Primary structure of pancreatic polypeptide from four species of perissodactyla (Przewalski's horse, zebra, rhino, tapir)

Pancreatic polypeptide (PP) has been purified from extracts of the pancreas of four species of odd-toed ungulates (Perissodactyla): Przewalski's horse, mountain zebra, white rhinoceros, and mountain tapir. The amino acid sequence of Przewalski's horse pancreatic polypeptide was established as Ala-Pro-Met-Glu-Pro-Val-Tyr-Pro-Gly-Asp¹⁰-Asn-Ala-Thr- Pro-Glu-Gln-Met-Ala-Gln-Tyr²⁰-Ala-Ala-Glu-Leu-Arg-Arg-Tyr-Ile-Asn-Met³⁰-Leu-Thr-Arg-Pro-Arg-Tyr·NH₂. Zebra PP was identical to Przewalski's horse PP, rhinoceros PP contained three substitutions relative to the horse (Ser for Ala¹, Leu for Met³, and Glu for Gln¹⁶), and tapir PP contained one substitution relative to the horse (Leu for Met³). On the basis of morphological characteristics and the fossil record, the rhinocerotids are classified with the tapirids in the suborder Ceratomorpha, whereas the horse and zebra belong to a separate suborder, Hippomorpha. On the basis of structural similarity of the PP molecules, however, it would appear that the tapir is more closely related to the horse than to the rhinoceros. These observations provide a further example of the need for extreme caution when inferring taxonomic or phylogenetic relationships between species from the structures of homologous peptides.