Galactitol is produced from galactose in a reaction catalyzed by aldose reductase. The abstraction of two carbons from D-xylulose-5-P yields the 3-carbon aldose glyceraldehyde-3-P. Therefore, glucose is both a hexose and an aldose, or an aldohexose. The reaction requires NADH and is catalyzed by aldose reductase. The term formose is a portmanteau of formaldehyde and aldose. It has one aldehyde group, and is thus part of the aldose family. It has an aldehyde group and is an aldose. This cleavage reaction generates the unusual aldose sugar erythrose-4-phosphate. The lactol functional group is prevalent in nature as component of aldose sugars. The ring opens to form a straight-chain aldose with an acidic C2 proton. Direct oxidation of an aldose affects the aldehyde group first. Topical administration of aldose reductase inhibitors have been shown to prevent the cataract in rats. Therefore, aldose reductase reduces galactose to its sugar alcohol form, galactitol. In that case, the compound is termed an aldose. Lastly, the compound reacts with Aldose Reductase (AKR1B1) to form PGF.