Riboflavin uptake in microvillous and basal membrane vesicles isolated from full-term human placentas.

Riboflavin uptake was characterized using membrane vesicles isolated from the apical (maternal-facing) and basal (fetal-facing) membranes of the syncytiotrophoblast from full-term human placentas. Equilibrium [3H]riboflavin uptake was insensitive to variations in incubation medium osmolarity in contrast to [3H]alanine uptake into an osmotically sensitive space. Osmotic insensitivity suggested riboflavin binding to a membrane component. The dissociation constant of riboflavin binding was similar in microvillous (Kd = 2 microM) and basal membrane vesicles (Kd = 1 microM). Binding capacity was significantly higher in microvillous membranes (Bmax = 11.9 pmol/mg protein). The relatively high affinity binding to the membrane vesicles may represent a first step in riboflavin transport.