Many sugars are ketones, known collectively as ketoses. Resorcinol is an analytical reagent for the qualitative determination of ketoses (Seliwanoff's test). Examples of heptoses include the ketoses, mannoheptulose and sedoheptulose. It is the simplest of all ketoses and has no chiral center or optical activity. Like most sugar isomerases, glucose isomerase catalyzes the interconversion of aldoses and ketoses. The table shows all aldoses with 3 to 6 carbon atoms, and a few ketoses. Ketoses of biological interest usually have the carbonyl at position 2. Aldoses can tautomerize to ketoses in a dynamic process with an enol intermediate. Aldoses can isomerize to ketoses through the Lobry-de Bruyn-van Ekenstein transformation. Examples include many sugars (ketoses), many steroids (e.g. testosterone), and the solvent acetone. Ketoses must first tautomerize to aldoses before they can act as reducing sugars. Aldehyde monosaccharides may be called aldoses; ketone monosaccharides may be called ketoses. Ketoses can isomerize to aldoses through the Lobry-de Bruyn-van Ekenstein transformation.