Molecular and kinetic parameters of sugar transport across the frog choroid plexus.

The nature of sugar transport across the "blood-cerebrospinal fluid barrier" has been investigated using anin vitro preparation of the frog posterior choroid plexus. The permeability of 41 sugars and related compounds was measured by the rapid osmotic procedure described previously. Sugar permeation was found to be stereospecific, inhibited by 1,5-difluoro-2,4-dinitrobenzene, insensitive to anoxia, and independent of the external alkali cation composition. In addition, the transport of a sugar was inhibited by structural analogues. Transport occurred equally well from the ventricular or serosal surface of the tissue, and the rate of transport could be described formally by Michaelis-Menten kinetics. The results were analyzed in terms of the conformation of the sugars in aqueous solution. Sugars which were transported have the d-glucose chair conformation. There is a good correlation between the affinity of the sugar for the transport system and the number of hydroxyl groups attached to the equatorial plane of the ring; d-glucose with five equatorial hydroxyl groups has the greatest affinity. It is concluded that sugar transport across the choroid plexus occurs by facilitated diffusion.