THE CONVERSION OF C14-LABELED SUGARS TO l-ASCORBIC ACID IN RIPENING STRAWBERRIES

Evidence that L-ascorbic acid' is formed in the intact rat via a sequence of hexose precursors has been presented by King and coworkers (14).Briefly, their technique involved the administration of uniformly labeled glucose-Cl', uniformly labeled glucuronolactone-C", glucose-l-C", or glucose-6-C" to chloretonized rats.The urinary ascorbic acid became labeled and results of their studies showed that the hexose carbon chain was preserved intact in this ascorbic acid, although an inversion of the chain had occurred; i.e., carbon 1 of glucose became carbon 6 of ascorbic acid.An independent and roughly simultaneous study of ascorbic acid bio-SyIdheBiS in the normal rat and also in cress seedlings was reported by Isherwood, Chen, and Mapson (5).They prepared a large number of compounds postulated as intermediates in the pathway from glucose or galactose to ascorbic acid.Each of these was then tested by subcutaneous injection into rats or by absorption into germinating cress seedlings.The increase in ascorbic acid excretion in the rat urine or in the amount of ascorbic acid present in the seedlings served as a measure of the precursor potential of the compound used.It was found that a scheme involving oxidation of glucose or galactose to the corresponding uranic acid derivative, followed by a reduction of the aldehydic carbon to a primary alcohol and oxidation of the lactone of this aldonic acid to the enediol structure of ascorbic acid, was best suited to the data.Such a pathway involved an inversion of the carbon skeleton, complementing the observations of King et al.In addition to the experiments conducted on intact organisms, Mapson, Isherwood, and Chen (6) were able to demonstrate the enzymatic conversion of L-galactonor-lactone to ascorbic acid by mitochondria from pea and mung bean seeds.Recently, Mapson and Isherwood (7) also reported briefly on the existence of a triphosphopyridine nucleotidedependent dehydrogenase from pea seedlings which catalyzes the reduction of the methyl ester of n-galacturonic acid to a derivative of L-galactonic acid.1 Hereafter, ascorbic acid will refer to L-ascorbic acid, glucose to D-glucose, galactoee to D-galactose, and glucuronate to n-glucuronate, unle~~s otherwise indicated.