Effect of urea and TMAO on Km of human aldose reductase for d-xylose (n = 3). ∗, Significantly different from control (P < 0.05). Additional significant differences with wild-type are TMAO versus urea and TMAO versus urea + TMAO. With C298A mutant, all other differences are also significant. Mean control values of Km are wild-type, 6.9 mM, and C298A, 228 mM.