Dual effects of mucus from the Japanese flounder (Paralichthys olivaceus) on blood coagulation: procoagulant and anticoagulant activities.

Fish skin mucus covers the body surface, serving as a physical and chemical first line of defense at the interface with the aquatic environment. Epidermal mucus provides a protective cushioning layer and contains numerous bioactive molecules involved in immune defense, antimicrobial activity, and tissue repair. During body-surface injury, direct contact between the plasma components and the mucus layer may influence plasma clotting. However, the role of epidermal mucus in this process remains poorly understood. This study investigated, under in vitro conditions, the effects of epidermal mucus from Japanese flounder (Paralichthys olivaceus) on plasma coagulation and examined coagulation-modulating components through biochemical fractionation. The addition of crude mucus to the flounder plasma shortened the fibrin clotting time and accelerated thrombin substrate cleavage under both prothrombin time-like and activated partial thromboplastin time-like conditions. Affinity fractionation of crude mucus using benzamidine-Sepharose showed that both the bound and unbound fractions retained procoagulant activity, suggesting that the observed activity was not solely attributable to serine protease-like components. Furthermore, size-based fractionation of the crude mucus using an ultrafiltration membranes and size-exclusion chromatography showed that procoagulant activity was distributed across multiple fractions in the approximate 29 to >2000 kDa range, whereas lower-molecular-weight fractions showed anticoagulant activity. These findings suggest that modulation of coagulation by mucus may contribute to the balance between local clot formation and the prevention of excessive coagulation at the epithelial surface.