AN ANALYSIS OF SOLUBLE STARCH SYNTHASE ISOZYMES FROM THE DEVELOPING GRAINS OF NORMAL AND SHX CV-BOMI BARLEY (HORDEUM-VULGARE)

Soluble starch synthase (SSS, EC 2.4.1.21) catalyzes formation of the alpha-1,4 bonds of amylopectin. It occurs in multiple isozymes which a re either type I, primer-independent in the presence of citrate, or ty pe II, always primer-dependent. To analyze the enzyme, a sensitive nat ive, gel assay was developed, monitoring ADP-[C-14]glucose incorporati on into insoluble alpha-glucan in the presence of either sodium citrat e or glycogen primer or both. Using this system, we observed multiple type I and type II forms in developing grains of barley (Hordeum vulga re L.) cv. Bomi, the relative activities of which vary with seed devel opment. At least one form comigrates in native gels with starch branch ing enzyme. Assays of the shx mutant, which is severely reduced in sta rch accumulation and in type I SSS activity, indicate that one type I isozyme becomes primer-dependent.