Abstract
Mung bean seeds possess a tetrameric galactose-binding protein that displays two types of activities: (a) a hemagglutinin activity, and (b) an alpha-galactosidase activity. This protein can be reversibly converted by pH changes from a tetrameric form, which possesses both enzymic and hemagglutinin activities, to a monomeric form which possesses enzymic activity only. This observation suggests that the enzymic phytohemagglutinin is an aggregated form of a monomeric alpha-galactosidase. The tetrameric alpha-galactosidase has a pH optimum of about pH 7.0, while the monomeric form displays a pH optimum of 5.6. Circular dichroism difference spectra and inhibition studies suggest that aggregation induces conformational changes in the subunits sufficient to alter their enzymatic properties. The possibility of in vivo changes in subunit equilibria, when combined with the accompanying alterations in activity, provides a new concept worthy of consideration with respect to the physiological role of phytohemagglutinins.
Highlights
The hemagglutinin assays were done at 0 “C for 30 min using trypsinked rabbitred blood cells as described previously (1)
Mung bean (Vigna radiata) seeds contain a tetrameric at -10 “C for about 2 years. In both cases the enzyme was galactose-binding protein (M, = 160,OOO) that displays both a maintained in 50 mM Tris-HC1, pH 7.0, 1mM galactose, and hemagglutinin activity and an a-galactosidase activity (1). 20% glycerol
After dialysis against buffer at pH 4.0 specific monosaccharides. This protein shows the aged preparation displayed a pH activity profile closely immunological cross-reaction with antibodies raised against resembling that of the fresh preparation
Summary
From theDepartment of Bwchemistv, University of California, Riuerside, California 92521. Mungbean seeds possess atetramericgalactosebinding protein that displays two types of activities: (a)a hemagglutinin activity, an(db)an a-galactosidase activity. This protein can be reversibly converted by pH changes from a tetrameric form, which possesses both enzymic and hemagglutinin activitietso, a monomeric form whicphossesses enzymic activityonly. Velocity sedimentation experiments were performed with a Beckman model E analytical ultracentrifuge equipped with an ultraviolet monochromator and scanning optics. Ultraviolet circular tn in i is an aggregated form of a monomeric a-galacto- dichroic spectra were obtained using a Cary Model 60 spectropolarisidase. The tetrameric a-galactosidase has a pH opti- meter with a Model 6002 circular dichroism attachment calibrated mum of about pH 7.0, while the monomeric form dis- with d-camphor-10-sulfonic acid. Combined with the accompanying alterations in activity, provides a new concept worthy of consideration
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