ATP and GTP Stimulate Membrane-bound but not Digitonin-solubilized -Glucan Synthases from Saprolegnia monoica

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Summary: Glucan synthases of Saprolegnia assayed in the presence of MgCl2 [(1→4)-β-glucan production] were stimulated by ATP or GTP, while the non-phosphorylating nucleotide analogues adenylyl (β,γ-methylene)diphosphonate and guanylyl (β,γ-methylene)diphosphonate did not stimulate glucan synthesis. The analogues adenosine 5′-O-(3-thiotriphosphate) and guanosine 5′-O-(3-thiotriphosphate) stimulated the enzyme activity but to a lesser extent than the nucleotides. The activators stimulated membrane-bound enzymes, particularly those equilibrating at the endoplasmic reticulum density in sucrose gradients. Glucan synthases solubilized by digitonin were not stimulated by nucleotides. Nucleotides, whose action seems to be related to the integrity of the phosphate groups, may produce their stimulative effect by increasing the translocation of the substrate through membranes to the enzymes.