Biosynthesis of Uridine Diphosphate D-Xylose. I. Uridine Diphosphate Glucuronate Carboxy-lyase of Wheat Germ^*

Abstract

Uridine disphosphate D-glucuronate carboxy-lyase from wheat germ has been purified 350-fold.The only products of enzyme action are uridine diphosphate D-xylose and C02. The enzyme has a pH optimum between 6.8 and 7.0. Km for uridine disphosphate D-glucuronate is about 3 X 10~4 m. P .L olymers of D-xylose abound in higher plants. It was long suspected that the metabolic pathway in plants leading from hexose to pentose involved C-6 decarboxylation. This was substantiated by studies of many workers using either intact plants (Altermatt and Neish, 1956;Neish, 1958;Loewus et al., 1958) or plant slices (Slater and Beevers, 1958). In these experiments the tissue was supplied with a suitably labeled hexose or uronic acid and the distribution of label in the pertinent metabolic products was determined. Involvement of sugar nucleotides in this conversion was suggested by the isolation of a mixture of uridine 5'-

Functional cloning and characterization of a UDP- glucuronic acid decarboxylase: The pathogenic fungus Cryptococcus neoformans elucidates UDP-xylose synthesis

“…Consistent with this mechanism, previous studies of UDP-GlcA decarboxylase activity from C. laurentii indicated an absolute requirement for exogenous NAD ϩ (23). However, studies of the enzyme from wheat germ showed neither activation by NAD ϩ nor inhibition by NADH (12). Further examination of preparations from wheat germ suggested that in fact NAD ϩ is tightly bound to the enzyme, resisting even the action of NADase (12).…”

“…However, studies of the enzyme from wheat germ showed neither activation by NAD ϩ nor inhibition by NADH (12). Further examination of preparations from wheat germ suggested that in fact NAD ϩ is tightly bound to the enzyme, resisting even the action of NADase (12). Lack of NADH dissociation also explains the observed retention of tritium label at C-4 throughout a reaction postulated to proceed via a 4-keto intermediate (12,23).…”

“…Further examination of preparations from wheat germ suggested that in fact NAD ϩ is tightly bound to the enzyme, resisting even the action of NADase (12). Lack of NADH dissociation also explains the observed retention of tritium label at C-4 throughout a reaction postulated to proceed via a 4-keto intermediate (12,23). The C. neoformans enzyme converted UDP-GlcA to UDP-Xyl in the absence of exogenous NAD ϩ , although activity was 2-fold higher in the presence of 1 mM NAD ϩ ( Table 2, compare standard reaction and reaction without NAD ϩ ).…”

“…subcellular localization. Plant and fungal enzymes are soluble (12,23) while the activity is associated with a particulate fraction in both cultured chondrocytes and hen oviduct (25,26). Consistent with these data, the expressed cryptococcal enzyme is found primarily in a soluble fraction, and the sequence has no regions suggestive of membrane spanning domains.…”

“…Although early studies of this enzyme indicated that it generated a mixture of UDP-Dxylose and UDP-L-arabinose, it was later shown that this was caused by a contaminating UDP-L-arabinose-4-epimerase; the UDP-GlcA decarboxylase itself produces only UDP-Xyl (12). This activity was partially purified from soluble fractions from both wheat germ (12)(13)(14) and a nonpathogenic species of cryptococcus, C. laurentii (15).…”