Human aldose reductase and human small intestine aldose reductase are efficient retinal reductases: Consequences for retinoid metabolism

Aldo-keto reductases (AKRs) are NAD(P)H-dependent oxidoreductases that catalyse the reduction of a variety of carbonyl compounds, such as carbohydrates, aliphatic and aromatic aldehydes and steroids. We have studied the retinal reductase activity of human aldose reductase (AR), human small-intestine (HSI) AR and pig aldehyde reductase. Human AR and HSI AR were very efficient in the reduction of all-trans-, 9-cis- and 13-cis-retinal (k cat/Km = 1100-10 300 mM-1 · min -1), constituting the first cytosolic NADP(H)-dependent retinal reductases described in humans. Aldehyde reductase showed no activity with these retinal isomers. Glucose was a poor inhibitor (Ki = 80 mM) of retinal reductase activity of human AR, whereas tolrestat, a classical AKR inhibitor used pharmacologically to treat diabetes, inhibited retinal reduction by human AR and HSI AR. Alltrans-retinoic acid failed to inhibit both enzymes. In this paper we present the AKRs as an emergent superfamily of retinal-active enzymes, putatively involved in the regulation of retinoid biological activity through the assimilation of retinoids from β-carotene and the control of retinal bioavailability.