Carbohydrate Metabolism of Citrus Fruits. I. Mechanisms of Sucrose Synthesis in Oranges and Lemons

Abstract

Tests with particulate preparations and fractionated extracts from oranges and lemons showed that soluble enzymes from orange pulp catalyzed the formation of sucrose from free fructose and uridine diphosphate glucose (UDPG). In lemons, the extracts were inactive but the same reaction was carried out by particulate material. None of the preparations showed significant formation of sucrose from UDPG and fructose-6-phosphate. In isolated tissues (excised vesicles and slices of flavedo or albedo) from lemons of all ages, label from glucose-C^ or fructose-C14 was incorporated into sucrose. Both hexose components were found to be labeled no matter which monosaccharide was supplied, although no labeling of the free monosaccharide pools occurred. This suggests that the labeling of the sucrose occurred from a phosphorylated form of fructose in these samples despite the failure to demonstrate such a reaction in the enzymatic experiments. In the intact, young fruit, labeled glucose was much more effective than fructose in the formation of sucrose. Here, also, the sucrose was labeled in both hexose moieties from either of the sugars supplied. In the intact fruit, feeding of glucose-C14 resulted in partial labeling of the free fructose pool. Therefore, it is possible that the free fructose might have contributed to the labeling of the sucrose. The isolated albedo of the lemon was far more active than the other tissues in the formation of sucrose from the labeled hexoses. This tissue maintained a relatively constant activity for sucrose formation throughout the growth of the fruit while the activity in the flavedo and vesicles fell off rapidly after early stages of growth.