Interaction of Ricin and RCA-I lectin with Human Serum Glycoproteins and Lung Cells

The plant protein ricin is one of the most toxic substances known and has been used in crimes internationally. Ricin binds to cell surface galactose as on glycoproteins and becomes endocytosed to kill cells. Blood contains high levels of galactosylated proteins which could bind to ricin and the related protein RCA-I from the same plant Ricinus communis. While blood proteins would be among targets for injected ricin, inhaled ricin would also bind to lung epithelial cells. Ricin and RCA-I are reported to have very similar or identical binding specificity. Thus, it is useful to increase the knowledge of proteins in blood and A549 cells that bind to ricin and RCA-I. Ricin and RCA-I staining of blots of electrophoresis gels of serum showed binding to serum proteins of 200-kDa and four between 75-kDa and 25-kDa for RCA-I and proteins between 150-kDa and 37-kDa for ricin which bound much less strongly. Both ricin and RCA-I bound to purified human immunoglobulin G heavy chain and transferrin but not to 2 HS glycoprotein which contains high amounts of sialic acid. Sambucus nigra agglutinin which binds sialic acid bound to 2 HS glycoprotein and transferrin but not to immunoglobulin G. Ricin bound to human lung A549 proteins of 150 kDa, 100 kDa and several proteins in human cell free salivary fluid between greater than 200 kDa to 25 kDa and binding to these proteins was diminished in the presence of serum. In fluorescence microscopy, RCA-I bound more strongly than ricin to A549 cells and this binding was diminished in the presence of human serum. RCA-I also bound much more strongly than ricin to human serum proteins coated on microtiter plate wells.