α-D-Glucuronidases were purified from the xylanolytic thermophiles and . This enzyme activity was found to be intracellular in each organism, with producing much greater total activity. The specific activities of the purified enzymes (10 U mg ; 1.7 U mg ) differed by a factor of approximately 5. For the determination of enzyme activities, 4--methyl-α-D-glucuronosyl-xylotriose was used as a substrate and the glucuronic acid released by α-D-glucuronidase action was quantified by a colorimetric procedure. 4--Methyl-α-D-glucuronosyl-xylotriose was the hydrolysis product that accumulated after exhaustive degradation of 4--methyl-α-D-glucuronoxylan with xylanases of . Hydrolysis of side chains in high-molecular-mass glucuronoxylan could not be detected. Neither of the enzymes was able to hydrolyse the chromogenic aryl-substrate -nitrophenyl-α-D-glucuronoside. Both α-D-glucuronidases have a dimeric structure, with monomeric molecular masses of 72 and 76 kDa for and of 71 kDa for . The pl was estimated to be 4.3 for each enzyme. While both enzymes exhibited a similar pH optimum (pH 5.5-6.5) they differed in their thermostabilities. At 60 °C, half-lives of 14 and 2.5 h, respectively, were determined for the α-D-glucuronidases of and . This description of α-D-glucuronidase activity in thermophilic anaerobic bacteria extends our knowledge of these enzymes, previously purified and characterized only in fungi.


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