Big dynorphin
Big dynorphin is an endogenous opioid peptide of the dynorphin family. It forms when a precursor protein prodynorphin is not fully processed, resulting in a 32-amino acid molecule containing both dynorphin A and dynorphin B sequences. It has the amino acid sequence: Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln-Lys-Arg-Tyr-Gly-Gly-Phe-Leu-Arg-Arg-Gln-Phe-Lys-Val-Val-Thr.
Big dynorphin is a highly efficiacious principal endogenous agonist at the human κ-opioid receptor. It produces one of the strongest responses at GPR139 receptor among dynorphins. Additionally, it uniquely modulates NMDA receptor and potentiates acid-sensing ion channel 1a.
It plays a role in pain and stress responses. It exhibits both anelgesic and pro-nociceptive properties. In mice it modulates memory and learning, and has anxiolytic properties, contributing to homeostatic regulation of opioid signaling in the central nervous system.
Fragments
Big dynorphin1-32 is the primary endogenously documented big dynorphin form, but various shorter big dynorphin fragments have been synthesized for research purposes. These include,, forms.Synthesis
Big dynorphin is generated through the proteolytic processing of prodynorphin, a 26-kilodalton precursor protein by proprotein convertase 1. The synthesis occurs within the neuronal cell body through translation of prodynorphin mRNA. Following translation, prodynorphin undergoes sequential processing by proprotein convertases, primarily PC1/3 and PC2, as well as the cysteine protease cathepsin L.Under normal circumstances, in the presence of carboxypeptidase E, prodynorphin is fully processed by sequential cleavage at dibasic amino acid sites to generate individual dynorphin peptides: dynorphin A1-17, dynorphin B, and α-neoendorphin. Big dynorphin forms when this proteolytic processing is incomplete, typically resulting from insufficient proprotein convertase activity or altered intracellular calcium levels during neurotransmitter release events. The 32-amino acid peptide comprises the complete dynorphin A sequence joined to the complete dynorphin B sequence, with two C-terminal amino acids.
Release mechanism
Peptide's release occurs primarily in a calcium-dependent manner through exocytosis of large dense core vesicles in the presynaptic terminal. Big dynorphin immunoreactivity has been detected in regions such as the nucleus accumbens, caudate nucleus, and hippocampus, with significant levels detected in cerebrospinal fluid. The ratio of big dynorphin to dynorphin B in human brain tissue is approximately 1:3.Pharmacology
Opioid receptors
Big dynorphin acts as a potent full agonist at the human κ-opioid receptor, exhibiting extremely high relative efficacy at this target. In one binding assay it demonstrated similar affinity to dynorphin A, but about 14 to 32-fold higher potency to activate G proteins than other dynorphin peptides. In contrast, other studies have suggested similar or higher potencies of other dynorphins.The peptide is also an agonist of other opioid receptors, It is 70-fold selective towards KOR over μ-opioid receptors and 200 over δ-opioid receptor.
In older guinea-pig ileum bioassay measuring native receptor function, big dynorphin shows approximately 10-20-fold reduced potency relative to dynorphin A, possibly due to conformational constraints of the larger peptide affecting receptor binding in peripheral tissue. This discrepancy suggests that big dynorphin's efficacy in human KOR systems may not translate directly to potency in peripheral tissue.
Big dynorphin could theoretically produce some of the classical pharmacological effects associated with KOR agonism such as dysphoria, dissociation, and sedation, but this has not been directly evaluated. Similarly biased signaling of big dynorphin at KOR has not yet been assessed.
GPR139 receptor
Big dynorphin is one of the activators of the GPR139, a G protein-coupled receptor, recently deorphanized as a dynorphin receptor. In a wide neuropeptide library screen it exhibited one of the highest efficacies among prodynorphin-derived peptides.At low concentrations, dynorphins predominantly activate canonical opioid receptors, while at higher concentrations, they additionally recruit GPR139. This receptor couples to Gq/11 G protein-mediated signaling, which is mechanistically opposite to the Gi/o inhibitory signaling of classical opioid receptors. This permits GPR139 to function as a molecular homeostatic brake: when dynorphin reaches high concentrations during intense stress or pain, simultaneous GPR139 activation counteracts excessive opioid receptor signaling through excitatory signaling pathways, preventing pathological over-inhibition of neuronal activity.
NMDA receptor
are modulated by big dynorphin, producing effects that are antagonist-insensitive to opioid receptor blockade. Intracerebroventricular injection of big dynorphin produced memory enhancement in passive avoidance tests, enhanced locomotor activity in the open field test, and anxiolytic-like effects that were blocked by the NMDA receptor antagonist MK-801 but resistant to nor-BNI, a selective KOR antagonist.In contrast, dynorphin A and dynorphin B at similar doses produced analgesia but did not produce the NMDA-dependent memory enhancement, anxiolysis, or locomotor stimulation.
Suggested mechanisms are interaction with the polyamine binding site or the NR2B subunit. The molecular basis for this atypical activity may involve the extended C-terminal region unique to big dynorphin, which contains multiple basic residues.
ASIC1a channel
Big dynorphin is a potent endogenous modulator of the acid-sensing ion channel 1a and current rescue following steady-state desensitization. In comparison, dynorphin A exhibits a approximately 1000-fold lower potency than big dynorphin at this target.Peptide potentiates ASIC1a current through a mechanism independent of both opioid and bradykinin receptor signaling. The potency correlates with big dynorphin's features, particularly its high net positive charge and abundance of arginine residues, with residues Arg6, Arg7. Big dynorphin rescues proton-gated currents and promotes acidosis-induced neuronal cell death in cultured cortical neurons, implicating it in pathological conditions involving cellular acidification and excitotoxicity.