Isolation of the Fourth Enzyme (Isomerase) of Histidine Biosynthesis from Salmonella typhimurium

A method is presented for the purification of the isomerase catalyzing the fourth step of histidine biosynthesis in Salmonella typhimurium. The enzyme was isolated from extracts of histidine auxotrophs which (when derepressed) produced 25 to 30 times the wild type levels of enzyme. The purification procedure involved fractionation with protamine sulfate and ammonium sulfate, gel filtration on Sephadex G-100, ion exchange chromatography on diethylaminoethyl Sephadex, and continuous flow high voltage electrophoresis. The final product is homogeneous with respect to size, with a weight average molecular weight of 29,000, but is composed of three enzymically active forms (in the ratio 90:9:1) which may be distinguished by their differing electrophoretic mobilities in alkaline polyacrylamide gels.