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 NADPdependent aldose reductase. l h e 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 In plants structural polysaccharides consist of, in addition to hexoses, mainly pentoses. Hemicellulose and pectins contain Xyl and Ara, whereas Rib and deoxyribose are the building blocks of RNA and DNA. Free pentoses, in contrast, are very rare in plants, although the recycling of pentoses from various cellular components may proceed via free sugars. Organisms growing on pentoses as the sole carbon source have to introduce these sugars into the oxidative pentose phosphate pathway; little is known about the introduction of free pentoses into this pathway. The three reaction sequences possible are depicted in Figure 1. The phosphorylation by a pentose kinase (Neufeld et al., 1960; Chan and Hassid, 1975) followed by a pentose phosphate isomerase (Woodruff and Wolfenden, 1979) has been reported in higher plants. The isomerization of a pentose to a pentulose (Pubols and Axelrod, 1959;Pubols et al., 1963) with subsequent phosphorylation by a ribulo-or xylulokinase was found in corn pollen (Zahnley and Axelrod, 1965). The reduction of a pentose to a sugar alcohol, the oxidation of the sugar alcohol to a pentulose, followed by the phosphorylation to either xylulose 5-phosphate or ribulose 5-phosphate, is well established in yeasts and fungi * Corresponding au thor; e-mail galdi@zedat.fu-berlin.de; fax 49-30-838-4313. 231 (Chiang and Knight, 1960), but has not been demonstrated in plants.The acidophilic and thermophilic red alga Galdieria sulphuraria is able to grow in the dark on at least six different pentoses as a sole carbon source. We have investigated the path of these sugars into the oxidative pentose phosphate pathway in this alga. Here we report the characterization and purification of an NADP-dependent aldose reductase.
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