Molecular cloning, sequence analysis and translation of proenkephalin mRNA from rat heart

Proenkephalin mRNA is abundant in rat cardiac ventricles but surprisingly low levels of opioid peptides or precursor forms derived from proenkephalin are present in tissue extracts. Proenkephalin mRNA in rat heart was characterized at the molecular level with the use of cDNA sequencing, in vitro translation, and primer extension. Two positive proenkephalin cDNA clones were obtained by screening approx. 20,000 recombinant phages from a heart cDNA library. Sequence analysis of the cDNA clones indicated that the heart transcript was the same form as in rat brain, but differed from the germ cell-specific testis transcript that utilizes a different transcriptional start site. Heart proenkephalin cRNA translated efficiently, resulting in the synthesis of a 35 kDa protein that was immunoprecipitated by an antibody specific to the protein. The transcriptional initiation sites utilized in the heart were the same as in the brain, based on primer extension studies. These data suggest that the proenkephalin transcript found in abundance in rat heart is the same form as found in the brain, and differs from the testis-type transcript. We conclude that the scant level of proenkephalin-derived peptides in the heart is not due to an intrinsic inability of the proenkephalin transcript to translate.