Orexin A Peptide 99% (CAS 205640-90-0), with the molecular formula C₁₅₂H₂₄₃N₄₇O₄₄S₄ and a molecular weight of 3561.12, is a linear polypeptide composed of 33 amino acids, containing an N-terminal pyroglutamic acid residue and two pairs of intrachain disulfide bonds. As an excitatory neuropeptide specifically secreted by the lateral hypothalamus, it regulates sleep-wake, energy metabolism, and reward pathways by activating OX1R/OX2R receptors. It possesses arousal-promoting, appetite-regulating, analgesic, and neuroprotective activities, making it a core tool peptide for research on narcolepsy, sleep disorders, and metabolic diseases.
The peptide code of disulfide bond cyclization
In its physical form, high-purity Orexin A Peptide (99%) is a white to off-white lyophilized powder. Fisher Scientific's product page indicates that the peptide has a purity of 99.7% and a molecular weight of 3561.10 Da. The precise pairing of the two sets of disulfide bonds in its sequence is crucial and a structural prerequisite for maintaining its biological activity. Synthetically synthesized Orexin A must undergo a specific oxidative folding process to ensure correct disulfide bond pairing; any mismatch will lead to a sharp decrease in receptor affinity. Regarding solubility, the peptide is readily soluble in sterile water, physiological saline, or phosphate buffer. According to BIOZOL's product information, each 100-microgram vial of lyophilized powder can be reconstituted with sterile deionized water or buffer solution, aliquoted, and then frozen to avoid repeated freeze-thaw cycles.
Regarding storage stability, the stringent requirement of 99% purity for Orexin A Peptide is standard for its use as a high-end biochemical reagent. The supplier explicitly requires that the lyophilized powder be stable for one year at -20°C and for two years at -80°C. The reconstituted solution can be stored at -20°C or -80°C for 6 months, but will degrade rapidly at 4°C or room temperature. Fisher's product page specifically emphasizes that the reconstituted peptide solution should not be repeatedly frozen and thawed; it should be immediately aliquoted into single-use vials after the first thawing, and each vial should be stored at -80°C. Due to the tendency of Orexin A to aggregate, its dissolution should be handled gently to avoid vigorous vortexing that generates bubbles, which can lead to irreversible aggregation of the peptide chains at the gas-liquid interface.
In terms of structural classification and molecular characteristics, the difference between Orexin A and Orexin B lies not only in their length and sequence but also in their receptor selectivity. Both are endogenous ligands for OX1R and OX2R. Orexin A has nanomolar-level high affinity for both OX1R and OX2R, but poor receptor selectivity; while Orexin B primarily binds to OX2R with high affinity. Therefore, Orexin A is often used as a broad-spectrum agonist when studying the function of specific receptor subtypes, while Orexin B serves as a selective OX2R tool drug.
Regarding quality control, the most critical parameter for Orexin A Peptide 99% is purity, requiring a minimum of 95% or 99%. Major impurities include deleted peptides, mismatched disulfide bond byproducts, and oxidative degradation products. Endotoxin testing must be performed to strictly control residual pyrogens; this is a prerequisite for its use in central nervous system administration experiments.
Activation logic of OX1R and OX2R
99% of the pleiotropic physiological functions of Orexin A Peptide are rooted in its interaction with two homologous G protein-coupled receptors. These two receptors have different tissue distributions and pharmacological properties. OX1R has selective affinity for Orexin A and is functionally closely related to the regulation of reward, drug-seeking behavior, and autonomic nervous responses. OX2R has equally high affinity for both Orexin A and Orexin B, and its function is mainly localized to regulating the stability of the sleep-wake cycle and the maintenance of wakefulness.
When Orexin A binds to its receptor, both preferentially couple to the Gq/11 protein. Upon receptor activation, phospholipase C is activated, catalyzing the hydrolysis of phosphatidylinositol 4,5-bisphosphate to inositol triphosphate and diacylglycerol. Inositol triphosphate triggers the release of calcium ions from the endoplasmic reticulum, instantaneously increasing intracellular calcium concentration; diacylglycerol, in turn, activates protein kinase C, phosphorylating downstream effector proteins and regulating ion channel activity. Through this calcium-protein kinase C signaling axis, Orexin A rapidly excites histaminergic neurons in the mammillary nucleus of the hypothalamus, noradrenergic neurons in the locus coeruleus of the brainstem, and serotonergic neurons in the raphe nuclei, forming a coordinated "wake-up network."
At the mechanistic level of maintaining wakefulness, OX2R plays an irreplaceable role. Both Orexin knockout mice and OX2R mutant mice exhibit a phenotype similar to narcolepsy-a sudden transition from active to REM sleep. Conversely, intraventricular injection of Orexin A dose-dependently prolongs wakefulness in mice and delays the onset of both non-REM and REM sleep. The underlying cellular mechanism is that Orexin A promotes cortical desynchronization by directly depolarizing the thalamic reticular nucleus, inhibiting the generation of sleep spindle waves in the thalamus, and simultaneously exciting cholinergic neurons in the basal forebrain. While Orexin A does not encompass all arousal functions, its role as a "fragile switch" in preventing sleep from intruding into wakefulness is irreplaceable.
In the perception of feeding and energy metabolism, Orexin A plays a multifaceted regulatory role by acting on the arcuate nucleus and paraventricular nucleus of the hypothalamus. Hunger and hypoglycemia are strong activation signals of the endogenous Orexin system. When blood glucose decreases, glucose-inhibitory Orexin neurons depolarize and increase their firing frequency, releasing Orexin A to the feeding center it innervates. Orexin A drives animals to forage by activating hypothalamic feeding-promoting neurons while inhibiting the satiety center. This signaling pathway explains why glucagon-like peptide-1 receptor agonists such as liraglutide are often accompanied by fatigue, partly because they indirectly reduce arousal levels by inhibiting the Orexin system.
The localization of Orexin A in the reward pathway explains how this traditionally considered "feeding-promoting peptide" is associated with drug addiction. Orexin neurons from the lateral hypothalamus project densely to the ventral tegmental area and nucleus accumbens. Orexin A stimulates dopamine release by activating OX1R neurons on dopamine neurons in the ventral tegmental area, enhancing the reward effect of addictive substances such as morphine and cocaine. OX1R antagonists have been shown to weaken conditioned positional preference in rats and alleviate withdrawal symptoms. This also provides a novel drug target for the treatment of substance use disorders.
Tool drugs for sleep disorders and neuropharmacology
The most fundamental use of Orexin A Peptide (99%) is as an "etiological tool" for narcolepsy. Orexin A levels in the cerebrospinal fluid (CSF) of patients with type 1 narcolepsy are significantly reduced, and in some patients, the molecule is completely undetectable in the CSF. In clinical studies, intraventricular or intravenous administration of exogenous Orexin A has been shown to improve excessive daytime sleepiness in narcolepsy patients, but its difficulty in crossing the blood-brain barrier limits its direct clinical application. Nevertheless, this molecule remains a positive control for validating the efficacy of new therapies.
In basic research on sleep and wakefulness mechanisms, Orexin A is an indispensable "perturbation tool." Stereotactic injection of nanograms of Orexin A into specific nuclei in rats allows for the study of the specific contributions of different brain regions to the regulation of wakefulness. Local administration of Orexin A has been shown to activate cholinergic neurons in the basal forebrain or to shift brain electrical activity from a slow-wave pattern to a rapid desynchronization pattern. Combining this peptide with selective OX1R or OX2R antagonists allows for a fine differentiation of the different roles of the two receptors in the initiation and maintenance of wakefulness. These experiments, spanning the past two decades of sleep research, serve as a data source for constructing modern models of the wakefulness system.
In the field of drug dependence research, Orexin A is widely used to explore the neural mechanisms of craving and relapse. By microinjecting Orexin A into brain regions such as the ventral tegmental area and central amygdala, researchers can simulate stress- or cue-induced relapse states and assess the excitability of dopamine neurons and drug-seeking behavior in animals. Co-injection with OX1R antagonists can determine the indispensability of the Orexin system in drug reward.
Orexin A is also a frequently used tool in studies of neural circuits related to mood and anxiety. In rodents, intraventricular injection of Orexin A induces physiological responses similar to alertness and high arousal, increases open-arm dwell time in the elevated cruciate maze, and exhibits an anti-anxiety effect. However, excessive Orexin signaling is also associated with stress-induced panic attacks. Blocking the Orexin system can alleviate excessive stress responses in anxiety models. OX2R antagonists have been explored in clinical trials for anxiety disorders and panic disorders.
In the field of energy metabolism regulation, Orexin A injection is often used to study the real-time regulation of lipid metabolism and insulin sensitivity by the central nervous system. Microinfusion of concentrated Orexin A into the third ventricle alters hepatic glucose output and skeletal muscle glucose uptake. The assessment of these acute effects is often used as strong evidence for evaluating centrally nervous system-driven metabolic improvements in experiments investigating the efficacy of glucagon-like peptide-1 receptor agonists or sodium-glucose cotransporter-2 inhibitors.
Frontier Exploration of Self-Assembly Materials and Therapeutic Interventions
In recent years, 99% of research surrounding Orexin A peptide has shifted from pulsed drug delivery interventions to structural-based supramolecular delivery systems. The cholesterol-based Orexin A conjugate reported in 2026 is representative of this direction. Researchers chemically synthesized a cholesterol moiety at the C-terminus or a specific site of Orexin A, causing it to self-assemble into a nanoscale fibrous network. When injected into the mouse brain, this self-assembled network significantly prolonged its residence time in brain tissue; the conjugate remained in the brain parenchyma for a much longer time than the free Orexin A peptide. This local residence can continuously activate its homologous receptor, reducing the trauma and infection risks associated with repeated intraventricular injections. This is the first proof-of-concept demonstration in the living animal brain of an action on a natural receptor based on a "functional neuropeptide self-assembly" strategy, and a prelude to Orexin A's leap from a simple biomolecule to a "smart material."
In the fields of drug reuse and receptor structural biology, Orexin A serves as a "gold standard" agonist and a positive control in the screening of small molecule agonists. With the crystal structures of OX1R and OX2R resolved, molecular docking techniques have become more precise. When screening lead compounds, researchers first perform Orexin A-induced calcium mobilization experiments to verify the reliability of the screening system, and then use this system to evaluate the activity of the synthesized molecules. Orexin A is commonly used to detect the blocking ability of small molecule chelators on downstream receptor signals, providing a benchmark for developing orally active non-peptide antagonists.
In gene therapy and cell regeneration therapy, Orexin A levels are biomarkers for monitoring the recovery of transplanted cell function. In studies of "clinical cure" of narcolepsy, after transplanting Orexin neurons derived from induced pluripotent stem cells into the mouse brain, the concentration of locally released Orexin A needs to be measured. Only when a threshold concentration is reached can it be determined that the transplanted cells have the ability to integrate into neural circuits and release functional neurotransmitters; exogenous recombinant Orexin A serves as a standard for setting this threshold.
As an active pharmaceutical ingredient, the Orexin A supply chain mainly consists of two tiers. High-end pharmaceutical-grade suppliers offer high-specification strains with purity exceeding 99% and endotoxin levels below 1.0 EU/mg, suitable for in vivo animal studies. Another tier offers research-grade strains with purity approximately 95% to 98%, intended for in vitro receptor binding assays or basic biochemical research. Significant differences exist in the performance of these two types in cell culture or animal injection; researchers must choose carefully based on their intended use. Given the extremely high molecular weight of this peptide, its synthesis involves multiple complex steps, including solid-phase peptide synthesis, disulfide bond oxidative folding, and reversed-phase high-performance liquid chromatography purification. The 9-fluorenemethyloxycarbonyl solid-phase synthesis strategy allows the amino acid chain to extend according to a predetermined sequence, while the folding process must be carried out at extremely dilute concentrations, precise pH, and redox potentials to prevent the formation of mismatched disulfide bonds. This is the technological root cause of the high production cost of Orexin A.
Conclusion
Orexin A Peptide 99% possesses a unique molecular structure with a 33-peptide linear backbone and two pairs of conserved disulfide bonds. This structure establishes a core mechanism for specific activation of OX1R/OX2R and synergistic regulation of multiple neural pathways, enabling sleep-wake regulation, energy metabolism balance, neuroprotection, and analgesia. It has become a benchmark tool peptide for hypothalamic excitatory neuropeptides. The disulfide bond stability at the molecular structural level, N-terminal receptor recognition, C-terminal signal activation, and high purity stability lay the structural foundation for high receptor affinity, metabolic stability, and comprehensive activity.
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References
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- Wang, L., et al. (2025). Orexin A as a neuroprotective and anti-inflammatory agent in neurodegenerative diseases. Journal of Neuroinflammation, 22(1), 123.