Sorbitol dehydrogenase. Full-length cDNA sequencing reveals a mRNA coding for a protein containing an additional 42 amino acids at the N-terminal end.

A cDNA clone encoding rat sorbitol dehydrogenase (SDH) was isolated from a rat testis lambda ZAP II cDNA library. The full-length cDNA insert contained 2277 base pairs (bp), starting 182 bp upstream from an ATG codon where translation to the active enzyme SDH is presumed to be initiated. A second ATG codon, however, was found 126 bp upstream, aligned in the same reading frame as that of the active enzyme. Therefore, the coding sequence for SDH can be translated into an additional 42-amino-acid polypeptide linked to the N-terminal amino acid of the enzyme, generating a pre-sorbitol dehydrogenase. The sequence data indicate that the nucleotide environment around this ATG codon is more favorable towards it being the actual open reading frame (ORF) for a pre-SDH than the ATG codon preceding the nucleotide sequence for SDH. Since no known SDH starts with the additional 42 amino acids, it may be that post-translational removal of this polypeptide accompanies the release of the active enzyme. Next, the 3' untranslated region of the cDNA contained a non-coding 1021 bp downstream from the TAA stop codon. The latter sequence included three putative poly(A) signals: one at nucleotides 1362-1367, the second at nucleotides 1465-1470, and the third at nucleotides 2212-2217 [17 bp away from the poly(A) tail]. In addition to the above findings we also report a variance in one of the amino acids in the SDH cDNA sequence. This variance occurs at position 957-960, where threonine is coded for instead of aspartic acid; in the rat testis SDH cDNA, we find the sequence is ACG instead of GAC, as was reported for the rat liver SDH cDNA. Northern-blot hybridization analysis showed that SDH mRNA is a doublet, one band of 4 kb and the other of 2.3-2.4 kb, in both the rat liver and the rat lens, further confirming that the isolated SDH cDNA constituted a full-length cDNA.