Absence of type 1 11β-hydroxysteroid dehydrogenase enzyme in koala liver

Abstract

The 11β-hydroxysteroid dehydrogenases (11β-HSDs) interconvert 11β-hydroxysteroids such as cortisol into 11-oxosteroids such as cortisone. In most mammals, 11β-HSD 1 is expressed predominantly in the liver and is active in both the oxidative (cortisol to cortisone) and dehydrogenase (cortisone to cortisol) directions, whilst 11β-HSD 2 is expressed predominantly in the kidney and functions as a pure oxidative enzyme. We have investigated 11β-HSD 1 activity in the Australian koala ( Phascolarctos cinereus) and have found no activity (either reductive or oxidative) in hepatic microsomes. Immunoblot analysis of koala hepatic microsomes, using an 11β-HSD 1 antibody raised against the mouse enzyme, failed to identify immunoreactive protein. Reverse transcriptase–polymerase chain reaction (RT-PCR) of koala liver mRNA and genomic PCR using primers designed against highly conserved regions of 11β-HSD 1 nucleotide sequences were also negative. Furthermore, Southern and Northern blot analysis of koala genomic DNA and mRNA, respectively, confirmed that the koala lacks the 11β- HSD 1 gene and gene transcript. These results support the fact that the lack of hepatic 11β-HSD 1 activity in the koala is due to the absence of the 11β- HSD 1 gene, and this absence is novel among mammalian species studied to date.