Characterization and Purification of an Aldose Reductase from the Acidophilic and Thermophilic Red Alga Galdieria sulphuraria

The acidophilic and thermophilic red alga Galdieria sulphuraria is able to grow heterotrophically on at least six different pentoses. These pentoses are reduced in the cell to pentiols by an NADP-dependent aldose reductase. The pentiols are then introduced into the oxidative pentose phosphate pathway via NAD-dependent polyol dehydrogenases and pentulokinases. The aldose reductase was purified 130-fold to apparent homogeneity by column chromatography. The enzyme is a homodimer of about 80 kD, as estimated by size-exclusion chromatography and from the sedimentation behavior. The Michaelis constant values for D-xylose (27 mM), D-ribose (29 mM), D-lyxose (30 mM), and D-arabinose (38 mM) were about three to five times lower than for the L-forms of the sugars. The activity of the enzyme with hexoses, deoxysugars, and sugar phosphates was only about 5 to 10% of the rate with pentoses. In the reverse reaction the activity was low and only detectable with pentiols. No activity was measured with NAD(H) as the cosubstrate in either direction.