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 abl e to grow heterotrophically on at least six different pentoses. These pentoses are reduced in the cell to pentiols by an NADP-dependent aldo se reductase. The pentiols are then introduced into the oxidative pent ose phosphate pathway via NAD-dependent polyol dehydrogenases and pent ulokinases. The aldose reductase was purified 130-fold to apparent hom ogeneity by column chromatography. The enzyme is a homodimer of about 80 kD, as estimated by size-exclusion chromatography and from the sedi mentation behavior. The Michaelis constant Values for D-xylose (27 mM) , D-ribose (29 mM), D-lyxose (30 mM), and D-arabinose (38 mM) were abo ut three to five times lower than for the L-forms of the sugars. The a ctivity of the enzyme with hexoses, deoxysugars, and sugar phosphates was only about 5 to 10% of the rate with pentoses. In the reverse reac tion the activity was low and only detectable with pentiols. No activi ty was measured with NAD(H) as the cosubstrate in either direction.