Active transport of riboflavin by the isolated choroid plexus in vitro

In vitro, the transport of [14C]riboflavin into and from the isolated choroid plexus, the anatomical locus of the blood-cerebrospinal fluid barrier, was studied. With concentrations of [14C]riboflavin of 0.7 microM (or greater) in the incubation medium, the choroid plexus accumulated [14C]riboflavin against a large concentration gradient by a process that did not depend on binding or intracellular metabolism of the [14C]riboflavin. The [14C]riboflavin accumulation process in isolated choroid plexus could be described by Michaelis-Menten transport kinetics (kt = 78 microM and Ymax = 1.65 mmol kg-1 (15 min)-1) and was inhibited by other flavins and probenecid but not by ribose, weak bases, or other B vitamins. The accumulation process was markedly depressed by iodoacetate and low temperatures. With a concentration of 0.08 microM [14C]riboflavin in the incubation medium, 28% of the [14C]riboflavin within the choroid plexus was converted to [14C]FAD or [14C]FMN intracellularly. Unlike the active transport of [14C]riboflavin into choroid plexus, accumulated [14C]riboflavin departed choroid plexus by a process independent of intracellular concentration or temperature. The efflux of [14C]riboflavin from choroid plexus could be described by first oder kinetics with a rate constant of -0.08 min-1.