This study identified the complement of soluble starch synthases (SSs) present in developing maize (Zea mays) endosperm. The product of the du1 gene, DU1, was shown to be one of the two major soluble SSs. The C-terminal 450 residues of DU1 comprise eight sequence blocks conserved in 28 known or predicted glucan synthases. This region of DU1 was expressed in Escherichia coli and shown to possess SS activity. DU1-specific antisera detected a soluble endosperm protein of more than 200 kD that was lacking in du1-mutants. These antisera eliminated 20% to 30% of the soluble SS activity from kernel extracts. Antiserum against the isozyme zSSI eliminated approximately 60% of the total soluble SS, and immunodepletion of du1-mutant extracts with this antiserum nearly eliminated SS activity. Two soluble SS activities were identified by electrophoretic fractionation, each of which correlated specifically with zSSI or DU1. Thus, DU1 and zSSI accounted for the great majority of soluble SS activity present in developing endosperm. The relative activity of the two isozymes did not change significantly during the starch biosynthetic period. DU1 and zSSI may be interdependent, because mutant extracts lacking DU1 exhibited a significant stimulation of the remaining SS activity.Plant starches are composed almost entirely of glucosyl units joined by either ␣(134) or ␣(136) glycoside bonds. In the amylopectin component of starch, these two types of linkage are located with a specific spatial distribution, and this defined branching pattern in turn determines higherorder structures (Gallant et al., 1997). The number of glucosyl units in each ␣(134)-linked chain and the locations of ␣(136) branch linkages that join the linear chains are expected to be determined by inherent properties of the starch biosynthetic enzymes. The specific mechanisms by which these enzymes determine architectural specificity, however, are not well understood.Three enzymatic activities are involved in starch synthesis from the glucosyl donor ADPGlc (for reviews, see Hannah et al

No abstract

Bioactive compound(s) extracted from cinnamon potentiate insulin activity, as measured by glucose oxidation in the rat epididymal fat cell assay. Wortmannin, a potent PI 3′-kinase inhibitor, decreases the biological response to insulin and bioactive compound(s) from cinnamon similarly, indicating that cinnamon is affecting an element(s) upstream of PI 3′-kinase. Enzyme studies done in vitro show that the bioactive compound(s) can stimulate autophosphorylation of a truncated form of the insulin receptor and can inhibit PTP-1, a rat homolog of a tyrosine phosphatase (PTP-1B) that inactivates the insulin receptor. No inhibition was found with alkaline phosphate or calcineurin suggesting that the active material is not a general phosphatase inhibitor. It is suggested, then, that a cinnamon compound(s), like insulin, affects protein phosphorylation-dephosphorylation reactions in the intact adipocyte. Bioactive cinnamon compounds may find further use in studies of insulin resistance in adult-onset diabetes.

No abstract

No abstract