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Plant Physiol. (1999) 120: 907-912
Biosynthesis of Ascorbic Acid in Kidney Bean.
L-Galactono-
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ABSTRACT |
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Hypocotyls of kidney beans
(Phaseolus vulgaris L.) accumulated ascorbate after
preincubation with a number of possible precursors, mainly
L-galactono-
-lactone (L-GL) and
L-gulono-
-lactone. The increase in the intracellular
ascorbate concentration was parallel to the high stimulation of the
L-GL dehydrogenase (L-GLD) activity measured in vitro using L-GL as a substrate and cytochrome
c as an electron acceptor. Cell fractionation using a
continuous linear Percoll gradient demonstrated that L-GLD
is associated with mitochondria; therefore, pure mitochondria were
isolated and subjected to detergent treatment to separate soluble from
membrane-linked proteins. L-GLD activity was mainly
associated with the detergent phase, suggesting that a
membrane-intrinsic protein is responsible for the ascorbic acid
biosynthetic activity. Subfractionation of mitochondria
demonstrated that L-GLD is located at the inner
membrane.
All higher plants contain high levels of vitamin C or ASC
distributed in many different cell compartments, such as the cytosol, mitochondria, chloroplast, and apoplast. The reducing activity of ASC
is responsible for many of its roles in plant physiology. Although the
function of ASC in plants is not completely understood, it has been
demonstrated to play a role in the defense mechanism as a free-radical
scavenger (Foyer et al., 1991 Despite these important functions of ASC in plant physiology, the
metabolic pathway leading to ASC biosynthesis is only partially known
(Smirnoff, 1996 Recently, L-GLD has been purified from potato (Ôba et
al., 1995 In this paper we provide evidence (for the first time to our knowledge)
that L-GLD is a membrane-intrinsic protein linked to the
inner membrane of mitochondria.
Growth Conditions
![]()
INTRODUCTION
Top
Abstract
Introduction
Methods
Results
Discussion
References
; Córdoba and González-Reyes,
1994
). ASC appears to be a growth-regulating agent in plant cells
(Arrigoni, 1994
; Córdoba-Pedregosa et al., 1996
).
). In animals ASC is synthesized by a microsomal L-GUL oxidase (EC 1.1.3.8) (Kiuchi et al., 1982
). However,
two biosynthetic routes have been suggested for plants. One involves L-GL as the last precursor, which is oxidized by a
dehydrogenase reaction in which Cyt c is used as an electron
acceptor (Ôba et al., 1994
; Wheeler et al., 1998
). This reaction
is catalyzed by L-GLD (EC 1.3.2.3). The other
alternative pathway involves D-Glc,
D-glucosone, and
L-sorbosone as intermediate precursors of ASC
biosynthesis (Loewus, 1988
; Loewus et al., 1990
; Saito et al., 1990
).
For yeast and plants, a third pathway that is catalyzed by
L-GL oxidase (EC 1.1.3.24), involves
L-GL and dioxygen as electron acceptors, and
yields ASC and hydrogen peroxide has been suggested (Baig et al., 1970
;
Bleeg and Christensen, 1982
).
) and cauliflower (Østergaard et al., 1997
) mitochondria. The
enzyme was also expressed in yeast, and the physiological relevance of
L-GL as the last precursor of the ASC biosynthesis pathway
in plants was emphasized. Both purified enzymes have the same molecular
mass (56 kD), although some other properties are different (e.g.
substrate specificity and inhibitor sensitivity).
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Methods
Results
Discussion
References
Isolation and Purification of Mitochondria
All procedures were performed at 0°C to 4°C. Hypocotyl fragments were mixed with ice-cold homogenization buffer (0.25 M Suc, 2 mM EGTA, 30 mM mercaptoethanol, 0.35 M D-mannitol, 0.1% BSA, and 30 mM Hepes, pH 7.6) in a ratio of 1 g fresh weight per milliliter of buffer and immediately homogenized in a blender using two 5-s strokes with an interval of 30 s. The homogenate was filtered on cheesecloth and centrifuged at 1,500g for 15 min to eliminate cell debris. The supernatant was again centrifuged at 14,000g for 20 min. The resulting supernatant consisted of the microsomal fraction and the pellet consisted of the crude mitochondrial fraction, which was resuspended in the homogenization buffer at pH 7.2 without mercaptoethanol.
). The crude mitochondrial fraction (1.5-3 mL at 12-24 mg of
protein) was added to 12 mL of a preformed 2% to 60% linear Percoll
gradient plus 0.25 M Suc, 2 mM EGTA, and 30 mM Hepes, pH 7.2, and centrifuged at 37,000g for
30 min. One-milliliter aliquots were then analyzed by marker enzymes
and electron microscopy to identify cell organelles. Where indicated,
homogenate was also fractionated directly by a linear gradient of
bxPercoll. The following marker enzymes were used: Cyt c
oxidase for mitochondrial inner membrane (Storrie and Madden, 1990
),
catalase for microbodies (Aebi, 1983
), NADPH-Cyt c
oxidoreductase for ER (Storrie and Madden, 1990
), pyrophosphatase for
tonoplast (Chanson, 1990
), and NADH-Cyt c oxidoreductase for
mitochondrial outer membrane (Moore and Proudlove, 1983).
Subfractionation of Pure Mitochondria
Procedures were according to the method of Greenawalt (1979)
1 buffer. Isolation buffer
consisted of 70 mM Suc, 220 mM mannitol, 0.05%
BSA, and 2 mM Hepes, pH 7.2. One milliliter of digitonin was immediately added and the mixture was incubated for 15 min. Eight
milliliters of isolation buffer was then added, and the suspension was
centrifuged in Eppendorf tubes at 12,000g for 10 min. The
pellet consisted of mitoplasts and the supernatant contained outer
membrane and intermembrane proteins. While the supernatant was stored
ice-cold, the pellet was diluted in 5 mL of isolation buffer plus 5 mg
mL
1 Lubrol PX and centrifuged at
144,000g for 60 min to obtain the inner membrane vesicles
(pellet) and the mitochondrial matrix (supernatant). The stored
supernatant was also centrifuged at 144,000g for 60 min to
obtain a pellet containing outer membranes and a supernatant containing
intermembrane space proteins.
Separation of Integral and Peripheral Membrane Proteins
Peripheral and integral proteins were separated as described by Bordier (1981)L-GLD Assay
L-GLD activity was determined according to the method of Ôba et al. (1994)
1). Cyanide
was included to prevent reoxidation of Cyt c by Cyt c oxidase present in mitochondrial fractions. No inhibition
of L-GLD by cyanide had been detected previously
(Ôba et al., 1994ASC Determination
Intracellular ASC concentration was determined in hypocotyls subjected to different experimental conditions by the bipyrydyl method described by Knörzer et al. (1996). Previously, an ASC standard calibration curve was run and an extinction coefficient of 12.3 mM
1 was obtained.
Protein Determination
Protein was determined using Coomassie Blue and the Bradford (1976)
-globulin was used as a standard.
| |
RESULTS |
|---|
|
|
|---|
In Vivo Correlation between ASC Concentration and L-GLD Activity
Hypocotyls accumulated ASC at a rate that depended on the precursor used in the incubation medium. After 48 h of treatment, ASC concentration from L-GL-preincubated hypocotyls increased about 6-fold with respect to control hypocotyls incubated in distilled water. Preincubation with L-GUL also increased the intracellular ASC concentration up to about 4 times that of control. Other compounds, such as D-Glc or D-Gal, led to ASC increases up to 1.3 times that of control hypocotyls (Fig. 1). L-GLD activity also increased at a rate depending on the precursor used in the incubation media. As expected, the highest activities were obtained after preincubation with L-GL (Fig. 2).
|
|
-lactone, Cyt c reduction was
significantly lower (23% and 14%, respectively) than that of the
L-GL-dependent reaction. Other possible electron
acceptors, such as Fe3+-EDTA,
NAD(P)+, or 2,6-dichlorophenolindophenol, used in
place of Cyt c did not serve as a substrate for the
reaction, since enzyme activity was undetectable. In other experimental
series, hypocotyl homogenates were incubated with
L-GL under continuous shaking but in the absence of Cyt c to determine whether oxygen alone would sustain the
reaction. However, the results were negative since neither ASC
(measured at a maximum absortion wavelength of 265 nm) nor hydrogen
peroxide (measured by using guaiacol and a peroxidase-coupled reaction) were produced.
L-GLD Is an Intrinsic Protein Located at the Inner Mitochondrial Membrane
A linear Percoll gradient previously calibrated using density markers was used to separate cell organelles from homogenates obtained from kidney bean hypocotyls. Subcellular fractions were clearly separated using this procedure. The L-GLD activity profile was parallel to that of Cyt c oxidase activity, which was used as mitochondrial enzyme marker. No L-GLD activity was detected in those fractions corresponding to the ER, tonoplast, or peroxisomes (Fig. 3). Therefore, our results suggest that L-GLD activity is associated with mitochondria.
|
ASC has been shown to play a relevant role in higher plant
physiology. However, details on its biosynthesis have not been thoroughly investigated, despite the fact that plants are the main nutritional source of vitamin C for human beings.
Received December 14, 1998;
accepted April 19, 1999.
Abbreviations:
ASC, ascorbic acid or ascorbate.
GL, galactono-
Aebi HE (1983) Catalase. In J Bergmeyer, M
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Bleeg HS,
Christensen F
(1982)
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González-Reyes JA
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González-Reyes JA,
Cañadillas MS,
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Paciolla,
C,
Tommasi F,
Liso R,
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Foyer CH, Lelandais M, Edwards EA, Mullineaux PM (1991) The role
of ascorbate in plants, interactions with photosynthesis, and
regulatory significance. In E Pell, K Steffens, eds, The
Active Oxigen/Oxidative Stress and Plant Metabolism. American Society
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Anderson JO,
McDaniel RG
(1980)
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488-495
Greenawalt JW
(1979)
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Nishikimi M,
Yagi K
(1982)
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Durner J,
Böger P
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Loewus MW,
Bedgar DL,
Saito K,
Loewus FA
(1990)
Conversion of L-sorbosone to L-ascorbic acid by a NADP-dependent dehydrogenase in bean and spinach leaf.
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Moore A,
L,
Proudlove MO
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Mitochondria and sub-mitochondrial particles.
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eds, Isolation of Membranes and Organelles from Plant Cells.
Academic Press, London, pp 153-184
Mutsuda M,
Ishikawa T,
Takeda T,
Shigeoka S
(1995)
Subcellular localization and properties of L-galactono-
Ôba K,
Fukui M,
Imai Y,
Iriyama S,
Nogami K
(1994)
L-Galactono-
Ôba K,
Ishikawa S,
Nishikawa M,
Mizuno H,
Yamamoto T
(1995)
Purification and properties of L-galactono-
Østergaard J,
Persiau G,
Davey MW,
Bauw G,
Van Montagu M
(1997)
Isolation of a cDNA coding for L-galactono-
Saito K,
Nick JA,
Loewus FA
(1990)
D-Glucosone and L-sorbosone, putative intermediates of L-ascorbic acid synthesis in detached bean and spinach leaves.
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(1996)
The function and metabolism of ascorbic acid in plants.
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Smith PK,
Krohn RI,
Hermanson GT,
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Provenzano MD,
Fujimoto EK,
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Olson BJ,
Klenk DC
(1985)
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View larger version (30K):
[in a new window]
Figure 4.
Distribution of Cyt c oxidase and
L-GLD in the subcellular fractions. Cyt c
oxidase and L-GLD activities were assayed in homogenate
(white bars), the microsomal fraction (light gray bars), the crude
mitochondria (dark gray bars), and the pure mitochondrial fraction
(black bars). Data represent specific activities for each analyzed
fraction.
, and the
results are shown in Table I.
Interestingly, the distribution of L-GLD was nearly
identical to that of Cyt c oxidase, a well-known intrinsic
protein, strongly suggesting that the biosynthetic ASC activity is due
to a protein directly linked to a mitochondrial membrane.
View this table:
Table I.
Effects of Triton X-114 on L-GLD
activity distribution
Pure mitochondrial fractions were subjected to Triton X-114 treatment,
and L-GLD activity was determined in the aqueous and
detergent phases. For comparative purposes, Cyt c oxidase
and NADH-Cyt c oxidoreductase activities were also
determined. Activities represent means ± SD from
three independent experiments. Ratio is defined as the proportion of
activity at the detergent phase with respect to the total (aqueous plus
detergent phases).
View this table:
Table II.
Distribution of L-GLD activity in
submitochondrial fractions
Submitochondrial fractions were isolated from pure mitochondria.
L-GLD, Cyt c oxidase (as a marker of inner
membrane), and NADH-Cyt c oxidoreductase (as a marker of
outer membrane) activities were determined. Data represent means ± SD from three independent experiments.
![]()
DISCUSSION
Top
Abstract
Introduction
Methods
Results
Discussion
References
, Baig et al. (1970)
, and De Gara et al. (1994)
showed correlations between L-GL or L-GUL additions to the culture
media and increased intracellular ASC.
); however, L-GL appears to be the last common immediate precursor of
ASC biosynthesis. Cyt c could not be replaced with other
electron acceptors such as Fe3+-EDTA,
NAD(P)+, or 2,6-dichlorophenolindophenol.
Moreover, dioxygen did not oxidize L-GL, nor was
hydrogen peroxide produced during the course of the reaction,
discounting the possibility that, like yeast enzyme (Bleeg and
Christensen, 1982
), hypocotyl enzyme may act as an oxidase.
, 1995
;
Mutsuda et al., 1995
; Østergaard et al., 1997
). However, its exact
localization has not as yet been investigated. Ôba et al. (1994)
reported the subcellular distribution of GLD in potato tubers after
linear Suc density gradient centrifugation, and found that the enzyme was mainly associated with mitochondria (but also with the ER and
peroxisomes). Mutsuda et al. (1995)
indicated that the enzyme was
associated with mitochondria of spinach leaves after linear Suc density
or discontinuous Percoll gradient centrifugation. Nevertheless, these
authors only investigated the enzyme distribution in intact
mitochondria and chloroplasts. Our results using a self-generated Percoll linear gradient showed that L-GLD in exclusively
asssociated with mitochondria; no activity was detected in membrane
fractions derived from microsomes, peroxisomes, or tonoplast.
, the
enzyme was firstly solubilized by sonication. Arrigoni et al. (1996)
used Tween 20 to solubilize the enzyme before in vitro assays, and
Mutsuda et al. (1995)
solubilized the enzyme using a variety of
detergents, such as Triton X-100, deoxycholate, or Chaps. We used
0.03% Triton X-100 in the in vitro assay to detect the GLD activity.
In the absence of the detergent the activity was negligible, suggesting
the possible association of L-GLD with membranes. To test
this possibility, pure mitochondria were subjected to Triton X-114
treatments according to the method of Bordier (1981)
. This experimental
procedure allowed us to separate integral from peripheral membrane
proteins. Our results indicate that L-GLD activity is found
mainly in the detergent phase. This was similar to Cyt c
oxidase and NADH-Cyt c oxidoreductase activities, which were
used as marker enzymes for integral inner and outer mitochondrial membranes, respectively. Thus, our results clearly indicate that L-GLD activity is due to a membrane-linked
protein.
1
This research was partially supported by the
Spanish Dirección General de Enseñanza Superior (project
no. PB95-0560).
![]()
FOOTNOTES
*
Corresponding author; e-mail fcordoba{at}uhu.es; fax 34-59-530175.
![]()
ABBREVIATIONS
-lactone.
GLD, galactono-
-lactone dehydrogenase.
GUL, gulono-
-lactone.
![]()
LITERATURE CITED
Top
Abstract
Introduction
Methods
Results
Discussion
References
-lactone conversion to ascorbate by lycorine.
J Plant Physiol
144:
649-653
-lactone dehydrogenase in spinach leaves.
Biosci Biotech Biochem
59:
1983-1984
-lactone dehydrogenase: partial characterization, induction of activity and role in the synthesis of ascorbic acid in wounded white potato tuber tissue.
Plant Cell Physiol
35:
473-478
-lactone dehydrogenase, a key enzyme for ascorbic acid biosynthesis, from sweet potato roots.
J Biochem
117:
120-124
-lactone dehydrogenase, an enzyme involved in the biosynthesis of ascorbic acid in plants.
J Biol Chem
272:
30009-30016
Copyright Clearance Center: 0032-0889/99/120//06
© 1999 American Society of Plant Physiologists
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