Marine by-products represent a promising source of bioactive peptides. This study aimed to isolate and characterize a low-molecular-weight peptide fraction with antioxidant activity from Argentine shortfin squid carcass by-products, and to evaluate in vitro its cytocompatibility and protective effects against corticosterone (CORT)-induced oxidative injury in rat adrenal pheochromocytoma (PC12) cells and human astrocyte (hACs) cells. Argentine squid antioxidant peptide (PASN) was obtained by size-exclusion chromatography and fractionation-based screening. PASN exhibited the strongest overall free-radical-scavenging activity and consisted predominantly of components below 1 kDa (211.73–1013.48 Da). Spectroscopic analyses indicated that enzymatic hydrolysis transformed its structure from a rigid triple-helix conformation to a more flexible conformation dominated by β-turns (50.78%) and random coils (17.38%). In addition, thermogravimetric analysis confirmed its excellent thermal stability, with an onset decomposition temperature as high as 244.81 °C, supporting its potential applicability in high-temperature food-processing matrices. In vitro assays demonstrated that PASN exhibited high biocompatibility and promoted proliferation of both PC12 cells and hACs, while significantly improving cell viability under CORT challenge. PASN also reduced lactate dehydrogenase (LDH) leakage (hACs: 38.31%; PC12: 31.17%) in both cell models and restored total superoxide dismutase (T-SOD) activity (hACs: 69.46%, PC12: 66.40%). Immunofluorescence further revealed that PASN rescued the expression of brain-derived neurotrophic factor (BDNF) (hACs: 35.23%, PC12: 12.50%) and glutamate decarboxylase (GAD1/2) (hACs: 102.66%, PC12: 31.31%), key markers associated with synaptic plasticity and GABAergic sleep regulation. Collectively, PASN is a thermally stable squid-derived peptide fraction that exerts antioxidant and cytoprotective effects in neural cell models in vitro and represents a promising sustainable candidate for nutraceutical development.

The compact muscle architecture and poor aqueous dispersibility of Sthenoteuthis oualaniensis protein limit its use as a food ingredient. This study developed a gel filtration chromatography (GFC)-guided papain–alcalase sequential hydrolysis strategy using target-window peak area ($A_{\mathrm{target}}$) as a peptide-distribution response. Box–Behnken optimization identified an enzyme dosage of 1110 U/g, a papain/alcalase mass ratio of 3:5, and 8 h hydrolysis, yielding an $A_{\mathrm{target}}$ of 0.0648 ± 0.0012 a.u. O-phthaldialdehyde (OPA)-derived degree of hydrolysis (DH) values were 10.72 ± 0.08%, 35.32 ± 0.12%, and 25.03 ± 0.37% for SPH-Pap, SPH-Alc, and SPH-opt, respectively, showing that the highest $A_{\mathrm{target}}$ did not coincide with the highest DH. The essential-to-total amino acid ratio remained 38.18–39.20%, while lysine and methionine changed modestly. SPH-opt maintained high solubility across a broad pH range and exhibited a peptide profile distinct from those of the single-enzyme hydrolysates. Radical-scavenging capacity was assessed only post-optimization. At 5 mg/mL, SPH-opt showed DPPH and ABTS scavenging rates of 23.75% and 22.84%, respectively; without external standards, these values support only relative within-study comparisons. Overall, $A_{\mathrm{target}}$ and DH provided complementary information on peptide distribution and bond cleavage, and no causal relationship between $A_{\mathrm{target}}$ and radical-scavenging capacity was established.