Novel characteristics of UDP-glucose dehydrogenase activities in maize: non-involvement of alcohol dehydrogenases in cell wall polysaccharide biosynthesis

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Abstract
UDP-glucose dehydrogenase (UDPGDH) activity was detected in extracts of maize cell-cultures and developing leaves. The reaction product was confirmed as UDP-glucuronate. Leaf extracts from null mutants defective in one or both of the ethanol dehydrogenase genes, ADH1 and ADH2, had similar UDPGDH activities to wild-type, showing that UDPGDH activity is not primarily due to ADH proteins. The mutants showed no defect in their wall matrix pentose:galactose ratios, or matrix:cellulose ratio, showing that ADHs were not required for normal wall biosynthesis. The majority of maize leaf UDPGDH activity had K m (for UDP-glucose) 0.5–1.0 mM; there was also a minor activity with an unusually high K m of >50 mM. In extracts of cultured cells, kinetic data indicated at least three UDPGDHs, with K m values (for UDP-glucose) of roughly 0.027, 2.8 and >50 mM (designated enzymes EL, EM and EH respectively). EM was the single major contributor to extractable UDPGDH activity when assayed at 0.6–9.0 mM UDP-Glc. Most studies, in other plant species, had reported only EL-like isoforms. Ethanol (100 mM) partially inhibited UDPGDH activity assayed at low, but not high, UDP-glucose concentrations, supporting the conclusion that at least EH activity is not due to ADH. At 30 μM UDP-glucose, 20–150 μM UDP-xylose inhibited UDPGDH activity, whereas 5–15 μM UDP-xylose promoted it. In conclusion, several very different UDPGDH isoenzymes contribute to UDP-glucuronate and hence wall matrix biosynthesis in maize, but ADHs are not responsible for these activities.