Mechanisms of vitamin B(12) absorption in breast-fed infants.
OBJECTIVE
The mechanisms of vitamin B(12) absorption in infants are unknown. We investigated whether haptocorrin (HC), a vitamin B(12) -binding protein in human milk, facilitates vitamin B(12) absorption during the neonatal period or if it occurs by a process similar to that in adults involving another vitamin B(12) -binding protein, intrinsic factor (IF).


METHODS
To determine whether HC or IF can deliver vitamin B(12) to the enterocyte, binding studies using Caco-2 intestinal cells in culture and purified human milk HC-[ (57)Co]vitamin B(12) or [(125)I]IF-vitamin B(12) were performed. Determination of IF secretion by infant stomach was investigated by a competitive ELISA on fecal extracts from breast-fed infants. Determination of receptors specific for IF-vitamin B(12) or HC-vitamin B(12) in infant intestine was achieved by ligand blot analysis using isolated brush border membrane vesicles (BBMV) from fetal and adult intestine and Caco-2 cells. PCR was performed to identify the IF receptor gene transcript in Caco-2 cells and fetal intestine.


RESULTS
Limited binding of both HC and IF to Caco-2 cells was observed; however, HC displayed affinity to low molecular weight proteins in BBMV from fetal intestine and Caco-2 cells while IF showed affinity for a 240 kDa protein in BBMV from fetal intestine and Caco-2 cells. IF receptor gene transcript was identified in fetal intestine and Caco-2 cells. An increase in IF excretion from breast-fed infants throughout early life was observed.


CONCLUSIONS
An IF-dependent vitamin B(12) absorption mechanism appears to be in place in breast-fed infants. However, IF levels may be too low in early life to participate in vitamin B(12) absorption; therefore, haptocorrin may mediate vitamin B(12) absorption until the absorption function can be taken over by a more mature IF system.
Adkins
Yuriko
Y
Department of Nutrition university of California, Davis, California 95616, USA.
Lönnerdal
Bo
B
D037263:Asialoglycoprotein Receptor / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D001692:Biological Transport; D001942:Breast Feeding; D018938:Caco-2 Cells; D004797:Enzyme-Linked Immunosorbent Assay; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007408:Intestinal Absorption / Q000502:physiology*; D007421:Intestine, Small / Q000378:metabolism*; D007437:Intrinsic Factor / Q000235:genetics / Q000502:physiology*; D008871:Microvilli / Q000378:metabolism; D008895:Milk, Human / Q000737:chemistry; D020133:Reverse Transcriptase Polymerase Chain Reaction; D014155:Transcobalamins / Q000235:genetics / Q000502:physiology*; D014805:Vitamin B 12 / Q000235:genetics / Q000378:metabolism / Q000493:pharmacokinetics* / Q000502:physiology*
D037263:Asialoglycoprotein Receptor; D014155:Transcobalamins; C036746:intrinsic factor-vitamin B 12 complex; D007437:Intrinsic Factor; D014805:Vitamin B 12