Intracellular levels of active steroid hormones are determined by their relative rates of synthesis and breakdown. In the case of the potent androgen dihydrotestosterone, synthesis from the precursor testosterone is mediated by steroid 5␣-reductase, whereas breakdown to the inactive androgens 5␣-androstane-3␣,17␤-diol (3␣-adiol), and androsterone is mediated by reductive 3␣-hydroxysteroid dehydrogenases (3␣-HSD) and oxidative 17␤-hydroxysteroid dehydrogenases (17␤-HSD), respectively. We report the isolation by expression cloning of a cDNA encoding a 17␤-HSD6 isozyme that oxidizes 3␣-adiol to androsterone. 17␤-HSD6 is a member of the short chain dehydrogenase/reductase family and shares 65% sequence identity with retinol dehydrogenase 1 (RoDH1), which catalyzes the oxidation of retinol to retinal. Expression of rat and human RoDH cDNAs in mammalian cells is associated with the oxidative conversion of 3␣-adiol to dihydrotestosterone. Thus, 17␤-HSD6 and RoDH play opposing roles in androgen action; 17␤-HSD6 inactivates 3␣-adiol by conversion to androsterone and RoDH activates 3␣-adiol by conversion to dihydrotestosterone. The synthesis of an active steroid hormone by back conversion of an inactive metabolite represents a potentially important mechanism by which the steady state level of a transcriptional effector can be regulated.

b-Carotene biochemistry is a fundamental process in mammalian biology. Aberrations either through malnutrition or potentially through genetic variation may lead to vitamin A deficiency, which is a substantial public health burden. In addition, understanding the genetic regulation of this process may enable bovine improvement. While many bovine QTL have been reported, few of the causative genes and mutations have been identified. We discovered a QTL for milk b-carotene and subsequently identified a premature stop codon in bovine b-carotene oxygenase 2 (BCO2), which also affects serum b-carotene content. The BCO2 enzyme is thereby identified as a key regulator of b-carotene metabolism.

Arabidopsis det2 mutants are small darkgreen dwarfs displaying pleiotropic defects in light-regulated development during multiple stages of the plant life cycle. The DET2 gene encodes a protein that shares Ϸ40% sequence identity with mammalian steroid 5␣-reductases and is implicated in the synthesis of a class of plant steroids, the brassinosteroids. Here we show that the DET2 protein, when expressed in human embryonic kidney 293 cells, catalyzes the 5␣-reduction of several animal steroid substrates and has similar kinetic properties to the mammalian steroid 5␣-reductase enzymes. Moreover, human steroid 5␣-reductases expressed in det2 mutant plants can substitute for DET2 in brassinosteroid biosynthesis. These data indicate that DET2 is an ortholog of the mammalian steroid 5␣-reductases and provide further evidence that brassinosteroids play an essential role in light-regulated plant development. The structural and functional conservation between DET2 and human steroid 5␣-reductases raise interesting issues concerning the evolutionary origin of the steroid hormone signaling system. Steroid hormones play important roles in mammalian development and homeostasis (1). While many steroids have been identified in plants (2), to date only the brassinosteroids (BRs) are widely distributed throughout the plant kingdom and elicit unique growth promoting activity when applied exogenously (3). Despite these properties, the physiological role of endogenous BRs in plant development has been unclear. We recently isolated the Arabidopsis DET2 gene by positional cloning (4) and found that DET2 encodes a 262-amino acid protein that shares significant sequence identity with mammalian steroid 5␣-reductases, enzymes that play crucial roles in steroid hormone metabolism and action by catalyzing an NADPHdependent reduction of the ⌬ 4,5 double bond in a variety of steroids (Fig. 1A) (5). Mutations in DET2, including a missense mutation at residue Glu-204, which is absolutely required for human steroid 5␣-reductase (hS5R) activity (5), lead to many defects in light-dependent Arabidopsis development that can be ameliorated by application of brassinolide, an active BR (4, 6). Based on these results, we proposed that DET2 encodes a 5␣-reductase in a BR biosynthetic pathway (Fig. 1B) and that BRs constitute an important class of plant hormones (4).Support for the postulated importance of BRs in plant development came from the identification of a second Arabidopsis gene, CPD (constitutive photomorphogenesis and dwarfism) (7). The CPD gene encodes a protein that shares sequence identity with several mammalian cytochrome P450 proteins including steroid hydroxylases. Mutations in CPD cause phenotypic defects that are similar to those of det2 mutations. Moreover, brassinolide treatment also restores a wild-type phenotype to cpd mutants. Rescue studies with intermediates in the BR biosynthetic pathway suggest that CPD may encode a steroid 23-hydroxylase (7).In mammals, steroid hormones are synthesized from cholesterol via pregnenolone thr...