alpha-D-glucuronidases from the xylanolytic thermophiles Clostridium stercorarium and Thermoanaerobacterium saccharolyticum.

alpha-D-Glucuronidases were purified from the xylanolytic thermophiles Clostridium stercorarium and Thermoanaerobacterium saccharolyticum. This enzyme activity was found to be intracellular in each organism, with T. saccharolyticum producing much greater total activity. The specific activities of the purified enzymes (10 U mg-1 T. saccharolyticum; 1.7 U mg-1 C. stercorarium) differed by a factor of approximately 5. For the determination of enzyme activities, 4-O-methyl-alpha-D-glucuronosyl-xylotriose was used as a substrate and the glucuronic acid released by alpha-D-glucuronidase action was quantified by a colorimetric procedure. 4-O-Methyl-alpha-D-glucuronosyl-xylotriose was the hydrolysis product that accumulated after exhaustive degradation of 4-O-methyl-alpha-D-glucuronoxylan with xylanases of C. stercorarium. 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 p-nitrophenyl-alpha-D-glucuronoside. Both alpha-D-glucuronidases have a dimeric structure, with monomeric molecular masses of 72 and 76 kDa for C. stercorarium and of 71 kDa for T. saccharolyticum. The pI 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 degrees C, half-lives of 14 and 2.5 h, respectively, were determined for the alpha-D-glucuronidases of C. stercorarium and T. saccharolyticum. This description of alpha-D-glucuronidase activity in thermophilic anaerobic bacteria extends our knowledge of these enzymes, previously purified and characterized only in fungi.