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The physiological role of steroid hormones in humans is well known, and the metabolic pathway and mechanisms of action are almost completely elucidated. The role of plant steroid hormones, brassinosteroids, is less known, but an increasing amount of data on brassinosteroid biosynthesis is showing unexpected similarities between human and plant steroid metabolic pathways. Here we focus our attention on the enzyme 5α-reductase (5αR) for which a plant ortholog of the mammalian system, DET2, was recently described in Arabidopsis thaliana. We demonstrate that campestenone, the natural substrate of DET2, is reduced to 5α-campestanone by both human 5αR isozymes but with different affinities. Solanum malacoxylon, which is a calcinogenic plant very active in the biosynthesis of vitamin D-like molecules and sterols, was used to study 5αR activity. Leaves and calli were chosen as examples of …