Abstract
Ribose-5-phosphate ketol-isomerase, an enzyme isomerizing ribose-5-phosphate to ribulose-5-phosphate, is isolated from Candida utilis which is grown in a medium containing xylose. The enzyme is also purified by means of fractionation with ammonium sulfate, acetone, and by DEAE-cellulose column chromatography. The enzyme has its optimum pH at 7.5 and optimum temperature at 50°C. Michaelis-Menten constant for d-ribose-5-phosphate is 7.38 × 10‒4 M and activation energy of the enzyme reaction is 10,525 calories. The enzyme activity is inhibited by p-CMB, EDTA and sodium pyrophosphate, and activated by the addition of magnesium ion. Extract of Candida utilis contains polyol: NAD oxidoreductase which catalyzes the conversion of polyols to the corresponding ketoses. By fractionation with ammonium sulfate and on DEAE-cellulose column chromatography, the purity of enzyme has been increased about 14-fold. The relatively high activity with both xylitol and sorbitol suggests that they may be the natural substances for the enzyme. Evidence suggests that this enzyme relates to the metabolism of d-xylose in Candida utilis.
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