Stimulation of mouse liver corticosteroid side chain isomerase by cobaltous and nickelous ions: evidence for an endogenous inhibitor of isomerase activity

Corticosteroid side-chain isomerase in the circulatory system

Corticosteroid side-chain (CSC) isomerase catalyzes ketol-aldol interconversion of the corticosteroid side chain. The enzyme was present in the blood of mouse, rat, guinea pig, chicken, pig, horse, sheep, cow, and human. The patterns of substrate specificity, measuring 3H-1H exchange of 21-tritiated forms of 11-deoxycorticosterone, corticosterone, and cortisol, were species specific. Based on enzyme activity and immunostaining of mouse blood fractions, red blood cells had the most isomerase activity, plasma had less, and white blood cells had low but highly variable levels of enzyme. Purified mouse liver CSC isomerase was found to be adsorbed by red blood cells. The results suggest that circulating CSC isomerase is derived in part from tissue sources and is in part an intrinsic blood enzyme.

Corticosteroid side-chain isomerase in mouse organs: kinetic and immunologic studies

We have investigated the distribution of corticosteroid side-chain (CSC) isomerase in the tissues of mice using as criteria its enzyme activity and immunoreactivity with monospecific polyclonal antibodies generated in rabbits. CSC isomerase was present in all organs examined. The liver and kidney contained the highest activity. The strain-dependent differences that we had previously reported for liver (i.e., BALB/c greater than C57BL/6) extended to the other organs, including the kidney, brain, heart, muscle, pancreas, testis, thymus, and lung. Western blot analysis showed a single antigen, identical in all tissues, corresponding in mobility to purified CSC isomerase. The intensities of the bands were generally proportional to enzyme activities. Titration of homogeneous enzyme with the IgG fraction of antiserum (unfractionated serum had some CSC isomerase activity) caused an increase in activity, followed by rapid inactivation after the addition of more antiserum. The broad distribution of CSC isomerase suggests that the ketol-aldol interconversion of the CSC may play a role other than, or in addition to, initiating metabolic inactivation of corticosteroids.

Purification and characterization of corticosteroid side chain isomerase

Corticosteroid side chain isomerase of rat liver catalyzes the interconversion of the ketol (20-oxo-21-ol) and aldol (20-hydroxy-21-al) forms of the corticosteroid side chain. The enzyme has now been purified to apparent homogeneity from rat liver cytosol by sequential chromatography on anionic, hydroxylapatite, and gel filtration columns. Ketol-aldol isomerization is followed by measuring the exchange of tritium from 21-tritiated steroids with water. The native enzyme is a dimer of MW 44,000. The isoelectric point is 4.8 +/- 0.1 pH units. The purified enzyme is stimulated by Co3+ or Ni2+. The enzyme utilizes 11-deoxycorticosterone, corticosterone, and 17-deoxycortisol as substrate but not cortisol, tetrahydrocortisol, and prednisolone. Tritium-water exchange of (21S)-[21-3H]DOC is a pseudo-first-order reaction; 21-3H exchange from the 21R isomer proceeds with first-order kinetics only after a lag associated with its epimerization to the 21S form.