UDP-D-glucuronic acid

UDP-D-glucuronic acid by the uridine 5 -(a-D-glucopyranosyluronic acid pyrophosphate) cyclase of L. minor. 

In another experiment with the particulate enzyme from the same plant he obtained 4-methyl D-glucuronosyl-D-galactose together with 4-methyl D-glucuronosyl-D-xylose. Kauss (43) also found that the particulate enzyme preparation from immature corn cobs contains, in addition to the methyl transferase, an enzyme which introduces the D-glucu-ronic acid group from UDP-D-glucuronic acid into hemicellulose B. 

In animals, UDP-D-glucuronic acid is the precursor it loses UDP and the D-glucuronic acid/D-glucuronolactone is reduced at C-l, forming L-gulonic acid/L-gulono-1,4-lactone. The lactone is oxidized by microsomal L-gulono-1,4-lactone oxidase to ascorbate. This enzyme is not expressed in primates, as they have lost biosynthetic capacity for ascorbate. 

A similar situation occurs in the formation of UDP-D-apiose (57) from UDP-D-glucuronic acid (55). This interesting rearrangement requires NAD, and is accompanied by a decarboxylation. The NAD might be expected to oxidize C-4, to facilitate the decarboxylation by formation of a /3-keto acid, and give UDP-L-t/ireo-pentos-4-ulose (56). A 1,3-shift between C-2 and C-4 would give an intermediate that could be reduced by the NADH first formed, to give UDP-D-apiose (57). This mechanism is supported by the carbon-atom movement... 

Scheme 11.—Biosynthesis of UDP-D-apiose from UDP-D-glucuronic Acid.

Interestingly, the sugar moiety of the postulated intermediate 54 is identical to that of the intermediate in the biosynthesis of UDP-d-apiose from UDP-D-glucuronic acid (27, see Scheme 11). 

These hemicelluloses are known to be biosynthesized by a corresponding synthetase, xylan synthetase, which is responsible for the formation of xylan from UDP-o-xylose, which is produced by the decarboxylation of UDP-d-glucuronic acid [20]. 

Roberts, R.M., and Cetorelli, J.J., 1973, UDP-D-glucuronic acid pyrophosphorylase and the formation of UDP-D-glucuronic acid in plants. In Loewus, F. (ed.), Biogenesis of Plant Cell Wall Polysaccharides. Academic Press, New York, pp. 49-68. 

An exploration of the sugar nucleotides in this alga disclosed the presence of UDP-D-galactose, GDP-L-galactose, UDP-D-glucose, GDP-D-mannose, and UDP-D-glucuronic acid. Moreover, AMP, UMP, GMP, IMP, ADP, UDP, IDP, NAD , NADP , and a new nucleotide, adenosine 3, 5 -pyrophosphate, were isolated. 

Lippel and Olson i reported that, after the intraportal injection of retinol-6,7- C2 to rats, retinyl jS-D-glucosiduronic acid appeared in the bile. Both l-0-retinoyl-/3-D-glucuronic acid and retinyl )8-D-glucosiduronic acid are also synthesized in vitro when washed, rat-liver microsomes are incubated with UDP-D-glucuronic acid and either retinoic acid or retinol, respectively. The synthesis of 1-O-ret-inoyl-)8-D-glucuronic acid was also demonstrated in microsomes of the kidney and in particulate fractions of the intestinal mucosa. 

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