Host-Pathogen Interactions: XXIII. The Mechanism of the Antibacterial Action of Glycinol, a Pterocarpan Phytoalexin Synthesized by Soybeans

Abstract

The biochemical basis for the ability of the pterocarpan phytoalexin glycinol (3,6a,9-trihydroxypterocarpan) to inhibit the growth of bacteria was examined. Glycinol at bacteriostatic concentrations (e.g. 50 micrograms per milliliter) inhibits the ability of Erwinia carotovora to incorporate [(3)H]leucine, [(3)H]thymidine, or [(3)H]uridine into biopolymers. Exposure of Escherichia coli membrane vesicles to glycinol at 20 micrograms per milliliter results in inhibition of respiration-linked transport of [(14)C]lactose and [(14)C]glycine into the vesicles when either d-lactate or succinate is supplied as the energy source. The ability of E. coli membrane vesicles to transport [(14)C]alpha-methyl glucoside, a vectorial phosphorylation-mediated process, is also inhibited by glycinol at 20 micrograms per milliliter. Furthermore, exposure of membrane vesicles to glycinol (50 micrograms per milliliter) at 20 degrees C results in the leakage of accumulated [(14)C]alpha-methyl glucoside-6-phosphate. The effects of the phytoalexins glyceollin, capsidiol, and coumestrol, and daidzein, a compound structurally related to glycinol but without antibiotic activity, upon the E. coli membrane vesicle respiration-linked transport of [(14)C]glycine and of [(14)C]alpha-methyl glucoside was also examined. Glyceollin and coumestrol (50 micrograms per milliliter), but not daidzein, inhibit both membrane-associated transport processes. These data imply that the antimicrobial activity of glycinol, glyceollin, and coumestrol are due to a general interaction with the bacterial membrane. Capsidiol (50 micrograms per milliliter) inhibits d-lactate-dependent transport of [(14)C]glycine but not vectorial phosphorylation-mediated transport of [(14)C]alpha-methyl glucoside. Thus, capsidiol's mechanism of antimicrobial action seems to differ from that of the other phytoalexins examined.