Disruption of the transsulfuration pathway as a sulfur-driven etiology of insulin resistance: proinsulin misfolding, disulfide bond deformation, and PDI dysregulation

Type 2 diabetes mellitus (T2DM), expected to exceed 700 million cases by 2045, is usually attributed to obesity and peripheral resistance but neglects insulin’s structural integrity. This review introduces the Sulfur-Insulin Deformation Hypothesis, positing T2DM as a sulfur metabolism disorder where mitochondrial suffocation disrupts the transsulfuration pathway [methionine to cysteine via cystathionine β-synthase (CBS) and γ-lyase (CGL)], depleting cysteine and glutathione (GSH), impairing protein disulfide isomerase (PDI) activity, and deforming insulin’s disulfide bonds (A6–A11, A7–B7, A20–B19) as a primary trigger of insulin resistance. A literature synthesis was conducted (1995–2025) across PubMed, Scopus, Web of Science, and Google Scholar, using Medical Subject Headings (MeSH) terms like “sulfur metabolism”, “insulin misfolding”, and “mitochondrial dysfunction”. From 1,202 articles, 113 studies were selected, including in vitro insulin folding models, animal metabolic stress data, human sulfur biomarker analyses, and trials of sulfur donors (e.g., N-acetylcysteine). Mitochondrial dysfunction reduces adenosine triphosphate (ATP), depleting cysteine and GSH by 30–73.8% (red blood cell GSH: 1.78 ± 0.28 µmol/g vs. 6.75 ± 0.47 µmol/g Hb, P < 0.001), elevating reactive oxygen species (ROS). This impairs PDI isoforms (PDIA1, PDIA3, PDIA4), disrupting insulin bonds; the A6–A11 bond loses 50–70% affinity [r = –0.65, P < 0.05 for homeostatic model assessment of insulin resistance (HOMA-IR)], hindering phosphoinositide 3-kinase-protein kinase B (PI3K-Akt) signaling and glucose transporter type 4 (GLUT4) translocation. In 225 T2DM patients, PDIA4 elevation correlated with glucose (r = 0.62, P < 0.01) and reduced sensitivity (r = –0.67, P < 0.01). PDIA4 inhibition [presenilin 1 (PS1), IC50 = 4 μM] cuts ROS by 50% (P < 0.01), lowers hemoglobin A1c (HbA1c) by 1.2% (P < 0.05), and boosts β-cell survival by 30% (P < 0.05). Redox-mediated chain splitting degrades 20% of insulin (0.40 nmol/kg/min) at –137 mV, modulated by GSH. The hypothesis redefines T2DM as a sulfur-driven structural disorder, unveiling the gut-mitochondria-sulfur-insulin axis and advocating sulfur-centric therapies (e.g., N-acetylcysteine, methylsulfonylmethane).