Biosynthetic Reactions in vitro Involving 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-  

Early investigations of apiose already reviewed showed that its biosynthesis involves D-glucuronic acid as an intermediate. Apiose is formed from D-glucuronic acid by loss of C-6, with a concomitant, in- 

Incubation of a cell-free extract of L. minor in the presence of NAD+ and UDP-D-[U-14C]glucuronic acid generated two compounds, each containing a different radioactive C5 sugar.96 The presence of apiose after hydrolysis was diagnosed by (a) paper chromatography, (b) specific complex-formation with benzeneboronic acid, (c) electrophoretic mobility, and (d) formation of its isopropylidene acetal. Treatment with ammonia at 0° showed that the compound was formed by a nonenzymic cyclization.96 Treatment with phosphoric diesterase plus phosphatase further supported an earlier report of the existence of a D-apiosyl cyclic phosphate.95 

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 

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

---