Cell-wall damage caused by mutations of cell-wall-related genes triggers a compensatory mechanism which eventually results in hyperaccumulation of chitin reaching 20% of the cell-wall dry mass. We show that activation of chitin synthesis is accompanied by a rise, from 1.3-fold to 3.5-fold according to the gene mutation, in the expression of most of the genes encoding enzymes of the chitin metabolic pathways. Evidence that GFA1, which encodes glutamine-fructose-6-Phosphate amidotransferase (Gfa1p), the first committed enzyme of this pathway, plays a major role in this process was as follows. Activation of chitin synthesis in the cell-wall mutants correlated with activation of GFA1 and with a proportional increase in Gfa1p activity. Overexpression of GFA1 caused an approximately threefold increase in chitin in the transformed cells, whereas chitin content was barely affected by the joint overexpression of CHS3 and CHS7. Introduction of a gfa1-97 allele mutation in the cellwall-defective gas1D mutant or cultivation of this mutant in a hyperosmotic medium resulted in reduction in chitin synthesis that was proportional to the decrease in Gfa1p activity. Finally, the stimulation of chitin production was also accompanied by an increase in pools of fructose 6-Phosphate, a substrate of Gfa1p. In quantitative terms, we estimated the flux-coefficient control of Gfa1p to be in the range of 0.90, and found that regulation of the chitin metabolic pathway was mainly hierarchical, i.e. dominated by regulation of the amount of newly synthesized GFA1 protein. In the search for the mechanism by which GFA1 is activated in response to cell-wall perturbations, we could only show that neither MCM1 nor RLM1, which encode two transcriptional factors of the MADS box family that are required for expression of cell-cycle and cell-wall-related genes, was involved in this process.

Alpha-amylase inhibitor (alpha-AI) from kidney bean (Phaseolus vulgaris L. cv Tendergreen) seeds has been purified to homogeneity by heat treatment in acidic medium, ammonium sulphate fractionation, chromatofocusing and gel filtration. Two isoforms, alpha-AI1 and alpha-AI1', of 43 kDa have been isolated which differ from each other by their isoelectric points and neutral sugar contents. The major isoform alpha-AI1 inhibited human and porcine pancreatic alpha-amylases (PPA) but was devoid of activity on alpha-amylases of bacterial or fungal origins. As shown on the Lineweaver-Burk plots, the nature of the inhibition is explained by a mixed non-competitive inhibition mechanism. Alpha-AI1 formed a 1:2 stoichiometric complex with PPA which showed an optimum pH of 4.5 at 30 degrees C. Owing to the low optimum pH found for alpha-AI activity, inhibitor-containing diets such as beans or transgenic plants expressing alpha-AI should be devoid of any harmful effect on human health.

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