INSIGHT INTO NAPHTHOQUINONE METABOLISM - BETA-GLUCOSIDASE-CATALYZED HYDROLYSIS OF HYDROJUGLONE BETA-D-GLUCOPYRANOSIDE

In plants, the naphthoquinone juglone is known to be involved in patho genic defence mechanisms, but it may also take part in plant developme ntal processes. This naphthoquinone can accumulate in a glycosylated f orm, namely hydrojuglone beta-D-glucopyranoside. The structural config uration of this compound was shown to be ,5-dihydroxy-4-naphthalenyl-b eta-D-glucopyranoside by means of MS, NMR and nuclear Overhauser effec t spectroscopy analyses. A hydrojuglone beta-D-glucopyranoside beta-gl ucosidase (EC 3.2.1.21) was purified to homogeneity from Juglans regia L. The enzyme catalysed the release of juglone from hydrojuglone beta -D-glucopyranoside with high specificity and showed Michaelis-Menten k inetics with K-m = 0.62 mM and V-max = 14.5 mu kat/mg of protein. This enzyme also showed a higher activity towards beta-D-fucosyl than beta -D-glucosyl bonds. The purified enzyme had an apparent M-r of 64000 by SDS/PAGE and a pI 8.9 by isoelectrofocusing PAGE. The purified enzyme was inhibited by several bivalent cations, such as Cu2+, Fe2+, Hg2+, and by D-glucono-1,5-lactone, showing non-competitive inhibition of th e mixed type.