Tetrahydroxyquinone is a black pigment isolated from Rubia tinctorum in which all four protons of the benzoquinone structure are substituted by hydroxyl groups. Tetrahydroxyquinone has a role as a radical scavenger and is keratolytic. In vitro, tetrahydroxyquinone induces apopoptosis of leukemia cells by reducing protein kinase B-​dependent survival signaling followed by apoptosis through the mitochondrial pathway. Tetrahydroxybenzoquinone is a moderately potent inhibitor of aldose reductase, with an IC50 of 23 μM.