Feingold (403) summarized the NDP-sugars identified in plants. Those that were not described above are ADP-L-Ara, GDP-L-Ara; ADP-ribose; GDP-Xyl, ADP-Gal, GDP-d — Gal; ADP-D-Man; UDP-Fructose, ADP-D-Fructose; UDP-D-digitoxose (2,6-dideoxy-D — ribohexose); TDP-GalA; UDP-2-deoxy-2-acetamido-D-Glc, UDP-2-deoxy-2-acetamido-D — Gal; UDP-cellobiose. In addition to these NDP-sugars, pectin consists of other sugar residues: aceric acid and DHA (deoxylyxoheptulopyranosilaric acid). The formation of these NDP — sugars is not well studied.

It is not clear if modifications of sugar residues on pectin (such as methylation or acety­lation) occur after transferring the sugar from the respective NDP-sugar. In chloroplasts for example, sulfolipid biosynthesis requires the activated sugar UDP-Glc-6-sulfonate, UDP 5′- diphospho-sulfoquinovose. In this case, the sugar-linked to NDP is modified with a sulfate group prior to the transfer of the sulfoquinovose. Whether such NDP-sugar modifications occur with NDP-sugars required for wall synthesis is unknown. While more is known about synthesis of activated sugars less is known about “catabolism” of NDP-sugars. Recently, work on ascorbic acid metabolism in plants revealed two mutants, vtc2 and vtc4, involved in the degradation of GDP-L-Gal. First, a GDP-L-Gal phosphorylase (vtc2, At4g26850) con­verts GDP-L-Gal and Pi into L-Gal-l-P and GDP (502) and subsequently a dephosphorylase

activity on L-Gal-1-P yields L-Gal (458) (vtc4, At3g02870). Interestingly, the phosphorylase converts GDP-d-G1c to d-G1c-1-P and GDP as well.