Isolation and complete amino acid sequence of two fibrinolytic proteinases from the toxic Saturnid caterpillar Lonomia achelous

The major toxic and fibrinolytic activity of the saliva and hemolymph of the larval form of Lonomia achelous was purified to homogeneity by a combination of metal chelate and affinity chromatography. Two apparent isozymes, Achelase I (213 amino acids, pI_calc = 10.55) and Achelase II (214 amino acids, pI_calc = 8.51), were sequenced by automated Edman degradation, and their C-termini confirmed by Fourier-transform mass spectrometry. The calculated molecular weights (22 473 and 22 727) correspond well to M_r estimates of 24 000 by SDS-PAGE. No carbohydrate was detected during sequencing. The enzymes degraded all three chains of fibrin, α > β ≫ γ, yielding a fragmentation pattern indistinguishable from that produced by trypsin. Chromogenic peptides S-2222 (Factor Xa and trypsin), S-2251 (plasmin), S-2302 (kallikrein) and S-2444 (urokinase) were substrates while S-2288 (broad range of serine proteinases including thrombin) was not hydrolyzed. Among a range of inhibitors Hg⁺², aminophenylmercuriacetate, leupeptin, antipain and E-64 but not N-ethylmaleimide or iodoacetate abolished the activity of the purified isozymes against S-2444. Phenylmethylsulfonyl fluoride, soybean trypsin inhibitor and aprotinin were less effective. The presence of the classic catalytic triad (histidine-41, aspartate-86 and serine-189) suggests that Achelases I and II may be serine proteinases, but with a potentially free cysteine-185 which could react with thiol proteinase-directed reagents.