Absence of soluble leptin receptor in plasma from dbPas/dbPas and other db/db mice

The leptin receptor (Ob-R) is alternatively spliced into at least five different RNAs designated Ob-R(a-e). Ob-R(a-d) predict receptors with a single transmembrane domain, and Ob-Re predicts a secreted form of the receptor. The presence of an approximately 120-kDa soluble leptin receptor in mouse plasma was confirmed by precipitation with leptin-Sepharose beads followed by immunobloting with anti-leptin receptor antibodies. The soluble leptin receptor is larger than that predicted by the primary sequence. Deglycosylation of the receptor with peptide N:glycosidase F results in a decrease in molecular mass to a size consistent with that of the primary sequence. The secreted receptor was present in plasma from wild type mice but was truncated in plasma from db3J/db3J and absent in dbPas/dbPas plasma. Although db3J/db3J mice are known to have a frameshift mutation at amino acid 625, the basis for the mutation in dbPas/dbPas mice was not known. Further studies indicated that dbPas/dbPas mice carry a duplication of exons 4 and 5 of Ob-R. This mutation introduces a premature stop codon into the protein at amino acid 281. The absence of Ob-R in db3J/db3J and dbPas/dbPas mice confirm the identify of the 120-kDa plasma protein as Ob-Re.