TOTAL POTENTIALLY AVAILABLE NUCLEOSIDES OF HUMAN-MILK BY STAGE OF LACTATION

Human milk-borne ribonucleotides reportedly have important physiological roles in breast-fed infants. Previous studies measured the free nucleotide content of human milk. To more fully evaluate the physiological capacity of nucleotides in human milk, we determined the monomeric and polymeric ribonucleotide and ribonucleoside content of milk pooled from 11 American women. Subsequently, we determined the total potentially available nucleosides (TPAN) of pooled and individual milk samples segregated by stage of lactation from 100 women in three European countries to test for effect of culture and diet. The methodology simulated in vivo digestion. Polymeric ribonucleotide (primarily RNA), monomeric ribonucleotide, and ribonucleoside-containing adducts (eg, uridine diphosphate hexose) were enzymatically hydrolyzed to their constituent ribonucleosides, the preferred form for absorption. Free and enzymatically liberated nucleosides were then measured by HPLC to yield the TPAN value. The mean (+/- SD) TPAN concentration of the 16 pooled European samples, derived from the 100 individual samples, was 189 +/- 70 mu mol nucleoside/L human milk (range 82-402 mu mol/L). The means (mu mol/L human milk) of each nucleoside were 38 for uridine, 88 for cytidine, 31 for guanosine, and 32 for adenosine. These values included the contribution from the cellular portion of human milk. Only one of the 16 pooled samples contained a measurable amount of inosine (4 mu mol/L). The potentially available ribonucleosides in the human milk samples were predominantly present as monomeric (36 +/- 10%) and polymeric (48 +/- 8%) nucleotides. This study demonstrates that the traditional measurement of the free nucleotide content of human milk (which accounts for neither polymeric nor cellular nucleotides) underestimates the total nucleotides available to the infant by greater than or equal to 50%.