In the African lungfish Met-enkephalin and Leu-enkephalin are derived fromseparate genes: Cloning of a proenkephalin cDNA

A full-length proenkephalin cDNA (accession number: AF232670) was cloned fr om an African lungfish (Protopterus annectens) brain cDNA library. The 1,35 1-bp African lungfish proenkephalin contains an open reading frame that cod es 266 amino acids and a stop codon. Within the sequence of lungfish proenk ephalin there are 5 pentapeptide opioid sequences (all YGGFM), 1 octapeptid e opioid sequence (YGGFMRSL) and 1 heptapeptide opioid sequence (YGGFMGY). A Leu-enkephalin sequence was conspicuously absent in lungfish proenkephali n. These results, coupled with observations on the organization of amphibia n proenkephalin and mammalian proenkephalin, indicate that among the Sarcop terygii (lobed finned fish and tetrapods), the appearance of a Leu-enkephal in sequence in proenkephalin may have evolved in either the ancestral amnio tes or the ancestral mammals, but not earlier in sarcopterygian evolution. Furthermore, the detection of neurons in the lungfish CNS that are only imm unopositive for Met-enkephalin, coupled with earlier anatomical studies on the presence of neurons in the lungfish CNS that are only immunopositive fo r Leu-enkephalin, indicates that a Leu-enkephalin-coding opioid gene must b e present in the CNS of the lungfish. This gene may be the lungfish form of prodynorphin. Given the phylogenetic position of the lungfish in vertebrat e evolution, the putative Leu-enkephalin-coding gene must have evolved in t he ancestral sarcopterygian vertebrates, or in the ancestral gnathostomes. The apparent slow rate of lungfish evolution makes these organisms interest ing models for investigating the evolution of the opioid/orphanin gene fami ly. Copyright (C) 2000 S. Karger AG, Basel.