Oxidation of corticosteroids to steroidal carboxylic acids by an enzyme preparation from hamster liver

Expanded substrate screenings of human and Drosophila type 10 17beta-hydroxysteroid dehydrogenases (HSDs) reveal multiple specificities in bile acid and steroid hormone metabolism: characterization of multifunctional 3alpha/7alpha/7beta/17beta/20beta/21-HSD

17beta-hydroxysteroid dehydrogenases (17beta-HSDs) catalyse the conversion of 17beta-OH (-hydroxy)/17-oxo groups of steroids, and are essential in mammalian hormone physiology. At present, eleven 17beta-HSD isoforms have been defined in mammals, with different tissue-expression and substrate-conversion patterns. We analysed 17beta-HSD type 10 (17beta-HSD10) from humans and Drosophila, the latter known to be essential in development. In addition to the known hydroxyacyl-CoA dehydrogenase, and 3alpha-OH and 17beta-OH activities with sex steroids, we here demonstrate novel activities of 17beta-HSD10. Both species variants oxidize the 20beta-OH and 21-OH groups in C21 steroids, and act as 7beta-OH dehydrogenases of ursodeoxycholic or isoursodeoxycholic acid (also known as 7beta-hydroxylithocholic acid or 7beta-hydroxyisolithocholic acid respectively). Additionally, the human orthologue oxidizes the 7alpha-OH of chenodeoxycholic acid (5beta-cholanic acid, 3alpha,7alpha-diol) and cholic acid (5beta-cholanic acid). These novel substrate specificities are explained by homology models based on the orthologous rat crystal structure, showing a wide hydrophobic cleft, capable of accommodating steroids in different orientations. These properties suggest that the human enzyme is involved in glucocorticoid and gestagen catabolism, and participates in bile acid isomerization. Confocal microscopy and electron microscopy studies reveal that the human form is localized to mitochondria, whereas Drosophila 17beta-HSD10 shows a cytosolic localization pattern, possibly due to an N-terminal sequence difference that in human 17beta-HSD10 constitutes a mitochondrial targeting signal, extending into the Rossmann-fold motif.

Corticosteroid side chain oxidations--3. Evidence for isomerization and direct oxidation reactions in the formation of steroid 21-oic acids by rabbit liver cytosol

Isomerase complexes that oxidize the alpha-ketol side chain of deoxycorticosterone (DOC) to pregnolic and pregnenoic acids without the addition of cofactors have been purified from rabbit and hamster liver cytosols. The isomerase complex in rabbit liver cytosol was partially resolved from a NAD-dependent dehydrogenase that oxidized the glycol side chain of 20 beta-dihydro DOC to the 20 beta-hydroxy-21-oic acid. Corticosterone (B) and 20 beta-dihydro B were less active substrates than DOC or 20 beta-dihydro DOC with the corresponding preparations. The rabbit resembles other rodent and human species in that isomerization and oxidation at C21 is the major pathway of acid formation whereas direct oxidation at C21 may only be of significance with metabolites that have the glycol side chain.

Corticosteroid side chain oxidations--II. Metabolism of 20-dihydro steroids and evidence for steroid acid formation by direct oxidation at C-21

Rabbits have been injected with 4-14C-labelled progesterone, deoxycorticosterone and corticosterone and the corresponding 20 beta-3H-reduced steroids (20-dihydro steroids) in order to compare the influence of oxidation at C-20 on the excretion of steroid acids. Both 20 beta-reduced progesterone and deoxycorticosterone were extensively oxidized at C-20 and metabolized to 20-oxo-21-oic acids devoid of tritium. A small proportion of the acidic metabolites of [20 beta-3H]dihydro deoxycorticosterone retained tritium. By contrast the majority of the metabolites of [20 beta-3H]dihydro corticosterone were tritiated and [11 beta,20 beta-3H]-dihydroxy-4-pregnene-3-one-21-oic acid was identified as a major acidic metabolite. These results indicate that the presence of a 11 beta-hydroxyl in 20 beta-dihydro corticosterone inhibits oxidation at C-20 and provides evidence for the direct oxidation of this corticosteroid at C-21 in this species.

An improved method for the chemical synthesis of steroidal 20-oxo-21-oic acids

A procedure is described for the chemical synthesis of steroidal-20-oxo-21-oic acids and -17 alpha(-hydroxy-20-oxo-21-oic acids. Corticosteroid derivatives containing the 20-oxo-21-aldehyde side chain are oxidized with freshly generated silver oxide in dilute aqueous sodium hydroxide.

Oxidation of corticosteroids to steroidal 20-hydroxy-21-oic acids by mouse liver

We have studied the enzyme catalyzed oxidation of 11-deoxycorticosterone to 20-hydroxy-3-oxo-4-pregnen-21-oic acid (pregnolic acid) in mouse liver. Enzyme activity, though variable, was higher than that of other species. Pregnolic acid was identified as the free acid, as the methyl ester by thin layer chromatography, and as the p-bromophenacyl ester by high performance liquid chromatography. With [4-14C, 21-3H]-DOC as substrate, exchange of tritium with water (interpreted as due to the reversible isomerization of the ketol to aldol form by the side chain) and the overall conversion of the ketol side chain to hydroxy acid was catalyzed by the post-microsomal supernatant fraction. Although we could not physically separate tritium exchange and acid production, pregnolic acid formation could be decreased or eliminated while tritium exchange was retained, consistent with our previous conclusion that isomerization to aldol was a precondition for acid formation. In preparations that made no acid, [4-14C]-DOC was recovered, depleted of tritium. The rate of exchange of [21S, 21-3H]-DOC with water was faster than [21R, 21-3H]-DOC. The stereochemistry of pregnolic acid at C-20 was 85-90% R (i.e.. 20 alpha-hydroxy-21-oic acid). The Km for isomerase with [21RS-21(3) H-DOC was 4.3 x 10(-5); Km for pregnolic acid formation was 8.0 x 10(-5) M. Corticosterone was oxidized to acid metabolites at 20% the rate of DOC.

Oxidation of cortisol to hydroxy acid metabolites by liver cytosol

Cytosols (post-microsomal supernatants) prepared from rat, hamster and mouse livers oxidized cortisol to 11 beta, 17, 20-trihydroxy-3-oxo-pregn-4-en-21-oic acids. Mouse liver enzymes yielded over 90% 20 alpha-hydroxy epimer from cortisol, 21-dehydrocortisol (11 alpha, 17-dihydroxy-3,20-dioxo-pregn-4-en-21-aldehyde), and 20 alpha-isocortisol (11 alpha, 17, 20 alpha-trihydroxy-3-oxo-pregn-4-en-21-aldehyde). The 20 beta-epimer of isocortisol yielded both 20 alpha- and 20 beta-hydroxy acid. Rat and hamster liver cytosols converted, 21-dehydrocortisol and 20 alpha-isocortisol to both 20 alpha and 20 beta-hydroxy acids, with the former predominant. The hamster enzyme oxidized 20 beta-isocortisol mainly to the 20 beta-hydroxy acid. The results support our conclusion that both 17 alpha-hydroxy and 17-deoxy corticosteroids are oxidized to hydroxy acids by similar pathways and that isosteroids are obligatory intermediates.