How Similar Are the Saliva Proteomes of Humans and Orangutans?

Objectives: The overall high genetic similarity of humans and orangutans, yet their different dietary lifestyles, make comparative studies of their salivary proteomes promising for understanding the functional significance of distinct salivary proteins. Studying salivary proteomes becomes even more critical as it helps understand how oral immune defense proteins adapt and respond to pathogen challenges. By elucidating the similarities and differences in the salivary proteomes of different primate species, we can deepen our understanding of primate biology, evolution, and the molecular basis of species-specific adaptations.
Methods: Here, we focused on exploring, cataloging, and comparing the salivary proteomes of humans and orangutans. For this study, we collected whole-mouth saliva samples from eight humans and four orangutans. For comparison, we used saliva samples that were collected in the past from two chimpanzees and three gorillas. For the analyses, we utilized label-free quantitative proteomic techniques as well as one-dimensional (1-D) and two-dimensional (2-D) gel electrophoresis combined with coomassie-blue and periodic-acid-Schiff stain to identify unique and similar molecular features among the proteins and glycoproteins in their saliva.
Results: We identified more than 150 secretory proteins commonly present in the saliva of humans and orangutans. Orangutans displayed additional variants of salivary cystatins and zinc-α2 glycoprotein compared to humans, while humans displayed additional variants of immunoglobulin heavy chain alpha compared to orangutans. Overall, the protein abundance, as well as banding and spot patterns of saliva proteins in orangutans, exhibited higher similarity with chimpanzees and gorillas than with humans.
Conclusions: This study provides a new evaluation of salivary proteins in nonhuman primates, highlighting both unique and shared features with humans. We hypothesize that the distinctive proteins in human saliva may have evolved in response to human lineage-specific dietary habits or exposure to specific pathogens.