UDP-apiose

Biosynthetic reactions forming apiose can be divided into decar-boxylative reactions resulting in the biosynthesis of UDP-apiose, and transglycosylation reactions resulting in the transfer of apiose from one compound to another. Formation of apiose in vitro has been extensively investigated in only two species of plants, namely, parsley (P. crispum) and duckweed (L. minor). Enzymes from these sources have been used in demonstrating the formation of apiose in vitro 4- ... 

UDP-apiose (5) is very unstable under a variety of conditions of pH and temperature this instability had previously prevented 7 the isolation of sufficient quantities of UDP-apiose for definitive identification. UDP-[U-l4C]apiose is degraded at pH 8.0 to uridine 5 -phosphate and 3-C-(hydroxymethyl)-a-D-[U-14C] erythrofuranosyl 1,2-phosphate at 80, 25, and 4°. The half-lives of UDP-[U-14C]apiose under these conditions are 31.6 seconds, 97.2 minutes, and 16.5 hours, respectively.7 The half-life of UDP-[U-,4C]apiose at pH 3.0 and 40° is 4.67 minutes. It is degraded7 to uridine 5 -pyrophosphate and D-[U-I4C]apiose. At pH 6.2-6.6 and 4°, degradation (of both the... 

While UDP-apiose was being identified, several workers began the purification and characterization of the enzyme(s) in cell-free extracts of L. minor and P. crispum that produce UDP-apiose. il 94,96,97,100-102 It had earlier been inferred that only one enzyme might be necessary to form UPD-apiose and UDP-D-xylose... 

The enzyme catalyzing the transfer of D-apiose from UDP-apiose to 4, 5-dihydroxyflavon-7-yl /3-D-glucopyranoside is commonly called apiin synthetase.7 Activity is measured by the formation of [14C]apiin from UDP-[U-14C]apiose. Apiin synthetase can be measured by the rapid separation and isolation, by poly(ethylenimine)-paper chromatography, of a product of the reaction, namely, [14C] apiin, from UDP-D-[U-14C]xylose and degradation products of UDP-[U-14C]apiose.31 There are reports of the isolation and purification of apiin synthetase from parsley leaves,31 from cell-suspension cultures of parsley,121 and from foxglove (Digitalis purpurea).31 Apiin synthetase isolated from parsley does not require metal ions, NAD+, or other cofactors, and is soluble. It is inhibited by several heavy metals, but not by tetra-N-... 

Decarboxylation steps are required for synthesis of UDP-xylulose and UDP-apiose (Fig. 21-1). Propose chemical mechanisms for these reactions. 

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