The Biosynthesis of d-Apiose in Lemna minor

Lemna minor incorporates radioactivity from n-glucose-l-14C, D-glucose-2-*4C, D-glucose-3 ,4-r4c, D-glucose-6-14C, sodium acetate-Z-14C, DL-serine-3-14C, and L-methionine-14CH8 into the n-apiose moiety of cell wall polysaccharide.The mechanism of biosynthesis of this hydroxymethyltetrose was studied by determining the distribution of 14C in n-apiose after feeding various labeled precursors.Labeled n-apiose was isolated from this tissue, and an aliquot was reduced to n-apiitol.These two compounds were then oxidized with periodate to formic acid, formaldehyde, and glycolic acid, and the radioactivity in each of the carbon atoms of n-apiose was determined.The results show that D-glucose can serve as a precursor for all of the carbon atoms of n-apiose in L. minor, whereas acetate and nL-serine are probably converted to n-glucose before being incorporated into n-apiose.The evidence obtained in this study indicates that n-apiose is derived from D-glucose by the loss of carbon atom 6, followed by a branching reaction which occurs without randomization, and converts either C-3 or C-4 of n-glucose to one of the hydroxymethyl groups of D-apiose. Apiose (3-C-hydroxymethyl-D-tetrose)was first isolated from parsley plants (Apium petroselinum) as a component of the glycoside apiin (1).It has since been shown to occur in a wide variety of plants, including Lemna minor, as a component of cell wall polysaccharide(2-5).Previous studies with parsley and L. minor established that carbon atoms 1 through 4 of D-glucose were incorporated into D-apiose, whereas carbon atom 6 of n-glucose was not utilized (6).The C-hydroxymethyl group may have been derived from either C-3 or C-4 of n-glucose (7).Radioactive compounds such as acetate-l-14C (7), acetate-2-14C (6), formate-r4C (7), and Lmethionine-14CH3 (6) were also effective precursors of n-apiose, indicating that a l-carbon donor might also be involved in the synthesis.Several mechanisms have been proposed for the formation of branched chain sugars.Hough and Jones have suggested that an aldol condensation between dihydroxyacetone and glycolaldehyde could form D-apiose (8).