Recombinant proricin binds galactose but does not depurinate 28 S ribosomal RNA

Preproricin transcripts microinjected into Xenopus oocytes were expressed and the product was segregated by the oocyte endoplasmic reticulum and core glycosylated. Recombinant proricin was soluble, stabilised by intramolecular disulfide bonds and biologically active in that it could bind to immobilized lactose (selectin 2) or immobilized asialofetuin. Affinity-purified proricin did not catalyse the depurination of 28 S ribosomal RNA unless it was reduced, when slight but significant activity was observed. Gel filtration of the reduced proricin fraction showed that this depurination activity was not associated with proricin. The activity was apparently due to ricin A chain released by reduction from mature ricin which was, in turn, generated from proricin, presumably via endogenous oocyte endoprotease activity.