Bee venom contains a variety of peptide constituents, including low-molecular-weight protease inhibitors. While the putativelow-molecular-weight serine protease inhibitor Api m 6 containing a trypsin inhibitor-like cysteine-rich domain was identifiedfrom honeybee (Apis mellifera) venom, no anti-fibrinolytic or anti-microbial roles for this inhibitor have been elucidated.In this study, we identified an Asiatic honeybee (A. cerana) venom serine protease inhibitor (AcVSPI) that was shownto act as a microbial serine protease inhibitor and plasmin inhibitor. AcVSPI was found to consist of a trypsin inhibitor-likedomain that displays ten cysteine residues. Interestingly, the AcVSPI peptide sequence exhibited high similarity to the putativelow-molecular-weight serine protease inhibitor Api m 6, which suggests that AcVSPI is an allergen Api m 6-like peptide.Recombinant AcVSPI was expressed in baculovirus-infected insect cells, and it demonstrated inhibitory activity against trypsin,but not chymotrypsin. Additionally, AcVSPI has inhibitory effects against plasmin and microbial serine proteases; however,it does not have any detectable inhibitory effects on thrombin or elastase. Consistent with these inhibitory effects, AcVSPIinhibited the plasmin-mediated degradation of fibrin to fibrin degradation products. AcVSPI also bound to bacterial andfungal surfaces and exhibited anti-microbial activity against fungi as well as gram-positive and gram-negative bacteria. Thesefindings demonstrate the anti-fibrinolytic and anti-microbial roles of AcVSPI as a serine protease inhibitor.

Serine protease inhibitors play a critical role in physiological processes and immune responses by regulating serine protease activities. Here we describe the molecular cloning and antimicrobial activities of a serine protease inhibitor from the mason bee, Osmia cornifrons (OcSPI). OcSPI consists of 405 amino acid residues and contains a potential reactive center loop (RCL) region in its C-terminus. Recombinant OcSPI was produced as a 64-kDa glycoprotein in baculovirus-infected insect cells and exhibited inhibitory activity against chymotrypsin. Additionally, OcSPI demonstrated inhibitory activity against microbial serine proteases, such as subtilisin A and proteinase K, but not against tissue plasminogen activator, thrombin, or plasmin. Recombinant OcSPI bound directly to Escherichia coli, Bacillus subtilis, and Beauveria bassiana and exhibited antimicrobial activity against both bacteria and fungi. Our results demonstrated the antimicrobial functions of OcSPI and suggest a role for OcSPI in the immune response of O. cornifrons.

Bumblebee venom serine protease inhibitors have been shown to inhibit plasmin activity. In this study, a bumblebee (Bombus ignitus) venom serine protease inhibitor (BiVSPI) that acts as an antimicrobial factor was identified. BiVSPI is a 55-amino acid mature peptide with ten conserved cysteine residues and a P1 methionine residue. BiVSPI was expressed in the venom gland and was present as an 8-kDa peptide in venom. Recombinant BiVSPI expressed in baculovirusinfected insect cells exhibited inhibitory activity against chymotrypsin, but not trypsin. BiVSPI also exhibited inhibitory activity against microbial serine proteases, such as subtilisin A (Ki 6.57 nM) and proteinase K (Ki 7.11 nM), indicating that BiVSPI acts as a microbial serine protease inhibitor. In addition, BiVSPI was also shown to bind directly to Bacillus subtilis, B. thuringiensis, and Beauveria bassiana, but not to Escherichia coli. Consistent with these results, BiVSPI exhibited antimicrobial activity against Gram-positive bacteria and fungi. These findings provide novel evidence for the antimicrobial function of this bumblebee venom serine protease inhibitor.

Insect-derived Kazal-type serine protease inhibitors exhibit thrombin, elastase, plasmin, proteinase K, or subtilisin A inhibition activity, but so far, no functional roles for bee-derived Kazal-type serine protease inhibitors have been identified. In this study, a bee (Apis cerana) venom Kazal-type serine protease inhibitor (AcKTSPI) that acts as a microbial serine protease inhibitor was identified. AcKTSPI contained a single Kazal domain that displayed six conserved cysteine residues and a P1 threonine residue. AcKTSPI was expressed in the venom gland and was present as a 10-kDa peptide in bee venom. Recombinant AcKTSPI Kazal domain (AcKTSPI-Kd) expressed in baculovirus-infected insect cells demonstrated inhibitory activity against subtilisin A (Ki 67.03 nM) and proteinase K (Ki 91.53 nM), but not against α-chymotrypsin or typsin, which implies a role for AcKTSPI as a microbial serine protease inhibitor. However, AcKTSPI-Kd exhibited no detectable inhibitory effects on factor Xa, thrombin, tissue plasminogen activator, or elastase. Additionally, AcKTSPI-Kd bound directly to Bacillus subtilis, B. thuringiensis, Beauveria bassiana, and Fusarium graminearum but not to Escherichia coli. Consistent with these findings, AcKTSPI-Kd showed antibacterial activity against Gram-positive bacteria and antifungal activity against both plant-pathogenic and entomopathogenic fungi. These findings constitute molecular evidence that AcKTSPI acts as an inhibitor of microbial serine proteases. This paper provides a novel view of the antimicrobial functions of a bee venom Kazal-type serine protease inhibitor.

Bee venom is a rich source of pharmacologically active substances. In this study, we characterized a B. terrestris venom Kunitz-type serine protease inhibitor (Bt-KTI). Bt-KTI consists of two exons encoding 82-amino acids (aa), including a predicted 24-aa signal peptide and a 58-aa mature peptide. Recombinant Bt-KTI was expressed as a 6.5-kDa peptide in baculovirus-infected insect cells. Bt-KTI showed no detectable inhibitory effect on factor Xa, thrombin, or tissue plasminogen activator. In contrast, Bt-KTI strongly inhibited plasmin, indicating that it acts as a plasmin inhibitor. The electrophoretic mobility shift assay showed that Bt-KTI binds to plasmin, indicating the formation of a plasmin-Bt-KTI complex. These results demonstrate that Bt-KTI acts as an antifibrinolytic agent, suggesting a role for Bt-KTI as an anti-bleeding agent.

Bee venom contains serine proteases and serine protease inhibitors. In this study, we identified a bumblebee (Bombus ignitus) venom Kunitz-type serine protease inhibitor (Bi-KTI) that acts as a plasmin inhibitor. Bi-KTI showed no detectable inhibitory effect on factor Xa, thrombin, or tissue plasminogen activator. In contrast, Bi-KTI strongly inhibited plasmin, indicating that it acts as an antifibrinolytic agent. The fibrin(ogen)olytic activities of B. ignitus venom serine protease (Bi-VSP) and plasmin in the presence of Bi-KTI indicate that Bi-KTI targets plasmin more specifically than Bi-VSP. These findings demonstrate a novel mechanism by which bumblebee venom affects the hemostatic system through the antifibrinolytic activity of Bi-KTI and through Bi-VSP-mediated fibrin(ogen) olytic activities, raising interest in Bi-KTI and Bi-VSP as potential clinical agents.

Bee venom is a rich source of pharmacologically active substances. In this study, we identified a bumblebee (Bombus ignitus) venom Kunitz-type serine protease inhibitor (Bi-KTI) that acts as a plasmin inhibitor. Bi-KTI showed no detectable inhibitory effect on factor Xa, thrombin, or tPA. However, it strongly inhibited plasmin, although this inhibitory ability was two-fold weaker than that of aprotinin. The activities of B. ignitusvenom serine protease (Bi-VSP) and plasmin in the presence of Bi-KTI indicate that Bi-KTI targets plasmin more specifically than Bi-VSP. These findings demonstrate a novel mechanism for bee venom by which Bi-KTI acts as an antifibrinolytic agent, raising interest in Bi-KTI as a potential clinical agent.