Proteolytic Targets and Clinical Benefits of Oral Nattokinase: Spike Protein and Amyloid Degradation, Fibrinolysis, and Cardiovascular Risk Reduction

Abstract Nattokinase (NK) is an orally administered subtilisin-family serine protease with reported activity across multiple interconnected proteolytic and cardiovascular targets, including degradation of SARS-CoV-2 spike protein, fibrinaloid amyloid microclots, fibrin clots, and classical amyloid substrates; enhancement of endogenous fibrinolysis; reductions in coagulation factors and blood pressure; and regression of carotid atherosclerotic plaque. SARS-CoV-2 spike protein can persist in human tissues and circulation for months to years after infection and mRNA vaccination, including in individuals with chronic post-vaccination illness, and possesses amyloidogenic properties capable of converting fibrinogen into fibrinolysis-resistant, thioflavin T-positive amyloid microclots. Direct human pharmacokinetic evidence demonstrates systemic circulation after oral dosing, with peak serum levels approximately 13 hours after administration. Three independent laboratories have reported proteolytic degradation of spike by natto-derived or related alkaline serine proteases: whole natto extract degraded recombinant receptor-binding domain and abolished viral infectivity in cultured cells, purified NK degraded full-length spike and the S2 subunit in a dose- and time-dependent manner across concentrations from 32 µg/mL to 7.8 ng/mL and degraded cell-surface spike, and an unrelated alkaline serine protease independently reproduced spike degradation, supporting a class-level proteolytic property. NK also targets amyloidogenic protein structures. Recombinant NK degrades fibrinaloid microclots in vitro in a dose-related manner. Classical cross-β amyloid substrates are likewise susceptible to NK, including Aβ40, Aβ42, human prion peptide fibrils, insulin fibrils, and β2-microglobulin fibrils, with degradation demonstrated across unrelated precursor proteins and, for Aβ, in Alzheimer disease model mice. These observations are consistent with a broader structural mechanism in which subtilisin-family and related proteases attack β-sheet-rich and exposed protein architectures rather than recognizing a single sequence-specific substrate. Fibrinolysis remains the best-established biological activity of NK: the enzyme directly cleaves fibrin independently of plasminogen, cleaves cross-linked fibrin with roughly six-fold the catalytic efficiency of plasmin in vitro and dissolves thrombus approximately four times as effectively at equivalent molar dose in vivo, inactivates plasminogen activator inhibitor-1, promotes tissue plasminogen activator release, and inhibits platelet aggregation and thrombus formation. Oral administration produces measurable systemic pharmacodynamic effects in humans, including increased D-dimer and fibrin/fibrinogen degradation products, reduced factor VIII activity, increased antithrombin, prolonged activated partial thromboplastin time, and reductions in fibrinogen, factor VII, factor VIII, and von Willebrand factor. Randomized human evidence also supports modest blood-pressure reduction, with a meta-analysis of six trials involving 546 participants reporting mean reductions of 3.45 mmHg systolic and 2.32 mmHg diastolic, potentially mediated in part by angiotensin-converting enzyme-inhibitory peptides generated from NK. Carotid imaging studies show a dose-ordered pattern: 2,000 FU daily was ineffective in a long-term placebo-controlled trial, 3,600 FU daily was ineffective in a lower-dose comparison group, whereas 6,000 FU daily reduced carotid plaque area by 36.6% over 26 weeks and 10,800 FU daily produced an approximately 36% reduction over 12 months, with common carotid intima-media thickness declining from 1.33 to 1.04 mm in the latter cohort. The similar plaque reductions observed at 6,000 and 10,800 FU suggest a possible therapeutic threshold near 6,000 FU rather than a continuously increasing dose-response. Prospective epidemiological data further associate higher natto intake with lower cardiovascular mortality, including hazard ratios of 0.75 for cardiovascular mortality and 0.68 for stroke mortality in the highest intake quartile. Across these findings, NK emerges as a multi-target proteolytic agent with established systemic fibrinolytic and cardiovascular activity and convergent preclinical evidence for degradation of spike protein and structurally diverse amyloid substrates. The principal unresolved translational question is whether orally administered NK reaches circulating or tissue spike, fibrinaloid microclots, and other amyloid targets at catalytically sufficient concentrations in humans and whether the degradation demonstrated experimentally produces measurable clinical benefit in vivo.