Purification and characterization of rice sucrose synthase isozymes

From an extract of milk-ripe stage rice grains, four sucrose synthase activities could be separated by a FPLC Mono Q column. They are considered isozymes because they show different electrophoretic mobilities in a non-denaturing gel. However, the migration rates of their subunits in SDS-PAGE were indistinguishable and had a molecular mass of 94 kDa. The native forms had identical molecular mass of 440 kDa, thus they were considered to be tetrameric but carrying different ionic charges. Ouchterlony assay indicates that they have the same epitopes. All isozymes use UDP as the best nucleoside diphosphate substrate. When characterized by the ratio of catalytic rates of sucrose synthesizing and cleaving reactions, the isozyme that had the slowest migration rate in PAGE had the smallest value.