There are three sources of proteins in human body: (a) food, (b) intestinal bacteria, and (c) body cells. Any protein molecule, irrespective of its source, ends with being enzymatically digested into a number of short peptide fragments. Almost all of the fragments are but intermediates on the way to their further digesting. Only a small proportion of such fragments is implicated in regulatory processes by interacting with target cells via receptors or otherwise.
The totality of issues related to peptide-mediated regulation may be reduced to four key questions:
- What are the mechanisms of the synthesis of the regulatory peptides (ribosomal and/or non-ribosomal)?
- What cells, tissues and organs produce the regulatory peptides?
- How are the regulatory peptides transported through barriers (intestinal wall, cell membrane, and nuclear membrane)?
- What are the mechanisms of interactions of the regulatory peptides with their target cells (receptor mediated or non-receptor mediated)?