In 1952, Dutton and Storey discovered that UDP-GlcA acts as a glucuronosyl donor in the synthesis of glucuronides by liver enzymes. In plants, UDP-GlcA is a key intermediate serving as a branch point between UDP-hexose (six carbons) and UDP-pentose (five carbons) sugars. UDP-GlcA is the precursor for UDP-D-xylose, UDP-L-arabinose, UDP-apiose, and UDP-galacturonic acid that contribute to synthesis of over 40% of cell wall polysaccharides. UDP-GlcA is made by (i) sequential phosphorylation of D-GlcA at C-1 by a membrane bound kinase (411) followed by a pyrophosphorylase activity that converts a-D-GlcA-1-P and UTP to UDP-GlcA, (ii) NAD-dependent oxidation of UDP-Glc to UDP-GlcA by UDP — Glc dehydrogenase (UGD, UDPGDH), and by a controversial pathway (iii) oxidation of UDP-Glc to UDP-GlcA by a bifunctional alcohol dehydrogenase ADH/UDPGDH.

In the 1960s the pathway from myo-inositol to cell wall glycans was proposed as a signif­icant metabolic pathway. Early experiments with 3H-inositol demonstrated that the label

was readily incorporated into cell wall polysaccharides (452). More recently, a labeling ex­periment with inositol in Arabidopsis showed that radioactivity is found only in the uronic acids, arabinose, and xylose that were released from wall glycans (453). The ability of inositol to drive the synthesis of GlcA was named “the myo-inositol oxidation pathway" We will discuss the myo-inositol and salvage pathways separately.