Peptide Research Glossary
Key terms in peptide research, pharmacology, and clinical science. Definitions focus on how these terms are used in the research literature, not dictionary definitions.
- Amino acid
- The molecular building block of peptides and proteins. Twenty standard amino acids are encoded by the human genome; each has a common backbone structure and a unique side chain that determines its chemical properties.
- See: Amino Acid Chemistry →
- Bioavailability
- The fraction of an administered dose that reaches systemic circulation in an active form. Oral bioavailability of most research peptides is very low due to gastrointestinal enzyme degradation. Subcutaneous injection typically achieves 70–100% bioavailability.
- See: Peptide Bioavailability Guide →
- DPP-4 (Dipeptidyl peptidase-4)
- A serine protease present in plasma and on cell surfaces that cleaves dipeptides from the N-terminus of peptides with proline or alanine at position 2. Responsible for the rapid degradation of native GLP-1 and GHRH. Inhibited by the gliptin class of diabetes drugs.
- GPCR (G protein-coupled receptor)
- A large family of cell-surface receptors that transmit signals through intracellular G proteins. Most peptide hormones and research peptides act through GPCRs. Named for their characteristic seven-transmembrane domain structure.
- Growth hormone secretagogue (GHS)
- A class of compounds studied for their ability to stimulate release of growth hormone from the pituitary gland, acting through the GHS receptor (GHS-R1a). Distinct from GHRH analogs, which act through a different receptor.
- See: GHS Overview →
- Half-life (t½)
- The time required for the plasma concentration of a compound to fall by 50% after administration. Most unmodified natural peptides have half-lives measured in minutes; synthetic modifications can extend this to hours or days.
- See: Peptide Half-Life and Stability →
- In vitro
- Literally "in glass" — referring to experiments conducted in cell cultures, test tubes, or other laboratory containers outside of a living organism. In vitro findings provide mechanistic hypotheses but do not establish that effects occur in a living system.
- See: How to Read a Research Study →
- In vivo
- Literally "within the living" — referring to experiments conducted in a living organism. Animal in vivo studies are closer to biological reality than in vitro work but do not automatically predict human outcomes.
- Peptidase / Protease
- Enzymes that cleave peptide bonds. Peptidases act on short peptides; proteases typically refer to enzymes acting on proteins. The gastrointestinal tract and blood plasma both contain active peptidases, which is why most peptides have poor oral bioavailability.
- Peptide
- A short chain of amino acids linked by peptide bonds. Typically defined as 2–50 amino acids in length. Longer chains that adopt stable three-dimensional structures are generally called proteins.
- See: What Are Peptides? →
- Peptide bond
- The covalent bond that links the carboxyl group of one amino acid to the amino group of the next. Formed by condensation (release of water). Cleaved by enzymes called peptidases or proteases.
- Randomized controlled trial (RCT)
- A clinical study design in which participants are randomly assigned to treatment or control groups. When properly blinded and pre-registered, the RCT is the strongest design for establishing whether an intervention causes a clinical effect.
- See: Understanding Clinical Trials →
- Receptor agonist
- A compound that binds to a receptor and activates it, producing a biological response. A full agonist produces the maximum response achievable for that receptor; a partial agonist produces a submaximal response even at full receptor occupancy.
- See: Peptide Receptor Binding →
- Receptor antagonist
- A compound that binds to a receptor without activating it, thereby blocking the receptor from being activated by endogenous ligands or other agonists. Does not produce a direct biological response but inhibits the response of other compounds.
- Solid-phase peptide synthesis (SPPS)
- The standard laboratory method for manufacturing synthetic peptides. Amino acids are added sequentially to a solid polymer resin support. The primary method used to produce research-grade peptides.
- See: How Peptides Are Synthesized →