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New Evidence of Similarity between Human and Plant Steroid Metabolism: 5-Reductase Activity in Solanum malacoxylon

Dipartimento di Fisiopatologia Clinica (F.R., G.D., M.S.), Unità di Endocrinologia, Università di Firenze, I-50134 Firenze, Italy; Dipartimento di Chimica Organica "Ugo Schiff" (A.G., N.C.), Polo Scientifico, Università di Firenze, I-50019 Sesto Fiorentino, Italy; and Dipartimento di Biotecnologie Agrarie (M.L.R.), Sezione di Genetica, I-50144 Firenze, Italy

Address all correspondence and requests for reprints to: Giovanna Danza, Dipartimento di Fisiopatologia Clinica, Unità di Endocrinologia, Università di Firenze, Viale G. Pieraccini 6, I-50134, Firenze, Italy. E-mail: g.danza{at}dfc.unifi.it.

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 5-reductase (5R) 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 5-campestanone by both human 5R 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 5R activity. Leaves and calli were chosen as examples of differentiated and undifferentiated tissues, respectively. Two separate 5R activities were found in calli and leaves of Solanum using campestenone as substrate. The use of progesterone allowed the detection of both activities in calli. Support for the existence of two 5R isozymes in S. malacoxylon was provided by the differential actions of inhibitors of the human 5R in calli and leaves. The evidence for the presence of two isozymes in different plant tissues extends the analogies between plant and mammalian steroid metabolic pathways.

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