Transcripts originating in intron 1 of the HSD11 (11β-Hydroxysteroid dehydrogenase) gene encode a truncated polypeptide that is enzymatically inactive

There is evidence for the presence in the kidney of more than one isoform of 11β-hydroxysteroid dehydrogenase (11HSD), an enzyme that interconverts cortisol and cortisone (in rodents, corticosterone and 11-dehydrocorticosterone). A specific isoform might arise from transcripts in the kidney that are known to originate in intron 1; translation from these transcripts is predicted to initiate at the codon that in the full-length rat enzyme encodes Met²⁷. Alignment of the full-length rat and human 11HSD sequences with other members of the short chain dehydrogenase family suggests that initiation of translation at Met²⁷ might yield a functional enzyme, since the amino-termini of most of these enzymes occur at equivalent positions. We confirmed that short transcripts are found in the kidney and are detectable at lower levels in the liver and lung. In vitro transcription and translation of short cDNA demonstrated that the AUG encoding Met²⁷ is indeed a functional initiation codon. However, Chinese hamster ovary cells transfected with short cDNA in the pCMV4 vector expressed apparently low levels of the corresponding truncated polypeptide and had no 11HSD activity. Thus, the functional significance of transcripts originating in intron 1 is unclear.