3β-HSD in songbird brain: subcellular localization and rapid regulation by estradiol.

The enzyme 3β-hydroxysteroid dehydrogenase/Δ5-Δ4 isomerase (3β-HSD) catalyzes the conversion of dehydroepiandrosterone to androstenedione, thereby playing a key role in sex steroid synthesis. In peripheral tissues, 3β-HSD is membrane-bound, is present in both mitochondria and microsomes, and is regulated differentially in these two subcellular compartments. In the brain, 3β-HSD is present, but its subcellular compartmentalization is unknown. Here, in Study 1, we examined the subcellular localization of 3β-HSD in the brain of a songbird, the zebra finch. In Study 2, in males and females, we determined whether 3β-HSD activity in different subcellular compartments is rapidly regulated by in vitro treatment with estradiol (E(2) ), which has many rapid effects on the brain. Brain 3β-HSD was enriched primarily in microsomes and secondarily in mitochondria and synaptosomes. In both males and females, E(2) treatment rapidly (within 5 min) inhibited 3β-HSD activity in both mitochondria/synaptosomes and microsomes, with greater inhibition in microsomes. We also assessed the activity of 5β-reductase, which acts on androstenedione. E(2) rapidly inhibited 5β-reductase activity in microsomes only. This is the first study to examine the subcellular localization of 3β-HSD in the brain, and the data demonstrate the importance of subcellular localization for the regulation of steroidogenic enzymes in the brain.