Influence of Free Fatty Acids, Lysophosphatidylcholine, Platelet-Activating Factor, Acylcarnitine, and Echinocandin B on 1,3-β-d-Glucan Synthase and Callose Synthesis

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Abstract
The activity of 1,3-.beta.-D-glucan synthase assayed in the presence of digitonin in a microsomal preparation from suspension-cultured cells of Glycine max can be fully inhibited by unsaturated fatty acids, trienoic acids being most effective. Lysophosphatidylcholine, platelet-activatig factor, acylcarnitine, and Echinocandin B can also fully inhibit the enzyme. Inhibition is observed both when the enzyme is activated by Ca2+ or by trypsinization. At low amounts some of the substances can also cause stimulation. These effects all may result from a displacement of certain endogenous phospholipids necessary for optimal activity of the 1,3-.beta.-D-glucan synthase. In the absence of digitonin the enzyme activity is greatly stimulated by lysophosphatidylcholine, platelet-activating factor, acylcarnitine, and Echinocandin B within a certain concentration range, presumably by rendering the microsomal vesicles permeable to the substrate and Ca2+. Dibucaine does not cause such an effect. Acylcarnitine and Echinocandin B at low concentrations can induce callose synthesis in vivo; this effect is enhanced by chitosan. At higher concentrations the two substances and polyunsaturated fatty acids cause severe electrolyte leakage. The effects are discussed in regard to the induction of callose synthesis by enforced Ca2+ influx, and its modulation by membrane lipids.