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Peptide Glossary
A–Z guide to peptide, laboratory, quality, and research terminology.
A
- Acetate Salt A counter-ion form produced by exchanging trifluoroacetate for acetate. It is the standard for approved peptide products and for sensitive cell-based work.
- Acetylation Capping a peptide's N-terminal amine with an acetyl group — adding 42.01 Da, removing a positive charge and blocking aminopeptidase attack.
- Activin Receptor Serine/threonine kinase receptors of the TGF-beta family — ActRIIA and ActRIIB plus type I partners ALK4/ALK7 — that transduce activin and myostatin signals through SMAD2/3.
- Aliquot A measured portion divided off from a larger stock. Splitting a reconstituted vial into single-use aliquots is the standard defence against repeated freezing.
- Amide (C-Terminal Amidation) Conversion of a peptide's C-terminal carboxylic acid into a primary amide (–CONH2), mimicking a natural modification and shifting the expected mass by about 1 Da.
- Amino Acid The monomer unit of every peptide: an alpha-carbon carrying an amino group, a carboxyl group and a variable side chain that dictates the residue's chemistry.
- AMPK AMP-activated protein kinase: a heterotrimeric energy sensor switched on when the AMP:ATP ratio rises, shifting cells from anabolic to catabolic metabolism.
- Amylin A 37-amino-acid hormone co-secreted with insulin from pancreatic beta-cells, acting at calcitonin-receptor complexes to signal satiation.
- Analog A deliberately modified version of a parent peptide — substituted, capped, truncated or conjugated — designed to change stability, potency or selectivity.
- Androgen Receptor A ligand-activated nuclear transcription factor (gene AR, Xq12) bound by testosterone and dihydrotestosterone, and the molecular target of every SARM.
- Antibody (Polyclonal vs Monoclonal) Polyclonals are a mixed population from an immunised animal recognising many epitopes; monoclonals descend from one B-cell clone and recognise a single epitope.
- Antiserum The clotted, cell-free serum fraction from an immunised animal — containing the raised antibodies plus every other serum immunoglobulin and protein.
B
- Bacteriostatic Water Sterile water containing 0.9% benzyl alcohol as a bacteriostatic preservative, which allows a reconstituted vial to be entered more than once.
- BDNF Brain-derived neurotrophic factor: a TrkB ligand central to synaptic plasticity, whose precursor proBDNF signals in the opposite direction through p75NTR.
- Bioavailability The fraction of an administered amount that reaches systemic circulation unchanged. For peptides it is the number that separates injectable formats from oral ones.
- Bioregulator (Khavinson Peptide) Very short synthetic peptides (2-4 residues) derived from Soviet-era tissue extracts, proposed to act tissue-specifically by binding DNA promoter regions.
- Biotin Label A vitamin-B7 tag conjugated to a peptide, giving an essentially irreversible handle for streptavidin- or avidin-based capture and detection.
- Blocking Peptide The free peptide matching an antibody's immunogen sequence, pre-incubated with the antibody so that genuine epitope-specific signal disappears.
C
- Certificate of Analysis (COA) The lot-specific document reporting what was tested on a peptide batch — purity, identity, appearance — and by whom, on a stated date.
- Cold Chain Unbroken temperature control from manufacture through transit to storage. For lyophilised peptides the requirement is real but far less absolute than for solutions.
- Compounding Pharmacy A licensed pharmacy that prepares medications to order under FDCA 503A (patient-specific) or 503B (outsourcing facility) — a category distinct from research supply.
- Cosmetic Peptide (Signal Peptide) Short peptides used in topical formulation, classed as signal, neurotransmitter-inhibiting, carrier or enzyme-inhibitor peptides — a formulation category, not a mechanism.
- Cyclic Peptide A peptide closed into a ring by a covalent bond. Cyclisation removes free termini, restricts conformation and typically improves protease resistance.
D
- DAC (Drug Affinity Complex) A maleimidoproprionic acid linker that couples a peptide covalently to albumin in circulation, stretching a minutes-long half-life into a multi-day one.
- Degradation Every chemical or physical route by which a peptide stops being the intended molecule — hydrolysis, deamidation, oxidation, racemisation and aggregation.
- Delta Sleep-Inducing Peptide (DSIP) A nine-residue peptide, Trp-Ala-Gly-Gly-Asp-Ala-Ser-Gly-Glu, isolated in 1977 from sleep-deprived rabbit brain perfusate; its receptor has never been identified.
- Disulfide Bridge A covalent sulfur–sulfur bond between two cysteine residues. It fixes a peptide's three-dimensional shape and is often essential to receptor recognition.
- Dual Agonist A single peptide engineered to activate two distinct receptors at a controlled potency ratio, rather than combining two separate molecules.
E
- ELISA Enzyme-linked immunosorbent assay: a plate-based immunoassay in which an enzyme-antibody conjugate converts a substrate to a measurable colour or luminescent signal.
- Endotoxin Lipopolysaccharide from Gram-negative bacterial cell walls. It is potent at picogram levels, heat-stable, and completely invisible to an HPLC purity figure.
- Excipient Any component of a formulation other than the active peptide: bulking agents, buffers, stabilisers and preservatives that shape how a vial looks and behaves.
F
- FDA-Approved vs Research-Grade Approval attaches to a specific finished product from a specific cGMP manufacturer for a specific indication — not to a molecule, and never to research-grade material.
- Fluorescent Label (FAM / FITC) Fluorescein-derived tags conjugated to synthetic peptides — FAM through a stable amide bond, FITC through a less stable thiourea — both excited near 495 nm.
- Follistatin A secreted activin-binding glycoprotein that neutralises activin, myostatin and several BMPs by encircling the ligand dimer and blocking receptor contact.
- Fragment (e.g. 176-191) A defined stretch of a larger parent protein, named by its residue numbers. A fragment carries only part of the parent's structure and rarely all its behaviour.
- Freeze–Thaw Cycle One round trip from frozen to liquid and back again. Each repetition concentrates solutes, shifts pH and creates interfaces where peptides aggregate.
G
- Gauge A needle sizing scale that runs backwards — the higher the number, the thinner the needle. It governs flow rate, shear on the peptide and coring of vial stoppers.
- Ghrelin Receptor (GHS-R1a) The class A GPCR bound by ghrelin and by every GHRP, notable for high constitutive activity and a requirement for acylated ligand.
- GHRH Growth hormone-releasing hormone: a 44-amino-acid hypothalamic peptide whose first 29 residues carry full activity, the basis of sermorelin and its analogs.
- GHRP Growth hormone-releasing peptides: short synthetic sequences acting at GHS-R1a, a receptor pathway entirely separate from GHRH.
- GIP Glucose-dependent insulinotropic polypeptide: a 42-residue incretin from duodenal K-cells, and the second receptor arm engaged by tirzepatide.
- GLP-1 Glucagon-like peptide-1: an incretin released from intestinal L-cells that acts at the GLP-1 receptor and is cleaved within minutes by DPP-4.
- GLP-1 Receptor Agonist An engineered molecule that activates the GLP-1 receptor while resisting DPP-4 cleavage, extending duration from minutes to hours or days.
- Glucagon Receptor A class B GPCR expressed mainly on hepatocytes, and the third receptor arm engaged by glucagon-containing multi-agonist research peptides.
- Gray-Market Vendor A supplier operating in the gap between research supply and unlicensed pharmacy — research-use labelling on the invoice, human-use signalling everywhere else.
- Growth Hormone Secretagogue Any compound that prompts the pituitary to release endogenous growth hormone, whether through the GHRH receptor, the ghrelin receptor, or both.
H
- Half-Life The time taken for a concentration to fall by half. For peptides it spans roughly two minutes to a week, and it is not the same as how long a vial stays good.
- HPLC High-performance liquid chromatography: the separation method behind the purity percentage on a peptide certificate, reported as peak area percent.
- Hyaluronic Acid A non-sulfated glycosaminoglycan of repeating disaccharide units whose biological behaviour depends strongly on chain length, supplied as sodium hyaluronate.
- Hydrophilic Water-loving. Most research peptides are hydrophilic, which is why they dissolve readily in aqueous diluent and why they cross biological membranes so poorly.
I
- I-125 Radiolabel Iodine-125 incorporated into a peptide's tyrosine residues, giving a high-specific-activity tracer for radioimmunoassay and receptor binding studies.
- IGF Binding Protein A family of six high-affinity carrier proteins that bind circulating IGF-1 and IGF-2, extending half-life while restricting receptor access.
- IGF-1 Insulin-like growth factor 1: a 70-amino-acid single-chain polypeptide that mediates most of growth hormone's downstream effects on tissue.
- Immunogen The material actually injected to raise an antibody — for short peptides, almost always a carrier-protein conjugate rather than the free peptide.
- In Vitro Literally "in glass" — experiments performed outside a living organism, from receptor binding and enzyme assays to cell and organoid culture.
- In Vivo Research conducted within a living organism, where absorption, distribution, metabolism, clearance and whole-system feedback all act on the compound at once.
- Incretin Gut-derived hormones — principally GLP-1 and GIP — that amplify glucose-stimulated insulin secretion after oral nutrient intake.
- Insulin Syringe Units (U-100) On a U-100 syringe 100 units equal 1 mL, so one unit is 0.01 mL. Units mark volume only; what that volume contains depends entirely on the reconstitution.
- Intranasal Delivery across the nasal mucosa. It bypasses first-pass hepatic metabolism and is the route behind the nose-to-brain hypothesis studied for several neuropeptides.
- Isoelectric Point (pI) The pH at which a peptide carries no net charge. It is also the pH at which the peptide is least soluble, which explains many failed reconstitutions.
L
- Linear Peptide An unbranched chain with free N- and C-termini and no ring closure. Most research peptides are linear, which makes them simple to make and quick to clear.
- Lipophilic Fat-loving. A lipophilic molecule or residue partitions preferentially into lipid phases, which governs membrane crossing, aggregation risk and albumin binding.
- Lot Number The batch identifier printed on a vial that links it to one production run and to the test results reported for that run.
- Luer-Lock A threaded syringe hub that screws the needle into place instead of relying on friction. It matters wherever back-pressure could pop a slip-fit connection apart.
- Lyophilized Freeze-dried under vacuum: water is frozen then sublimed away, leaving a dry cake that is far more stable in transit and storage than a solution.
M
- Mannitol A crystalline sugar alcohol used as a bulking agent in freeze-dried vials. It gives low-mass peptides a visible, mechanically sound cake.
- Mass Spectrometry The identity test on a peptide certificate: measured mass-to-charge ratios are converted to molecular mass and matched against the sequence's theoretical value.
- Matrikine Peptide fragments liberated by proteolysis of extracellular matrix proteins — collagen, elastin, laminin — that act as signalling molecules the intact protein does not.
- Micrograms vs Milligrams One milligram equals 1,000 micrograms. Vials are labelled in mg, literature reports mcg, and the factor of a thousand between them is the most common lab slip.
- Molarity Moles of solute per litre of solution. Cell and receptor work is specified in molar units, so a mg/mL vial concentration has to be converted before use.
- Molecular Weight (Da) Mass expressed in daltons. It is the bridge between the milligrams on a vial label and the moles a calculation needs, and the basis of identity confirmation.
- Myostatin (GDF-8) GDF-8, a TGF-beta superfamily ligand secreted by muscle that restrains muscle mass by signalling through ActRIIB/ALK4-5 and SMAD2/3.
N
- NAD+ Nicotinamide adenine dinucleotide: a redox cofactor that is also consumed as a substrate by sirtuins, PARPs and CD38, which is why its tissue level is regarded as rate-limiting.
- Native Sequence The amino acid sequence exactly as it occurs in a named species, with no substitutions, caps or conjugates. Native is a claim about identity, not about durability.
- Neurotrophin A four-member family of secreted dimeric growth factors — NGF, BDNF, NT-3 and NT-4/5 — that signal through Trk receptor tyrosine kinases and the shared p75NTR.
- NMN Nicotinamide mononucleotide: the immediate biosynthetic precursor to NAD+, one enzymatic step away in the mammalian salvage pathway.
- Nootropic A term coined in 1972 for compounds meeting five specific criteria including very low toxicity; in modern use it is a loose category label rather than a mechanism.
P
- PDRN Polydeoxyribonucleotide: a defined-length mixture of purified DNA fragments, usually salmon-derived, studied through adenosine A2A receptor and nucleotide salvage mechanisms.
- PEGylation Covalent attachment of polyethylene glycol chains to a peptide, raising its effective size so the kidney filters it more slowly and proteases reach it less easily.
- Peptide A chain of amino acids joined by peptide bonds — conventionally 2 to about 50 residues, above which the same molecule is usually called a protein.
- Peptide Bond The amide linkage formed when one amino acid's carboxyl group condenses with the next amino group — the covalent backbone of every peptide.
- PPAR-delta A ligand-activated nuclear receptor that pairs with RXR and drives transcription of fatty-acid oxidation genes in muscle, the molecular target of GW-501516.
- Preclinical All research conducted before a compound is given to humans — cell work, animal pharmacology, ADME and GLP toxicology — the stage nearly all research peptides occupy.
- Purified IgG The immunoglobulin G fraction isolated from antiserum — by Protein A/G capture (all IgG) or by antigen-affinity chromatography (target-specific IgG only).
- Purity The main HPLC peak's share of total detected peak area — a relative measure of chromatographic homogeneity, not a measure of how much peptide is in the vial.
R
- Recombinant Produced by an engineered living host — bacteria, yeast or mammalian cells — carrying the gene for the target. The route of choice above roughly 50 residues.
- Reconstitution Dissolving a lyophilized peptide in a chosen solvent to reach a known, calculable concentration in mg/mL — a laboratory preparation step, not an administration step.
- Research Use Only (RUO) A labelling designation marking material as intended for laboratory investigation only, not for diagnostic procedures or administration to humans or animals.
- REV-ERB Heme-binding nuclear receptors (NR1D1/NR1D2) that act as transcriptional repressors in the core circadian loop and connect clock timing to metabolic genes.
- RIA (Radioimmunoassay) Radioimmunoassay: a competitive immunoassay in which radiolabelled antigen competes with sample analyte for limited antibody, read as radioactivity rather than colour.
S
- SARM (Selective Androgen Receptor Modulator) A non-steroidal ligand of the androgen receptor engineered for tissue-selective transcriptional activity; none is FDA-approved, and all are research-use-only.
- Sirtuin A family of seven NAD+-dependent protein deacylases (SIRT1-7) that couple the cell's NAD+ supply to transcription, DNA repair and mitochondrial function.
- Solid-Phase Peptide Synthesis (SPPS) Merrifield's method: the chain grows anchored to a resin bead, C-terminus first, with excess reagents washed away after each residue is added.
- Somatostatin The inhibitory hypothalamic peptide that suppresses growth hormone release, existing as 14- and 28-residue forms acting at five receptor subtypes.
- Species Reactivity The list of species whose version of a target a reagent recognises — and the distinction between reactivity that was tested and reactivity merely predicted from homology.
- Stability How well a peptide retains its intended chemical structure over time under defined conditions — a property of the storage state, not of the molecule alone.
- Sterile Water Preservative-free water for injection: appropriate where benzyl alcohol would interfere, but the resulting solution should be treated as single-entry.
- Subcutaneous Delivery into the adipose layer beneath the dermis. In animal research it is the default parenteral route for peptides because it forms a slow-release depot.
- Sublingual Troche A soft lozenge designed to dissolve under the tongue, releasing its contents across the sublingual mucosa rather than through the stomach and liver.
- Synthetic (SPPS) Built chemically rather than biologically, one residue at a time on a solid support. This is how nearly every research peptide under 50 residues is made.
T
- Tetrapeptide A peptide of exactly four amino acid residues linked by three peptide bonds — a length that recurs across bioregulators, matrikines and cosmetic actives.
- TFA Salt The default counter-ion form of synthetic peptides. TFA can account for a tenth to a quarter of vial mass and is cytotoxic in some cell assays.
- Third-Party Testing Analysis performed by an independent laboratory with no stake in the result — the difference between a supplier's own claim and verified evidence for a lot.
- Tripeptide Three amino acid residues joined by two peptide bonds — the shortest length at which sequence-specific receptor and metal binding is routinely observed.
- Triple Agonist A single peptide engineered to activate three receptors — typically GIP, GLP-1 and glucagon — from one proglucagon-family backbone.