Bdellins A/B
Dual-family proteinase inhibitors with exceptional neurotrophic potency
Last updated: March 14, 2026
Mechanism Disclaimer
Bdellins comprise two structurally distinct groups of proteinase inhibitors isolated from Hirudo medicinalis salivary gland secretion. Despite sharing protease targets (trypsin, plasmin, acrosin), Group A and Group B belong to entirely different structural families — a remarkable example of convergent functional evolution within a single organism.
Classification and Structure
| Property | Group A (Bdellastatin) | Group B (Bdellin B3) | HMW Bdellin B |
|---|---|---|---|
| Molecular weight | 6,300 Da (59 aa) | 5,000 Da | 20,000 Da |
| Structural family | Antistasin family | Non-classical Kazal-type | Kazal-type (extended) |
| Disulfide bonds | 5 S-S bonds | 3 S-S bonds | Multiple |
| Homology | 29% to antistasin | 55% to LDTI | Extended C-terminal domain |
| Reactive site | P1 Lys34 | Canonical Kazal loop | Kazal loop + membrane binding |
Bdellin B3 spans only 37 amino acids between its first and last cysteine residues, making it among the shortest Kazal-type inhibitors known. The high-molecular-weight bdellin B (20 kDa) features an extended C-terminal domain that enables membrane binding — unique among leech Kazal-type inhibitors.
Inhibitory Specificity
Trypsin
Ki = 0.1 nM. Sub-nanomolar inhibition of trypsin-like serine proteases. Biological function: modulates trypsin activity in the leech intestinal canal, where Aeromonas symbionts secrete trypsin-like enzymes for blood digestion.
Plasmin
Ki = 0.1 nM. Potent plasmin inhibition prevents premature fibrinolysis of the blood meal within the leech crop. In the host context, plasmin inhibition modulates tissue remodeling and inflammatory proteolysis.
Acrosin
Acrosin is a trypsin-like serine protease from sperm acrosome. Bdellin inhibition of acrosin reflects the broad-spectrum anti-trypsin activity rather than a specific reproductive biological function of the leech.
Neurotrophic Activity
Highest Neurotrophic Potency of Any Individual SGS Component
Bdellin B
60% EAI increase at 0.05 ng/mL — the single most potent neurite growth stimulator among all tested SGS components in chick embryo dorsal root ganglia (Chapter 7).
Bdellastatin
48% EAI increase at 0.01 ng/mL — achieving comparable neurotrophic activity at a 5-fold lower concentration, indicating even higher specific activity.
EAI = Explant Area Index, a standardized measure of neurite outgrowth from dorsal root ganglia explants.
Recombinant Production and Applications
Recombinant Bdellastatin
Moser et al. (1998) achieved heterologous expression of bdellastatin in Saccharomyces cerevisiae (yeast). The yeast expression system was necessary to ensure proper formation of the five disulfide bonds critical for antistasin family fold — bacterial expression systems failed to produce active protein.
Anti-Inflammatory Potential
By inhibiting trypsin-like proteases at sub-nanomolar concentrations, bdellins contribute to the suppression of tissue degradation during inflammatory responses. Their dual anti-inflammatory and neurotrophic activities suggest multifunctional therapeutic potential in neuroinflammatory conditions.