Radiolabeling of lectin from Robinia pseudoacacia by reductive methylation

Lectin from the bark of Robinia pseudoacacia was radiolabeled by reductive methylation. Reductive methylation of the lectin with [14C]formaldehyde and a reducing agent, sodium borohydride, resulted in incorporation of 14C into the lectin but with considerable inactivation of carbohydrate binding activity. Dialysis of the labeled lectin against the buffer containing Ca2+ and Mn2+ partially restored the activity. Higher incorporation of 14C into the lectin was observed when sodium cyanoborohydride was used as a reducing agent. In this case, more than eighty percent of the labeled lectin retained carbohydrate binding activity. Autoradiography showed that the radiolabeled lectin had a molecular mass of 29 kDa, corresponding to that of the intact lectin subunit.