PHOSPHORYLATED ALPHA(1-]4)GLUCANS AS SUBSTRATE FOR POTATO STARCH-BRANCHING ENZYME-I

The possible involvement of potato (Solanum tuberosum L.) starch-branc hing enzyme I (PSBE-I) in the in vivo synthesis of phosphorylated amyl opectin was investigated in in vitro experiments with isolated PSBE-I using P-33-labeled phosphorylated and H-3 end-labeled nonphosphorylate d alpha(1 --> 4)glucans as the substrates. From these radiolabeled sub strates PSBE-I was shown to catalyze the formation of dual-labeled (H- 3/P-33) phosphorylated branched polysaccharides with an average degree of polymerization of 80 to 85. The relatively high molecular mass ind icated that the product was the result of multiple chain-transfer reac tions. The presence of alpha(1 --> 6) branch points was documented by isoamylase treatment and anion-exchange chromatography. Although the i nitial steps of the in vivo mechanism responsible for phosphorylation of potato starch remains elusive, the present study demonstrates that the enzyme machinery available in potato has the ability to incorporat e phosphorylated alpha(1 --> 4)glucans into neutral polysaccharides in an interchain catalytic reaction. Potato mini tubers synthesized phos phorylated starch from exogenously supplied (PO43-)-P-33 and [U-C-14]G lc at rates 4 times higher than those previously obtained using tubers from fully grown potato plants. This system was more reproducible com pared with soil-grown tubers and was therefore used for preparation of 33P-labeled phosphorylated alpha(1 --> 4)glucan chains.