Occurrence of thiol proteinases in the eggs of the silkworm, Bombyx mori

In the crude extract of matured eggs of the silkworm (Bombyx mori), proteolytic activity was detected only in the acidic pH region with maximal activity at pH 3.5, and this activity was maintained throughout the development. No appreciable activity was observed in the neutral to alkaline pH region either in matured eggs or in eggs at early embryonic stages. Two molecular forms of proteinases active at pH 3.5 were obtained from matured eggs, and their molecular weights were estimated to be over 16,000 and about 68,000 respectively. An additional form of 40,000 molecular weight appeared in eggs just before hatching. They were strongly inhibited by thiol proteinase inhibitors, such as p-chloromercuribenzoate (pCMB), p-chloromercuriphenyl sulfonate (pCMPS), and N-[N-(L-3-trans-carboxyoxirane-2-carbonyl)-L-leucyl]agmatine (E-64), whereas pepstatin, diisopropylphosphorofluoridate, and EDTA were without effect, suggesting that they are thiol enzymes. They hydrolyzed casein, bovine serum albumin, egg albumin, and gamma-globulin. They could not hydrolyze alpha-N-benzoyl-DL-arginine p-nitroanilide, alpha-N-benzoyl-DL-arginine beta-naphthylamide, and several other synthetic substrates. These thiol proteinases are different from animal tissue thiol proteinases hitherto known.