We purified recombinant glucose-6-phosphate isomerase from Pyrococcus furiosus using heat treatment and Hi-Trap anion-exchange chromatography with a final specific activity of 0.39 U mg(-1). The activity of the glucose-6-phosphate isomerase for L: -talose isomerization was optimal at pH 7.0, 95 degrees C, and 1.5 mM Co(2+). The half-lives of the enzyme at 65 degrees C, 75 degrees C, 85 degrees C, and 95 degrees C were 170, 41, 19, and 7.9 h, respectively. Glucose-6-phosphate isomerase catalyzed the interconversion between two different aldoses and ketose for all pentoses and hexoses via two isomerization reactions. This enzyme has a unique activity order as follows: aldose substrates with hydroxyl groups oriented in the same direction at C2, C3, and C4 > C2 and C4 > C2 and C3 > C3 and C4. L: -Talose and D: -ribulose exhibited the most preferred substrates among the aldoses and ketoses, respectively. L: -Talose was converted to L: -tagatose and L: -galactose by glucose-6-phosphate isomerase with 80% and 5% conversion yields after about 420 min, respectively, whereas D: -ribulose was converted to D: -ribose and D: -arabinose with 53% and 8% conversion yields after about 240 min, respectively.