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Characterization of spinach leaf α-l-arabinofuranosidases and β-galactosidases and their synergistic action on an endogenous arabinogalactan-protein

Authors


Y. Hirano, Y. Tsumuraya and Y. Hashimoto (corresponding author)

Abstract

Two β-galaclosidases (β-Galase-I and -II, EC 3.2.1.23) and two α-l-arabinofuranosidases (α-l-Arafase-I and -II. EC 3.2.1.55). were purified from mesophyll tissues of spinach (Spinacia oleracea L.), using chromatography on DEAE-cellulose, lactose-conjugated Sepharose CL-4B, and Sephadex G-100, or on hydroxylapatite and Sephadex G-150. The apparent molecular mass (Mr) of β-Galase-I and -II, respectively, were estimated to be 38 000 and 58 000 on SDS-PAGE and 64 000 and 60 000 on gel-permeation chromatography, indicating that the former was a dimeric protein. The isoelectric points of β-Galase-I and -II were 6.9 and 5.2, respectively. Both enzymes hydrolyzed maximally p-nitrophenyl (PNP) β-galactoside at pH 4.3, and were activated about 2-fold in the presence of BSA (100 μg ml−1). The activity of both enzymes was inhibited strongly by heavy metal ions and p-chloromercuriberszoate (p-CMB). d-Galactono-(1→4)-lactone and d-galactal served as potent competitive inhibitors for the enzymes. β-Galase-I and -II could be distinguished from each other in their relative rates and kinetic properties in the hydrolysis of aryl β-galactosides as well as of lactose and galacto-oligosaccharides. In particular. β-Galase-I exhibited a preferential exowise cleavage of β-1,6-galactotriose and β-1.3-galactan. α-l-Arafase-l (Mr 118000) and -II (M, 68 000) were optimally active on PNP α-l-arabinofuranoside at pH 4.8 and gave Km values of 1.2 and 2.2 mM. respectively. l-Arabino-(1 → 4)-lactone. Ag+, and SDS acted as inhibitors for the isozymes. α-lArafase-I was characterized by its activity to hydrolyze PNP β-d-xylopyranoside besides PNP α-l-arabinofuranoside. inhibition by d-xylose and d-glucono-(1 → 5)-lactone. and less sensitivity to Hg2+. Cu2+, and p-CMB. Sugar beet arabinan was hydrolyzed rapidly by α-lArafase-II at one-half the rate for PNP α-larabinofuranoside, while the polysaccharide was less susceptible to α-lArafase-I. A spinach leaf arabinogalactan-protein was practically resistant to the action of β-Galases, but its susceptibility to the enzymes increased remarkably after prior hydrolysis with α-lArafase-Il.

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