Inhibition of Mung Bean UDP-Glucose: (1→3)-β-Glucan Synthase by UDP-Pyridoxal

UDP-pyridoxal competitively inhibits the Ca(2+)-, cellobiose-activated (1-->3)-beta-glucan synthase activity of unfractionated mung bean (Vigna radiata) membranes, with a K(i) of 3.8 +/- 0.7 micromolar, when added simultaneously with the substrate UDP-glucose in brief (3 minute) assays. Preincubation of membranes with UDP-pyridoxal and no UDP-glucose, however, causes progressive reduction of the V(max) of subsequently assayed enzyme and, after equilibrium is reached, 50% inhibition occurs with 0.84 +/- 0.05 micromolar UDP-pyridoxal. This progressive inhibition is reversible provided that the UDP-pyridoxylated membranes are not treated with borohydride, indicating formation of a Schiff's base between the inhibitor and an enzyme amino group. Consistent with this, UDP-pyridoxine is not an inhibitor. The reaction of (1-->3)-beta-glucan synthase with UDP-pyridoxal is stimulated strongly by Ca(2+) and, less effectively, by cellobiose or sucrose, and the enzyme is protected against UDP-pyridoxal by UDP-glucose or by other competitive inhibitors, implying that modification is occurring at the active site. Pyridoxal phosphate is a less potent and less specific inhibitor. Latent (1-->3)-beta-glucan synthase activity inside membrane vesicles can be unmasked and rendered sensitive to UDP-pyridoxal by the addition of digitonin. Treatment of membrane proteins with UDP-[(3)H]pyridoxal and borohydride labels a number of polypeptides but labeling of none of these specifically requires Ca(2+) and sucrose; however, a polypeptide of molecular weight 42,000 is labeled by UDP-[(3)H]pyridoxal in the presence of Mg(2+) and copurifies with (1-->3)-beta-glucan synthase activity.