Ascorbic acid conjugates isolated from the phloem of Cucurbitaceae

• Published in: Phytochemistry (Oxford) Vol. 69; no. 9; pp. 1850 - 1858
• Main Authors: Hancock, Robert D., Chudek, John A., Walker, Paul G., Pont, Simon D.A., Viola, Roberto
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Ltd 01-06-2008; Elsevier
• Subjects: Antioxidant; Ascorbic Acid - chemistry; Ascorbic Acid - metabolism; Biological and medical sciences; Cell physiology; Chemical constitution; Chemical Phenomena; Chemistry, Physical; Chromatography, High Pressure Liquid; Courgette; Cucurbita pepo; Cucurbitaceae; Cucurbitaceae - genetics; Cucurbitaceae - metabolism; Fundamental and applied biological sciences. Psychology; Glucose - metabolism; Glucoside; Hydrolysis; Kinetics; Magnetic Resonance Spectroscopy; Mass Spectrometry; Molecular Structure; NMR; Oxidation-Reduction; Phloem; Phloem - metabolism; Plant physiology and development; Plasma membrane and permeation; Structure; Transport; Zucchini; Zucchini; NMR; Phloem; Courgette; Cucurbitaceae; Glucoside; Cucurbita pepo; Structure; Antioxidant; Transport; Mass spectrometry; Ascorbic acid; Exudate; Fruit; Enzyme; Glucose; Hydrolysis; Absorption; Dicotyledones; L-Ascorbate oxidase; Angiospermae; Isolation; Above ground plant part; Spermatophyta; Yield; Oxidoreductases; Cucurbita pepo;Cucurbitaceae;Courgette;Zucchini;Structure;Antioxidant;Transport;Phloem;Glucoside;NMR;Mass spectrometry; Resins
• ISSN: 0031-9422; 1873-3700
• DOI: 10.1016/j.phytochem.2008.03.022

A series of
l-ascorbic acid analogues were isolated from phloem exudates of
Cucurbitaceae fruit. The most abundant analogue from
Cucurbita pepo was a 6-glucosyl conjugate. The potential role of ascorbate glycosides in long-distance transport is discussed in relation to the polymer trap theory of phloem loading.
Analysis of phloem exudates from the fruit of
Cucurbitaceae revealed the presence of several compounds with UV–visible absorption spectra identical to that of
l-ascorbic acid. In
Cucurbita pepo L. (zucchini), the compounds could be isolated from phloem exudates collected from aerial parts of the plant but were not detected in whole tissue homogenates. The compounds isolated from the phloem exudates of
C. pepo fruit were eluted from strong anion exchange resin in the same fraction as
l-ascorbic acid and were oxidised by ascorbate oxidase (E.C. 1.10.3.3). The major compound purified from
C. pepo fruit exudates demonstrated similar redox properties to
l-ascorbic acid and synthetic 6-
O-glucosyl-
l-ascorbic acid (6-GlcAsA) but differed from those of 2-
O-glucosyl-
l-ascorbic acid (2-GlcAsA) isolated from the fruit of
Lycium barbarum L. Parent and fragment ion masses of the compound were consistent with hexosyl-ascorbate in which the hexose moiety was attached to C5 or C6 of AsA. Acid hydrolysis of the major
C. pepo compound resulted in the formation of
l-ascorbic acid and glucose. The purified compound yielded a proton NMR spectrum that was almost identical to that of synthetic 6-GlcAsA. A series of
l-ascorbic acid conjugates have, therefore, been identified in the phloem of
Cucurbitaceae and the most abundant conjugate has been identified as 6-GlcAsA. The potential role of such conjugates in the long-distance transport of
l-ascorbic acid is discussed.