11β-Hydroxysteroid dehydrogenase studied in vivo in man with cortisol-4-¹⁴C-11-³H

The dynamics of cortisol labeled at C-4 with ¹⁴C and at C-11 with ³H was studied in man. A large primary isotope effect was demonstrated. Tritium appeared rapidly in body water and about 50% of this isotope was removed from the substrate in less than 2 hours. The tritium species had a 40 minute half-life in comparison with a 60 minute half-life for the ¹⁴C-cortisol. The role played by the kidney was examined by simultaneous sampling of blood from the radial artery and renal vein. During an initial 40 minutes the renal parenchyma metabolized about 5% of the tritium species and transferred the hydrogen isotope to water. Because of the large isotope effect this is a minimal estimate of the in vivo activity of the renal 11β-hydroxysteroid dehydrogenase.