Starch-biosynthesis enzymes producing the less stickiness of boiled rice of indica variety

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Boiled rice of indica variety is less sticky compared with that of japonica variety, being suggested to be due to a higher amount of very long chains of indica amylopectin. Therefore, we examined branching enzyme from rice endosperm of indica and japonica varieties because the enzyme, out of enzymes involving starch biosynthesis, may have an important role to produce short and very long chains of amylopectin. Among six varieties examined here of indica and japonica rices, Saikai 184 variety had a typical type of indica amylopectin and Saikai 194 had that of japonica amylopectin having a small amount of very long chains. Immature endosperms (about 20 days after pollination) of Saikai 184 and Saikai 194 had at least two isoforms of branching enzyme with a various amount when examined by activity staining after gel electrophoresis. A major isoform was purified to be essentially free of amylase by chromatography on butyl-Toyopearl 650S,DEAE-Sepharose FF,Mono Q,Superose 12 and amylose-Sepharose 6B.Both preparations showed a similar ratio of the activities determined by the phosporylase stimulation assay and the branch linkage assay, and were found to transfer preferentially long chains when they were incubated with reduced amylose as a substrate. This type of chain transfer resembled that for maize BE-I isoform, but differed from that for maize BE-II isoform and for kidney bean enzyme which preferentially transferred short chains. However, the rice isoform and maize BE-I differed in producing a relatively high amount of chains with d. p. 10-12 and of a chain with d. p. 6, respectively. This result might imply that such an intrinsic chain transfer action determines chain-length distribution of amylopection in plant origin. The less stickiness of indica boiled rice might be clarified by further experiments on catalytic functions of starch synthase and /or minor isoform of branching enzyme.