Biologically active A-chain of the plant toxin ricin expressed from a synthetic gene in Escherichia coli.

To assess the biological activity and pharmacokinetic properties of nonglycosylated ricin A-chain (RA), we have obtained the polypeptide following expression of a synthetic 842-bp RA gene in Escherichia coli. Expression of the gene was carried out using the phage T5 PN25 promoter fused to the E. coli lac operator. The RA polypeptide was synthesized in a completely soluble form and was purified in one step by immunoabsorption. It was shown to be as cytotoxic for a human cell line as both native RA and chemically deglycosylated native RA. Reconstituted whole ricin and an immunotoxin containing the recombinant RA were also biologically active. Immunotoxins made with recombinant and deglycosylated RA had similar clearance rates in vivo showing, after a short period of rapid elimination, stabilities far higher than that of an immunotoxin made with native RA. Our results show that the complete elimination of sugar side chains from the RA is not sufficient to entirely eradicate the rapid initial in vivo clearance of RA-based biologicals.