Some homogeneity studies on commercial preparations of allegedly crystalline phosphomannose isomerase

Commercial preparations of what is labeled as “Crystalline Phosphomannose Isomerase” were subjected to chromatographic, electrophoretic and ultracentrifugal analysis to determine the degree of their homogeneity. The results suggest that these preparations do not contain more than 5 to 10% of their total protein content as phosphomannose isomerase and that they must owe their crystallinity to other enzymatic or nonenzymatic protein(s). In both column chromatography and polyacrylamide gel electrophoresis 10 to 16 different protein components were discernible, only one of which had phosphomannose isomerase activity. Sedimentation velocity ultracentrifugation allowed the distinction of only two uniformly shaped schlieren peaks, reflecting the unsatisfactory nature of this technique as a tool for probing the purity of grossly inhomogeneous protein solutions.