A function of the allantoic sac in the metabolism of uteroferrin and maternal iron by the fetal pig.

The possible roles of the allantoic sac in metabolism of uteroferrin, the iron-containing, purple phosphatase, were examined. This protein, which originates in the maternal endometrium, was measured in allantoic fluid by a specific double-antibody radioimmunoassay and by its beta-mercaptoethanol-activated acid phosphatase activity. Total uteroferrin reaches a maximum between days 60 and 75 of pregnancy, when it constitutes up to one-third of the total protein, and then declines towards term. Fluid volume, total protein and total iron also reach a maximum around day 60. Two-dimensional polyacrylamide-gel electrophoresis was employed to identify proteins in allantoic fluid. Of the proteins detected, at least three basic and two acidic polypeptides are also characteristic of maternal uterine secretions. The remaining proteins appeared identical to those found in fetal serum and included transferrin, albumin and alpha-fetoprotein. Although uteroferrin acid phosphatase activity is stable when incubated in buffer, the enzyme loses activity in sterile allantoic fluid collected at all stages of pregnancy. Experiments with 59Fe-uteroferrin have shown that allantoic fluid promotes iron loss from uteroferrin and that the metal appears in transferrin. Thus the allantoic sac may serve not only as a depot for uteroferrin accumulation but as a site of active iron metabolism.