Endogenous Peptides

Endogenous peptides are peptides produced naturally within the body. They serve as hormones, neurotransmitters, immune mediators, antimicrobials, and structural signals. The human body produces hundreds of distinct peptides that regulate virtually every physiological process — from growth and metabolism to pain, mood, digestion, and immune defense.

Understanding endogenous peptides is essential context for peptide therapy: most therapeutic peptides are either synthetic versions of natural peptides, modified analogs designed for improved pharmacokinetics, or fragments of larger endogenous proteins that retain specific biological activities.

Major categories of endogenous peptides

Peptide hormones

The classical endocrine peptides — produced by specialized glands, released into the bloodstream, and acting on distant target organs:

• Insulin (51 amino acids) — pancreatic beta cells; glucose regulation. The first peptide hormone discovered (1921) and the first produced recombinantly (1982).
• Glucagon (29 amino acids) — pancreatic alpha cells; raises blood glucose. Counter-regulatory to insulin.
• GHRH (44 amino acids) — hypothalamus; stimulates pituitary growth hormone release. Sermorelin is a synthetic analog of GHRH(1-29).
• GLP-1 (30 amino acids) — intestinal L-cells; incretin hormone that stimulates insulin secretion, slows gastric emptying, and promotes satiety. Semaglutide and liraglutide are stabilized GLP-1 analogs.
• Oxytocin (9 amino acids) — hypothalamus; social bonding, uterine contraction, lactation.
• Vasopressin/ADH (9 amino acids) — hypothalamus; water retention, blood pressure regulation.
• ACTH (39 amino acids) — anterior pituitary; stimulates cortisol release from adrenal glands. Semax is a synthetic analog of the ACTH(4-7) fragment.

Gastric and gut peptides

The GI tract is the largest endocrine organ in the body, producing dozens of peptide signals:

• Ghrelin (28 amino acids) — the "hunger hormone"; stimulates appetite and GH release via the GHS-R receptor. Ipamorelin and GHRP-6 are synthetic ghrelin mimetics.
• GIP (42 amino acids) — glucose-dependent insulinotropic peptide; the other incretin. Tirzepatide is a dual GIP/GLP-1 agonist.
• BPC (Body Protection Compound) — a peptide sequence found in human gastric juice. BPC-157 is a synthetic pentadecapeptide derived from this protein, stabilized for research use.
• VIP (28 amino acids) — vasoactive intestinal peptide; regulates GI motility, blood flow, and immune function.

Antimicrobial peptides

The innate immune system's first-line defense — peptides that directly kill bacteria, viruses, and fungi:

• LL-37 (37 amino acids) — the only human cathelicidin; disrupts microbial membranes, modulates immune cell recruitment, and promotes wound healing.
• Defensins (29-45 amino acids) — alpha and beta defensins expressed in epithelial cells, neutrophils, and Paneth cells; broad-spectrum antimicrobial activity.
• KPV (3 amino acids) — the C-terminal tripeptide of alpha-MSH; anti-inflammatory peptide that inhibits NF-κB in intestinal epithelial cells.

Structural and signaling peptides

Peptide fragments released during tissue turnover that signal repair:

• GHK (3 amino acids) — a tripeptide released during collagen breakdown that modulates expression of 4,000+ genes; the copper complex (GHK-Cu) is used in wound healing and skincare.
• Matrikines — peptide fragments from extracellular matrix degradation that signal fibroblasts to produce new collagen. Matrixyl (palmitoyl pentapeptide-4) mimics this endogenous feedback loop.

Age-related decline

A central rationale for peptide therapy is that endogenous peptide production declines with age:

• GHRH and GH decline ~14% per decade after age 30 (somatopause)
• Melatonin production decreases as pineal function declines
• Thymosin Alpha-1 output drops with thymic involution
• GHK levels fall from ~200 ng/mL at age 20 to ~80 ng/mL by age 60
• Endorphin baseline levels decline, contributing to reduced pain tolerance and mood regulation

This decline is not pathological — it is a normal feature of aging. But it provides the pharmacological rationale for peptide supplementation: restoring peptide signaling toward youthful levels may partially restore the physiological functions those peptides regulate.

Exogenous peptides: analogs, fragments, and mimetics

Therapeutic peptides relate to their endogenous counterparts in three ways:

Direct replacement — administering the natural peptide itself (oxytocin, GHK-Cu). Limited by rapid degradation and short half-life.

Stabilized analogs — modified versions with improved pharmacokinetics. Semaglutide is GLP-1 with amino acid substitutions and a fatty acid chain that extends half-life from 2 minutes to 7 days. Semax adds a Pro-Gly-Pro tail to ACTH(4-7) for protease resistance.

Fragments — isolated active regions of larger peptides. BPC-157 is a 15-amino-acid fragment of a larger gastric protein. TB-500 is the actin-binding fragment of 43-amino acid Thymosin Beta-4. Fragments are cheaper to synthesize and may retain the desired activity while losing unwanted effects.

Mimetics — synthetic molecules that activate the same receptor but have unrelated structures. Ipamorelin mimics ghrelin at the GHS-R receptor but is structurally distinct from ghrelin itself.

The endogenous peptide landscape provides the map; therapeutic peptides are the routes drawn on it. Knowing what the body already produces — and how that production changes with age, disease, or injury — is the foundation for rational peptide use.