Purification and characterisation of serum retinol binding proteins and the transport of Vitamin A across the human placenta
Since retinol is insoluble in aqueous media, transport of vitamin A to epithelial tissues from its storage sites in the liver, is mediated by a specific plasma carrier protein called Retinol-Binding Protein (RBP). There have been controversial views expressed by different workers as to the mechanism of active transfer of retinol through the lipid bilayer of target cell membrane. The supply of Vitamin A and its derivatives from mother to foetus is of vital importance for fetal growth and development. Placenta plays an important role in the supply of these nutrients to the fetus. Since little is known about the manner of conveyance of these retinoids from the maternal circulation to the foetus, a study of the placental transport of vitamin A is important and the current work is aimed to investigate these mechanisms. In order to study the retinol transport mechanisms, as a first step, a method to isolate RBP from both adult blood, as well as from the foetal blood, was developed in this laboratory. For the first time foetal RBP has been isolated from placental blood and its characteristics compared with that of adult RBP. This study found that no significant differences exist between the two RBPs. In addition, the heterogeneity observed in certain RBP preparations was explained by reviewing the methodologies for RBP purification. In the second part of this work, transport of retinol across the placental membrane was studied using BeWo cell monolayers grown on special membrane plates as a model. These results have been discussed and the existing theories on retinol transport have been reviewed. In conclusion, this study has shown that there are no significant differences between foetal RBP and adult RBP. Although the possibility of a simple diffusion mechanism cannot be ruled out this study found that transport of retinol across BeWo cells mainly takes place through a receptor-mediated mechanism.