Retinol transport proteins and concentrations in human amniotic fluid, placenta, and fetal and maternal sera.

1. The proteins binding retinol, and retinol concentrations, were determined in amniotic fluid, placental cytosol and in the fetal and maternal circulation. 2. In non-pregnant women, plasma retinol was almost exclusively found in a transthyretin-retinol-binding-protein complex whereas, in pregnant women, retinol-binding-protein-bound retinol was observed not complexed to transthyretin. This latter fraction increased in concentration with fetal age. These two fractions were the major retinol-protein complexes in amniotic fluid and their relative amounts changed with progress of gestation. 3. In fetal blood both of these fractions were again found, with higher proportions of retinol-binding-protein-bound retinol in the umbilical artery than in the umbilical vein.