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SBTX, a new toxic protein distinct from soyatoxin and other toxic soybean [Glycine max] proteins, and its inhibitory effect on Cercospora sojina growth.

Vasconcelos IM, Morais JK, Siebra EA, Carlini CR, Sousa DO, Beltramini LM, Melo VM, Oliveira JT

Laboratório de Toxinas Vegetais, Departamento de Bioquímica e Biologia Molecular, Universidade Federal do Ceará, CEP 60451-970, Fortaleza, CE, Brazil.

SBTX, a novel toxin from soybean, was purified by ammonium sulfate fractionation followed by chromatographic steps DEAE-Cellulose, CM-Sepharose and Superdex 200 HR fast-protein liquid chromatography (FPLC). Lethality of SBTX to mice (LD(50) 5.6mg/kg) was used as parameter in the purification steps. SBTX is a 44-kDa basic glycoprotein composed of two polypeptide chains (27 and 17kDa) linked by a disulfide bond. The N-terminal sequences of the 44 and 27kDa chains were identical (ADPTFGFTPLGLSEKANLQIMKAYD), differing from that of 17kDa (PNPKVFFDMTIGGQSAGRIVMEEYA). SBTX contains high levels of Glx, Ala, Asx, Gly and Lys and showed maximum absorption at 280nm, epsilon(1cm)(1%) of 6.3, and fluorescence emission in the 290-450nm range upon excitation at 280nm. The secondary structure content was 35% alpha-helix, 13% beta-strand and beta-sheet, 27% beta-turn, 25% unordered, and 1% aromatic residues. Immunological assays showed that SBTX was related to other toxic proteins, such as soyatoxin and canatoxin, and cross-reacted weekly with soybean trypsin inhibitor and agglutinin, but it was devoid of protease-inhibitory and hemagglutinating activities. The inhibitory effect of SBTX on growth of Cercospora sojina, fungus causing frogeye leaf spot in soybeans, was observed at 50mug/ml, concentration 112 times lesser than that found to be lethal to mice. This effect on phytopathogenic fungus is a potential attribute for the development of transgenic plants with enhanced resistance to pathogens.

Published 28 April 2008 in Toxicon, 51(6): 952-63.
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