The steroid-induced 3α-hydroxysteroid:NAD+-oxidoreductase from Pseudomonas testosteroni (ATCC 11996) exhibits reversible, concentration-dependent monomer-dimer transitions. The dimerization is demonstrable by gel chromatography and by precipitations with ammonium sulphate in concentrated solutions of the enzyme. The dimer formation leads to a considerable decrease in the specific activity, especially when androsterone is used as substrate. In this case the formation of dimers results in a 90 % reduction of the specific activity found with the monomers. The described effect is demonstrable, but less pronounced, when tetrahydrocortisone or deoxycholate are the substrates.
Higly purified preparations of the 3α-hydroxysteroid dehydrogenase prepared by repeated isoelectric focusings at different pH gradients have shown the enzyme to be composed of at least two isoenzymes. One major part focusing at pH 6.2 and a minor part focusing at pH 5.8.