SUBSTRATE-SPECIFICITY OF MAIZE BETA-GLUCOSIDASE

Maize (Zea mays L.) beta-glucosidase (beta-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-2H-1,4-benzoxazin-3(4H)-one (DIMBOA-glc) and this substance was hydrolyzed to glucose and DIMBOA by the enzyme. 4-Methylumbelliferyl-beta-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-beta-D-glucopyranosides 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.