New Tools for Molecular Imaging of Redox Metabolism: Development of a Fluorogenic Probe for 3α-Hydroxysteroid Dehydrogenases

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Abstract
A new fluorogenic substrate was developed for 3α-hydroxysteroid dehydrogenases (3α-HSD), including the human enzymes implicated in important physiological functions (androgen deactivation, neurosteroid activation). While ketone 5 is nonfluorescent, the corresponding alcohol exhibits high fluorescence with emission maximum at 510 nm, thus constituting a redox optical switch. This study began with a chemical concept of a ketone−alcohol optical switch which guided the synthesis of a focused array of compounds. Subsequently, seven compounds were selected (1−7) on the basis of their optical and chemical (stability) properties and were submitted to a screen against a panel of dehydrogenase enzymes. Probe 5 was found to be highly selective for bacterial, rat, and human 3α-HSD enzymes. The kinetic parameters were obtained for human 3α-HSD enzyme (type 2 isozyme, AKR 1C3; Km = 2.5 μM, kcat = 8.2 min-1). Remarkably, comparison to 5α-dihydrotestosterone (5α-DHT, Km = 26 μM, kcat = 0.25 min-1, Figure 4), a likely physiological substrate in prostate, revealed that synthetic probe 5 is in fact a far better substrate for this enzyme. Structure 5 represents an exciting lead for the development of a redox imaging probe.