Inhibitory Effects of Mulberry Leaf Lectins to Silkworm Proteases

Abstract

Proteases from digestive fluid of Nang-lai strains of the Thai silkworm, Bombyx mori were partially purified by 40% ammonium sulfate fractionation and p-aminobenzidine agarose affinity chromatography. Six proteases named P1-P6 were obtained and P4, P5 and P6 were purified approximately 21,18 and 17 folds, respectively. The P4, P5 and P6 proteases had an equal apparent subunit molecular weight of 24 kDa and native molecular weight of 66 kDa. They exhibited the alkaline optimum pH at pH 9.0, 11.0 and 11.5, respectively. Their proteolytic activities were inhibited by specific trypsin inhibitors including leupeptin, tyrosyl-L-lysine chloromethylketone and soybean trypsin inhibitor suggesting that they were trypsin-like proteases. Inhibitory effect of two lectins from mulberry leaf, MLL1 and MLL2, to the proteases were studied. It was found that MLL1, in comparison to MLL2 and ConA, had greater inhibitory effects towards the protease P4 and a lesser inhibitory effect towards the protease P5 and P6. The inhibition of the proteases by both mulberry leaf lectins was not recovered by the specific lectinbinding sugars, N-glycolylneuraminic acid and N-acetylgalactosamine.