Carbohydrate and enzyme distribution in protoplasts from valencia orange juice sacs

Abstract

The subcellular distribution of neutral sugars, organic acids and their metabolic enzymes was investigated in protoplasts from ‘Valencia’ orange juice sacs. The vacuole was found to contain 70% of the malic acid, 75% of the fructose and glucose, 89% of the citric acid and 100% of the sucrose. Of the enzymes assayed, α-mannosidase, phosphohexoisomerase and phosphoglucomutase showed activity in the vacuole fraction at 100, 5 and 29%, respectively. The activities of acid and neutral invertases, UDPG pyrophosphorylase and both ATP and PPi phosphofructokinase were present only in the cytoplasmic fraction. The remaining enzymes (sucrose synthase, hexokinase, fructokinase and aconitase) showed no activity in any of the samples assayed (protoplast, vacuole and cytoplasm). From the data presented, it appears that vacuolar sugars are the major form of carbohydrate supplying energy to the mature juice sac cells. Possible mechanisms of sugar transport across the tonoplast prior to their oxidation are discussed.