Human milk is a rich source of RNA, free nucleotides (NT) and nucleosides (NS). To determine the uptake of different NS sources by the intestinal epithelium, jejunal explants from suckling piglets were cultured in a medium supplemented with a mixture of NS (adenosine, cytidine, guanosine, inosine, uridine; 10 mg/l each), a mixture of five NT (AMP, CMP, GMP, IMP, UMP; 7 mg/l each) or RNA (60 mg/l), respectively. Aliquots from the media were taken at different times (0·5, 2, 5, 15, 30, 60, 180 min). NS and NT concentrations were analysed in the different supernatants at those periods using solid-phase extraction followed by HPLC. When explants were cultured in the presence of NS the concentration of these compounds, excepting cytidine, rapidly decreased, suggesting that they are efficiently taken up. When explants were incubated in the presence of NT, the total concentration of these compounds decreased while the total concentration of NS increased, suggesting that enterocytes efficiently hydrolyse NT into NS. Likewise, when explants were incubated in the presence of RNA, the total concentration of both NT and NS increased, indicating that intestinal explants are able to hydrolyse RNA to NT and then to NS in the absence of luminal enzymes. In conclusion, the jejunum of piglets at weaning is able to hydrolyse RNA and free NT to NS, and NS, excepting cytidine, are efficiently taken up by the small intestine. These results suggest that the current concentration of NT used to supplement infant formulas should be reconsidered.Intestine: Nucleic acids: Nucleosides: Nucleotides: Pig explants Nucleotides (NT) are low molecular weight intracellular compounds, which play key roles in nearly all biochemical processes (Stryer, 1995). Nucleosides (NS) and NT are naturally present in all foods of animal and vegetable origin as free NS and NT, and as nucleic acids; soluble NS and NT, and RNA and DNA are present in milk from various mammals, contributing up to 20 % of the non-protein nitrogen (Gil & Uauy, 1989Leach et al. 1995;Gil, 2001). Although NT deficiencies have not been related to any particular disease, dietary NT have been reported as beneficial for infants because they positively influence lipoprotein and PUFA metabolism, cellular and humoral immunity, and intestinal growth, development and repair (Carver & Walker, 1995; Gil & Uauy, 1996;Cosgrove, 1998;Sánchez-Pozo et al. 1998;Gil, 2002;Aggett et al. 2003;Buck et al. 2004;Schaller et al. 2004;Hawkes et al. 2006). However, there is a lack of information about the bioavailability of dietary NT and derivatives in infants.Digestion of nucleic acids and NT clearly occur in man and animals. Most dietary NT are ingested in the form of nucleoproteins from which the nucleic acids are liberated in the intestinal tract by the action of proteolytic enzymes. Pancreatic ribonuclease and deoxyribonuclease degrade RNA and DNA into a mixture of mono-, di-, tri-and polynucleotides.
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. JPGN 35:685-690, 2002.
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