Screening, isolation and characterization of lectins extracted from mushrooms indegenous to Southern Africa

Abstract

Lectins are among a large number of proteins produced by mushrooms. Mushroom lectins with important biological functions have been isolated and studied. However, none of the studies were reported on lectins isolated from mushrooms indigenous to southern Africa. A galactose-specific lectin from one of the common mushroom species in southern Africa, Schizophyllum commune, was isolated and characterized. Initially, twenty mushroom samples were collected and their crude extracts screened for lectin activities. Assays involved in the screening procedures included heamagglutination, carbohydrate inhibition, enzyme linked glycoprotein (ELGA) and various stability assays. Four different mushroom samples exhibited positive lectin activities with varying stabilities towards thermal treatment and susceptibility to proteolytic degradation. Further screening assays resulted in ZHR1 being selected for identification and purification of the lectin. This was due to its ability to agglutinate rabbit erythrocytes. In addition to its activity being destroyed after 3 hours of treatment with trypsin-NIPAAM conjugate, the activity of this lectin was also completely destroyed after an hour incubation in boiling water. In contrast to other mushroom extracts assayed, heamagglutination activity of the crude extract of ZHR1 was not inhibited by glycoproteins only but also by the sugars such as galactose, lactose and mannose. ZHR1 was identified as S. commune. S. commune lectin (ScL) was purified using affinity chromatography on a fetuin-agarose column and further purified using gel-filtration chromatography on Biogel P-100 column. ScL was characterized as a glycosylated, galactose-specific dimeric lectin with a molecular weight of approximately 32 and 33 kDa. ScL is a thermolabile lectin which loses its activity as early as 5 minutes after being incubated at 60°C. Anti-ScL antibodies, to be employed in screening for the presence of ScL in the protein extracts, were developed in the rabbits and their interaction with ScL was detected using the double immunodiffusion assays whereas their specificity towards the lectin was detected using Western blot. ScL is one of the first mushroom lectins to be isolated and studied in southern African region. If the lectin is found to be exhibiting important biological functions, ScL can be of commercial importance in the region.