Properties of the extracellular acidic proteases of Dichelobacter nodosus. Stability and specificity of peptide bond cleavage.

Dichelobacter nodosus, a Gram negative obligate anaerobe and causative organism of ovine footrot, secretes a family of extracellular acidic serine proteases with pI's in the range of 5.2 to 5.6, and a basic serine protease with a pI of approximately 9.5. The acidic proteases show optimum activity at pH 8 and require a divalent metal ion (eg. Ca) to maintain structural integrity. In the presence of EDTA or conditions that cause protein unfolding, the proteases undergo rapid and complete autolysis. The proteases were stable to heating to about 50 degrees C for 30 min but at higher temperatures, activity was rapidly lost; virulent proteases V1 and V2 were slightly more stable (by about 5 degrees C) than benign proteases B2 and B3. The effect of various protease inhibitors on the D. nodosus acidic proteases was the same except that the inhibitor, chymostatin, markedly inhibited protease V5 but not proteases V1-3 or B1-B4. Cleavage of the oxidized insulin B-chain showed that the specificity of proteases V1-V3 and B1-B4 was identical but that it was distinct from that of proteases V5/B5.