Preliminary kinetic characterization of xylose reductase and xylitol dehydrogenase extracted from Candida guilliermondii FTI 20037 cultivated in sugarcane bagasse hydrolysate for xylitol production

Candida guilliermondii FTI 20037 was cultured in sugarcane bagasse hydrolys ate supplemented with 2.0g/L of (NH4)(2)SO4, 0.1 g/L of CaCl2 . 2H(2)O, and 20.0 g/L of rice bran at 35 degreesC; pH 4.0; agitation of 300 rpm; and ae ration of 0.4, 0.6, or 0.8 vvm. The high xylitol production (20.0 g/L) and xylose reductase (XR) activity (658.8 U/mg of protein) occurred at an aerat ion of 0.4 vvm. Under this condition, the xylitol dehydrogenase (XD) activi ty was low. The apparent K-M for XR and XD against substrates and cofactors were as follows: for XII, 6.4 x 10(-2)M (xylose) and 4.5 x 10(-3) mM (NADP H); for XD, 1.6 x 10(-1)M (xylitol) and 9.9 x 10(-2) mM (NAD+). Because XR requires about 10-fold less xylose and cofactor than XD for the condition i n which the reaction rate is half of the V-max, some interference on the ov erall xylitol production by the yeast could be expected.