Organ specificity of isoforms of starch branching enzyme (Q-enzyme) in rice [Oryza sativa]

The activity and isoenzymes of starch branching enzyme or Q-enzyme in the developing endosperm were compared with those in the leaf blade, leaf sheath, culm and root of rice plants. Q-enzyme from each of these organs could be resolved into two fractions, QE I and QE II, by column chromatography on DEAE cellulose. However, the ratio of the activity of QE I to that of QE II varied considerably among the organs. The Q-enzyme from the endosperm was specific for that organ in that the enzyme activity, on the basis of either fresh weight or soluble protein content, was about 100- to 1,000-fold higher than those from the other organs. Moreover, in the endosperm, the activity of QE I was markably higher than that of QE II as compared with the relative levels in other organs. Native polyacrylamide gel electrophoresis followed by activity staining revealed that the QE II fraction was composed of multiple isoforms. The endosperm contained two isoforms, QE IIa and QE IIb. After electrophoresis on a native polyacrylamide gel, QE IIa was detected only in the extract of endosperm, whereas QE IIb was present in extract of all organs examined. The antiserum raised against QE IIa from the endosperm cross-reacted to a considerable extent with QE IIb from the same organ. However, the antiserum failed to recognize any isoforms of QE II from the other organs.