Purification and Characterization of Two Major Toxic Proteins from Seeds of Ah-us precatorius*

A purification procedure is described for two major toxic proteins from seeds of Abrus precatorius by successive chromatography on diethylaminoethyl-Sephadex A-50, carboxymethyl-cellulose, and DEAE-cellulose. The two toxins obtained both appeared homogeneous as judged by electrophoresis and analytical ultracentrifugation. Abrin A, the more positively charged protein obtained from the DEAE-cellulose chromatography, can be crystallized as needles from ammonium sulfate solution by the free interface diffusion technique; under various similar conditions, the other fraction, abrin C, failed to yield crystals. X-ray precession photographs have indicated that abrin A crystallizes in an orthorhombic unit cell of symmetry P212121 and dimensions a = 74.9, b = 269.8, and c = 70.3 A. The amino acid composition of abrins A and C are different. Molecular weights determined by sedimentation equilibrium are 60,100 for A as compared to 63,800 for C. After A has been treated with 2-mercaptoethanol at 100“ for 2 min, polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate gives rise to one strong (corresponding to apparent subunit molecular weight of 32,000) and two much weaker bands. This is in sharp contrast with the pattern for C, two bands of approximately equal intensity, corresponding to apparent subunit molecular weights of 33,000 and 28,000. The two proteins also behave differently in affinity chromatography on a Sepharose 4B column. Abrin C at 1 pg per ml concentration level agglutinates sheep erythrocytes, whereas abrin A shows no such activity even at 40 pg per ml concentration level. Abrin C exhibits a more toxic effect on mice than abrin A. These differences in physicochemical and biological properties for the two abrins are presented in detail.