Xylitol Accumulation Research Articles

Effect of aerobiosis on fermentation and key enzyme levels during growth of Pichia stipitis, Candida shehatae and Candida tenuis on d-xylose

The relationship between the degree of aerobiosis, xylitol production and the initial two key enzymes of d-xylose metabolism were investigated in the yeasts Pichia stipitis, Candida shehatae and C. tenuis. Anoxic conditions severely curtailed growth and retarded ethanol productivity. This, together with the inverse relationship between xylitol accumulation and aeration level, suggested a degree of redox imbalance. The ratios of NADH- to NADPH-linked xylose reductase were similar in all three yeasts and essentially independent of the degree of aerobiosis, and thus did not correlate with their differing capacities for ethanol production, xylitol accumulation or growth under the different conditions of aerobiosis. Under anoxic conditions the enzyme activity of Pichia stipitis decreased significantly, which possibly contributed to its weaker anoxic fermentation of xylose compared to C. shehatae.

Aldose reductase and polyol in cultured pericytes of human retinal capillaries

Aldose reductase has been found in several tissues involved in the various complications of longterm diabetes. Since the loss of intramural pericytes is typical of early microangiopathy of the diabetic retina, the presence of aldose reductase and its activity were investigated in pericytes cultured from capillaries of human retinas. Verification that contaminating cells had been removed was made on the basis of the pericytes' distinctive appearance in culture, inability to internalize acetylated-low-density lipoprotein, and immunoreactivity for muscle actin. Using pure cultures of human pericytes, the presence of aldose reductase was demonstrated immunohistochemically with antibodies directed against human placental aldose reductase, and aldose reductase activity was shown biochemically by monitoring the accumulating of xylitol in cells incubated with xylose. Xylitol accumulation was inhibited by the inclusion of an aldose reductase inhibitor in the xylose-containing medium.

Effects of the aldose reductase inhibitor sorbinil on the isolated cultured rat lens

The isolated cultured rat lens has been used to examine the effects of the aldose reductase inhibitor sorbinil on lenticular polyol accumulation and sugar cataract formation. Lenses incubated in medium containing 35 mmol/L glucose accumulated sorbitol over a seven-day period without the appearance of overt opacities. Sorbitol accumulation was inhibited in a dose response fashion by sorbinil with an IC 50 of 3.1 × 10 −6 mol/L. In lenses incubated in the presence of 29.5 mmol/L xylose, xylitol accumulation was accompanied by an increase in the water content of the lens and the development of a classical sugar cataract. All of these effects could be prevented by the addition of sorbinil to the culture medium. Complete inhibition of cataract formation required greater than an 80% inhibition of the xylitol accumulation. Reversal of a preformed xylose cataract by sorbinil could be achieved if the inhibitor was added at the stage of cortical opacities (20 h). Cataract progression proceeded normally over the next 48 hours and then the lens slowly began to clear. The rate of the reversal was dependent on the dose of sorbinil.

Effect of slow feeding of xylose on ethanol yield byPachysolen tannophilus

With slow feeding of xylose to a batch fermentation byPachysolen tannophilus, the yield of ethanol from xylose was improved to 0.41 g/g (80% of theoretical) with a maximum ethanol concentration of 26.5 g/L at 120 h. This is a 41% improvement on the ethanol yield observed for batch fermentations without slow feeding. The optimum level of xylose in the medium was determined to be between 5 and 8g/L; xylose at greater than 10 g/L leads to xylitol accumulation, whereas xylose below 3 g/L permits ethanol to be oxidized to acetate. This latter effect is exacerbated by increased aeration.

Xylitol dehydrogenase fromPachysolen tannophilus

NAD-xylitol-dehydrogenase (EC 1.1.1.9) from Pachysolen tannophilus was investigated in relation to xylitol byproduction during xylose fermentation by this yeast. For this purpose the enzyme was partially purified by a combination of affinity chromatography and fast liquid protein chromatography. The enzyme catalyzes an equilibrium reaction which at physiological pH values favours the accumulation of xylitol. The kinetics of the enzyme were shown to be Michaelis-Menten type with respect to both reaction directions. The activity of the enzyme was shown to be under the influence of the ‘catabolic reduction charge’ (NADH/NAD + NADH) and ATP. The apparent equilibrium constant of the enzyme may explain the considerable byproduction of xylitol during xylose fermentation by P. tannophilus .