Characterisation of 11β-hydroxysteroid dehydrogenases in feline kidney and liver

Abstract11β-Hydroxysteroid dehydrogenases type 1 and 2 (11β-HSD1 and 11β-HSD2) are microsomal enzymes responsible for the interconversion of cortisol into the inactive form cortisone and vice versa. 11β-HSD1 is mainly present in the liver, and has predominantly reductase activity although its function has not yet been elucidated. 11β-HSD2, present in mineralocorticoid target tissues such as the kidney, converts cortisol into cortisone. Reduced activity due to inhibition or mutations of 11β-HSD2 leads to hypertension and hypokalemia resulting in the Apparent Mineralocorticoid Excess Syndrome (AMES). Like humans, cats are highly susceptible for hypertension. As large species differences exist with respect to the kinetic parameters (Km and Vmax) and amino acid sequences of both enzymes, we determined these characteristics in the cat. Both enzyme types were found in the kidneys. 11β-HSD1 in the feline kidney showed bidirectional activity with predominantly dehydrogenase activity (dehydrogenase: Km 1959±797 nM, Vmax 766±88 pmol/mg*min; reductase: Km 778±136 nM, Vmax 112±4 pmol/mg*min). 11β-HSD2 represents a unidirectional dehydrogenase with a higher substrate affinity (Km 184±24 nM, Vmax 74±3 pmol/mg*min). In the liver, only 11β-HSD1 is detected exerting reductase activity (Km 10462 nM, Vmax 840 pmol/mg*min). Sequence analysis of conserved parts of 11β-HSD1 and 11β-HSD2 revealed the highest homology of the feline enzymes with the correspondent enzymes found in man. This suggests that the cat may serve as a suitable model species for studies directed to the pathogenesis and treatment of human diseases like AMES and hypertension