Endocrine Abstracts

The 5-reductases are steroid metabolising enzymes that saturate the C4=C5 bond of the steroid A-ring, and their substrates include androgens, glucocorticoids, and bile acids. 5β-reductases (SRD5A1 & SRD5A2) convert testosterone to the more potent androgen 5β-dihydrotestosterone, and carry out the first step in glucocorticoid clearance, generating 5β-dihydrocortisol from cortisol. 5β-reductase (AKR1D1) is also able to carry out the first step of glucocorticoid clearance, converting both cortisol and cortisone to 5β-dihydrocortisol and 5β-dihydrocortisone but, in contrast to 5β-reductases, it converts testosterone to a low activity intermediate, 5β-dihydrotestosterone. In addition, it catalyses an essential step in bile acid synthesis. The subcellular localisation of steroid metabolising enzymes is thought to have a role in determining their activity and substrate preference. In this regard, hepatic SRD5A1 is reported to be localised in either the nucleus, cytoplasm or both. However, evidence for the subcellular localisation of SRD5A2 and AKR1D1 in hepatocytes is lacking. To determine whether SRD5A2 and AKR1D1 are localised to the nucleus or cytoplasm, nuclear and cytoplasmic fractions were isolated from HepG2 hepatoma cells. The purity of the fractions was confirmed by Western blot, using Lamin A/C as a nuclear and β-tubulin as a cytosolic marker. Similar to SRD5A1, SRD5A2 was detected in both cytoplasmic and nuclear fractions. In contrast, AKR1D1 was detected only in the cytoplasmic fractions. Further work is required to determine whether the localisation of SRD5A1 and 2 impact on their substrate preference, and to determine the localisation of AKR1D1 within the cytoplasm.