Phoratoxin, a Toxic Protein from the Mistletoe Phoradendron tomentosum subsp. macrophyllum (Loranthaceae)

Abstract

In a simplified procedure for isolation of phoratoxin, the dried plant material (stems and leaves) was extracted with 2% aqueous acetic acid. The extract was treated with polyamide to remove colored impurities and subjected to gel filtration on Sephadex G‐25. The high‐molecular‐weight fraction was chromatographed on SE‐Sephadex, using a gradient elution technique, whereby pure phoratoxin was obtained.The material was homogeneous by standard criteria of protein purity. The molecular weight, determined in the ultracentrifuge by the sedimentation equilibrium method, is 5000. Phoratoxin has the following amino acid composition (residues/mole): alanine 2, arginine 3, aspartic acid 4, 1/2‐cystine 6, glycine 6, histidine 1, Isoleucine 3, leucine 1, lysine 4, phenylalanine 1, proline 2, serine 5, threonine 5, tryptophan 1, tyrosine 1, valine 1. The molecular weight, calculated from the amino acid composition, is 4884. The material contains no reducing sugars, amino sugars or sialic acids. The LD50, determined by intraperitoneal injection in mice, is 0.57 ± 0.05 mg/kg bodyweight.