Inhibition and Ultraviolet-Induced Chemical Modification of UDP-Glucose:(1,3)-β-Glucan (Callose) Synthase by Chlorpromazine

Abstract

UDP-glucose:(1,3)-ft-glucan (callose) synthase (CS) from storage tissue of red beet (Beta vulgaris L.) was strongly inhibited by the phenothiazine drug chlorpromazine (CPZ). In the absence of ultraviolet irradiation, CPZ was a noncompetitive inhibitor with 50% inhibitory concentration values for plasma membrane and solubilized CS of 100 and 90 um, respectively. Both the Ca2"-and Mg2"-stimulated components of CS activity were affected. CPZ inhibition was partially alleviated at saturating levels of Ca2", but not Mg2", suggesting that CPZ interferes with the Ca2"-binding site of CS. Binding experiments with ["4C]CPZ, however, showed strong nonspecific partitioning of CPZ into the plasma membrane, providing evidence that perturbation of the membrane environment is probably the predominant mode of inhibition. Ultraviolet irradiation at 254 nm markedly enhanced CPZ inhibition, with complete activity loss following exposure to 4 Mm CPZ for 2 min. Inhibition followed a pseudo-first order mechanism with at least three CPZ binding