Maize leaf and kernel starch synthases and starch branching enzymes

Abstract

Soluble starch synthases and branching enzymes were partially purified from developing leaves and kernels of maize using DEAE-cellulose chromatography. One form of starch synthase and two forms of branching enzyme were detected in leaves as compared to two forms of starch synthase and three forms of branching enzyme isolated from the kernels. The starch synthase fraction from the leaves and the first starch synthase fraction from the kernels showed greater activity in reactions containing various glycogens as primers than in those containing amylopectin. In addition, both were capable of synthesizing a polyglucan in the absence of an added primer but in the presence of sodium citrate and bovine serum albumin (citrate-stimulated starch synthesis). The second starch synthase fraction from kernels showed greater activity with amylopectin as primer and had no citrate-stimulated activity. We suggest that the leaf enzyme and endosperm starch synthase I are the same enzyme and that it is ‘constitutively’ expressed. Branching enzymes from leaves and kernels differed not only in their elution profiles but also their stimulation of phosphorylase a (assay A) and amylose branching (assay B) activities. A minor branching enzyme fraction from leaves (leaf branching enzyme I) eluted from the DEAE-cellulose column after the addition of a salt gradient, whereas branching enzyme I from kernels eluted in the buffer wash prior to the application of the gradient. However, the ratios of assay A to assay B suggested that branching enzyme I from leaves was catalytically similar to branching enzyme I from the kernels. The major leaf branching enzyme (branching enzyme II) eluted at the same position from the DEAE-cellulose column as endosperm branching enzyme IIa. These enzymes had similar ratios of activity (Assay A/Assay B). The cross reaction of leaf branching enzymes with antisera prepared against maize endosperm branching enzymes in immunodiffusion experiments and enzyme activity neutralization experiments further demonstrated the relationship of the leaf and endosperm branching enzymes.