Cat proenkephalin-A does not contain the opioid octapeptide

The sequence of a large cDNA fragment of proenkephalin-A from the cat adrenal medulla was obtained using reverse transcription followed by polymerase chain reaction, and cloning. This cDNA encompasses the region normally containing ail the opioid peptides, except the C-terminal heptapeptide. As with other species, cat proenkephalin-A contains four conserved copies of (Met(5))-enkephalin, and one of (Leu(5))-enkephalin, flanked by processing sites of paired basic amino acids. However, significant differences were found in the nucleotide and deduced amino acid sequences in the region of the octapeptide. In particular, the essential tyrosyl residue is substituted by a histidyl residue, making it unlikely that the cat equivalent would have opioid activity. Furthermore, the peptide is not flanked by paired basic residues, suggesting it is not processed.