Functional characteristics of aquaporin 7 as a facilitative glycerol carrier.

Aquaglyceroporins, which constitute a subgroup of aquaporin (AQP) water channels, had been believed to serve as channels for glycerol as well as for water. However, our recent studies have indicated that AQP9 and AQP10 operate in a carrier mode, which is of saturable nature, for glycerol transport. Assuming that such a functional characteristic could also be shared by AQP7, another aquaglyceroporin, we examined its glycerol transport function. The specific transport of glycerol by human AQP7, which was stably expressed in Madin-Darby canine kidney II cells, was indeed highly saturable, indicating the involvement of a carrier mode of operation mechanism. Kinetic analysis indicated that the specific transport conformed to Michaelis-Menten kinetics with the Michaelis constant of 11.9 µM and was not associated with a nonsaturable transport component as an indication of a simultaneous channel mode of operation, which was previously indicated for AQP10. AQP7-specific glycerol transport was furthermore found to be specifically inhibited by several compounds analogous to glycerol and operate without requiring either Na(+) or H(+). These characteristics of the carrier mode of AQP7 operation suggest that it is a facilitative carrier for glycerol and, possibly, also for analogous compounds, providing a novel insight into its operation mechanism.