Tracking the evolution of the proenkephalin gene in tetrapods.

In gnathostomes there is remarkable consistency in the organization of the proenkephalin gene. This opioid precursor encodes seven opioid (YGGF) sequences: five pentapeptide sequences, a met-enkephalin-7 sequence and a met-enkephalin-8 sequence. Yet, within vertebrate lineages there can be distinct sets of pentapeptide opioids (YGGFM or YGGFL).

Analyzing the evolution of beta-endorphin post-translational processing events: studies on reptiles.

In many cartilaginous fishes, most ray-finned fishes, lungfishes, and amphibians, the post-translational processing of POMC includes the monobasic cleavage of beta-endorphin to yield an opioid that is eight to ten amino acids in length. The amino acid motif within the beta-endorphin sequence required for a monobasic cleavage event is -E-R-(S/G)-Q-. Mammals and birds lack this motif and as a result beta-endorphin(1-8) is a not an end-product in either group.