Vitamin B12 binding proteins in amniotic fluid.

The cobalamin-binding proteins in amniotic fluid are determined by assaying the unsaturated cobalamin-binding capacity of haptocorrin (R-proteins, cobalophilin, or transcobalamin I-like protein and any coexisting intrinsic factor) and non-haptocorrin. Forty-five specimens obtained by amniocentesis, and ninety-two obtained at parturition are analysed. The haptocorrin cobalamin-binding capacity increases rapidly after a gestational age of about 15 weeks. At parturition the concentration varies from 1.4 to 26.8 nmol/l. The non-haptocorrin cobalamin-binding capacity is less than 1% of the total binding capacity. The unsaturated cobalamin-binding capacities do not correlate significantly with birth weight, placental weight, or total protein concentration of the amniotic fluid. The ratio between haptocorrin concentration in amniotic fluid and maternal plasma is about 3:1. The unsaturated cobalamin-binding capacity of haptocorrin in amniotic fluid is significantly higher in pregnancies with female than with male fetuses, and is relatively high in one case of twins. It is suggested that haptocorrin in amniotic fluid originates mainly from the fetus, and may have a bacteriostatic effect.