Substrate specificity of maize β-glucosidase

Abstract

Maize ( Zea mays L.) β-glucosidase (β-glu) is a homodimer of 60-kDa monomers and localized in the plastid. Numerous glycosides were tested as substrates. When possible, K m and V max values were determined. The major natural substrate in methanol extracts of young maize seedlings was the glucoside of 2,4-dihydroxy-7-methoxy-2 H-1,4-benzoxazin-3(4 H)-one (DIMBOA-glc) and this substance was hydrolyzed to glucose and DIMBOA by the enzyme. 4-Methylumbelliferyl-β- d-glucoside was the best substrate ( K m = 0.14 mM) for which kinetic data were obtained. Several aryl glycosides ( K m = 0.39−6.32 mM) and n- octyl-β- d- glycopyranosides were also substrates. Various compounds were also tested as inhibitors. When inhibition was observed, K i and/or K i′ values were estimated. Monosaccharides were poor inhibitors. The best competitive inhibitors were d-gluconic acid lactone, dhurrin, and (DIMBOA) ( K i values < 1 mM). Other glucosides, aglycones, and aglycone analogues were found to inhibit the enzyme competitively. Tryptamine was a mixed inhibitor ( K i = 33.9 mM, K i′ = 15.7 mM). The enzyme has a broad substrate specificity and can cleave many glycosides with hydrophobic aglycones; the best inhibitors and substrates are those that resemble the natural substrate DIMBOA-glucoside.