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Tomato ( Lycopersicon esculentum Mill) fruit tissue was found to contain two isoforms of a- d -mannosidase (EC. 3.2.1.24), which were purified by ion-exchange chromatography on DEAE sephadex A-50 followed by gel filtration on sephadex G-100. The two isoforms were named isoform I and II (minor and major forms, respectively) which differed in their abundance, their elution profile on both ion-exchange and gel filtration column chromatography. Isoforms I and II showed 6 and 24% of the total activity, respectively. Both the isoforms had an acid pH optimum (4.5) and were thermally stable at 65° for up to 15 min. Isoform I showed a broad temp optimum between 55 and 65° whereas isoform II had an optimal activity at 65°. The K m values of isoforms I and II for p -nitrophenyl-a- d -mannopyranoside were 1.11 and 1.05 mM, respectively. The activity of isoform I was inhibited by Hg 2 (1 mM) whereas isoform II was inhibited by Cu 2 (0.1 mM) and Hg 2 (1 mM). Both the isoforms were free of other glycosidases. Purified isoform II had a SDS M r of ca. 38,000. |
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