine kinase, which use the Jak-Stat cascade as a major signalling pathway. Hormones and receptors are thought to have arisen as a result of gene duplication and subsequent divergence early in vertebrate evolution. Mammalian growth hormone and prolactin show a slow basal evolutionary rate of change, but with episodes of accelerated evolution. These occurred for growth hormone during the evolution of the primates and artiodactyls and for prolactin in lineages leading to rodents, elephants, ruminants, and man. Placental lactogen has probably evolved independently on three occasions, from prolactin in rodents and ruminants and from growth hormone in man. Receptor sequences also show variable rates of evolution, corresponding partly, but not completely, with changes in the ligand. A principal biological role of growth hormone, the control of postnatal growth, has remained quite consistent throughout vertebrate evolution and is largely mediated by insulin-like growth factors. Prolactin has many and diverse roles. In relation to lactation, the relative roles of growth hormone and prolactin vary between species. Correlation between the molecular and functional evolution of these hormones is very incomplete, and it is likely that many important functional adaptations involved changes in regulatory elements, for example, altering tissue of origin or posttranscriptional processing, rather than change of the structures of the proteins themselves.

Update on host defense and immunonutrients

Neonatal intensive care in the past three decades has provided exciting modalities for improving the survival of critically ill neonates. There remains a great need for improving the quality of life for these survivors. In this article, the role the developing GI tract and its microenvironment play in the well-being of the neonate has been emphasized. Future therapies based on manipulation of the GI tract and its microenvironment by functional foods, immunonutrients, or pharmacologic agents may have effects not only during the neonatal period, but also throughout the individual's entire life.