Tracer studies on ascorbic acid formation in plants

Abstract

Tracer studies in which D-glucose-1- 14C, -2- 14C or -6- 14C; D-galactose-1- 14C; D-glucuronic acid-6- 14C; D-glucuronolactone-1- 14C or -6- 14 C; D-galacturonic acid-1- 14C; D-galacturonate-U- 14C methyl ester; myo-inositol-2 14C; or D-glucose-1-t or -6-t are administered to intact plant tissues, usually to the detached ripening strawberry or the detached parsley leaf, reveal the existence of two processes of L-ascorbic acid formation in higher plants. In one process, D-glucose and related sugars are utilized as their hexose phosphates. Oxidation proceeds at carbon 1 of the sugar to form the carboxyl group at carbon 1 of L-ascorbic acid. Except for partial equilibration of the hexose phosphate with triose and pentose phosphates, the six carbon chain is utilized without cleavage. Carbon 5 is epimerized from the D- to the L- configuration during the conversion. Carbon 6 remains reduced throughout the process. The other process of L-ascorbic acid formation proceeds from exogenously supplied D-glucuronolactone or D-galacturonate methyl ester. Here, the carboxyl group at carbon 6 of the uronic acid derivative is conserved, becoming carbon 1 of L-ascorbic acid. The aldehydic group of the uronic acid derivative is reduced to a primary alcohol corresponding to carbon 6 in the L-ascorbic acid. There is no cleavage of the carbon chain during the conversion. D-Glucuronic acid supplied exogenously or generated enzymatically within the plant tissue from myo-inositol is not utilized for L-ascorbic acid synthesis. The physiological significance of the two processes is discussed.