Purification and characterization of cyanide hydratase from the phytopathogenic fungus Gloeocercospora sorghi

Abstract

Previous studies have demonstrated that fungal pathogens of cyanogenic plants produce cyanide hydratase (CHT, EC 4.2.1.66), which converts HCN to formamide. Production of CHT in these fungi is thought to be a means of circumventing cyanide toxicity, and CHT is thus believed to be an important pathogenicity trait. In the present study, 13 species of fungi were assayed for CHT production, and all 7 species that were pathogens of sorghum, a cyanogenic plant, produced this enzyme. CHT was purified to apparent homogeneity from one of these sorghum pathogens, Gloeocercospora sorghi. The enzyme had a K m of 12 m m for KCN. Enzymatically functional CHT was obtained only as a large molecular entity of greater than 300 kDa. However, a polypeptide of approximately 45 kDa was identified as the only component of purified CHT detectable by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 45-kDa polypeptide band could be resolved into three isozymes of p I 6.1, 6.3, and 6.5. Antibodies raised against the 45-kDa polypeptide inhibited the G. sorghi CHT activity and showed high specificity in Western blots to a polypeptide of approximately the same size. The evidence suggests that functional G. sorghi CHT is an aggregated protein that consists of 45-kDa polypeptides. A CHT with similar properties was also found in the fungus Colletotrichum graminicola, another pathogen of sorghum.