Aquaporin 9 (AQP-9) belongs to a family of membrane proteins that allow passage of water and certain other solutes through biological membranes.¹ The family is composed of 13 members (AQP-0 to AQP-12). Little is known about the function of the two newest members, AQP-11 and AQP-12.

The aquaporins can be divided into two functional groups based on their permability characteristics: the aquaporins that are permeated primarily by water (AQP-1, AQP-2, AQP4, AQP-5, AQP-6, AQP-8), and the aquaglyceroporins that are permeated by water and other small solutes such as glycerol. This last group includes AQP-3, AQP-7, AQP-9 and AQP-10.

The proteins present a conserved structure of six transmembrane domains with intracellular N- and C-termini. The functional channel is a tetramer but each subunit has a separate pore and therefore the functional channel unit contains four pores.²

AQP-9 is expressed in several tissues including the brain, liver, testis and spleen. In the liver AQP-9 has a central role in glycerol metabolism as demonstrated in experiments using AQP-9 knockout mice: mice deficient in AQP-9 expression showed enhanced plasma levels of glycerol and triglycerides.³