Enzymatic synthesis and purification of uridine diphosphate [14C]galacturonic acid: a substrate for pectin biosynthesis.

Pectins are complex polysaccharides that contain 1,4-linked alpha-D-galactosyluronic acid residues found in the primary wall of all higher plant cells. The pectic polysaccharides play critical roles in cell wall structure and in plant growth and development. As a first step in studying pectin biosynthesis a method was developed to routinely generate and purify UDP-[U-14C]galacturonic acid (UDP-[14C]GalA), the nucleotide sugar substrate for homogalacturonan biosynthesis. UDP-[14C]GalA was enzymatically synthesized by 4-epimerization of commercially available UDP-[U-14C]glucuronic acid (UDP-[14C]GlcA) using a particulate preparation from radish roots. The resulting mixture of UDP-[14C]GalA and UDP-[14C]GlcA was separated by high-performance anion-exchange chromatography using a Dionex CarboPac PA1 anion-exchange column. The UDP-sugars were detected by their absorbance at 262 nm or by pulsed amperometric detection following postcolumn addition of NaOH. The yield of UDP-[14C]GalA obtained using this procedure was 16% of the starting UDP-[14C]GlcA. Establishment of a reliable method to synthesize and purify UDP-[14C]GalA will facilitate the identification and purification of the galacturonosyltransferase(s) involved in pectin biosynthesis.