Serum Vitamin B 12 Content and UBBC in Gravid and Fetal Rabbits

It is well known that human serum vitamin B₁₂ decreases progressively during pregnancy (1-3) with lowest levels present at term. Within 4-6 weeks post partum, the maternal serum B₁₂ level returns to normal or higher (4-5). In general, fetal cord blood contains more vitamin B₁₂ than comparable maternal blood (3). The unsaturated vitamin B₁₂ binding capacity (UBBC) of the mother increases during pregnancy and returns to normal within a month of parturition (6, 7). Similar phenomenon have also been found in rats (1). Although pregnant women and gravid experimental rats appear to follow the same pattern with regard to serum vitamin B₁₂, no definitive rationale has as yet been offered to explain these effects.

Rabbits are known to contain the highest serum vitamin B₁₂ content and UBBC of all laboratory mammals so far tested (8). Because rabbits are often used as models for fetal development, it was of interest to determine the serum B₁₂ and UBBC during gravidity and in the fetus.

Materials. Samples. Sera of gravid New Zealand rabbits, fetuses near term and sera of both sexes during growth and development were obtained commercially.¹ Sera from New Zealand strain rabbits that were maintained in the laboratory were used for comparative purposes. Sera of normal pregnant women were obtained a few hours prior to parturition and cord blood was obtained from their normal healthy infants. All sera were maintained frozen at −20° until analyzed.

Vitamin B₁₂ assay. Vitamin B₁₂ was determined by a competitive binding assay using porcine intrinsic factor (IF)² as the binding protein, high specific activity ⁵⁷Co labelled cyanocobalamin (15 μCi/μg B₁₂-60 μCi/μg B₁₂)³ and equilibrium dialysis briefly described as follows:

Dilute 10 μl of rabbit serum (or use 0.5 ml of undiluted human serum) to 0.5 ml with saline.