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PubMed1998

Different renal effects of two inhibitors of catechol-O-methylation in the rat: entacapone and CGP 28014.

Dopamine is a natriuretic hormone that is abundantly synthesized in the kidney and is involved in sodium homeostasis. It is metabolized by monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT) to form 3-methoxytyramine and dihydroxyphenylacetic acid (DOPAC) and finally homovanillic acid (HVA). In order to investigate whether dopamine metabolism is involved in renal sodium regulation, we tested the renal effects of the nitrocatechol entacapone (COMT inhibitor), in comparison with those of the pyridine derivative CGP 28014, in the anaesthetized rat. Entacapone injection resulted in a more than 5-fold increase in sodium excretion, while the renal excretion of dopamine only transiently increased by 20%. DOPAC excretion showed a more than 2-fold increase which persisted throughout the study. Pretreatment with the selective dopamine DA1-receptor antagonist SCH23390 reduced the entacapone-induced natriuretic response by 69%. Glomerular filtration rate (GFR) and mean arterial blood pressure (MAP) remained unchanged. Injection of CGP 28014 did not produce a natriuretic response; nevertheless, both dopamine and DOPAC excretion increased by 78% and more than 2-fold, respectively. GFR and MAP remained unchanged. In conclusion, COMT inhibition using entacapone results in a mainly DA1 receptor mediated natriuresis involving inhibition of tubular transport processes, supporting a role for dopamine metabolism in sodium homeostasis. Although CGP 28014 increases the renal excretion of both dopamine and DOPAC it does not affect renal sodium handling indicating a different mechanism of action.

Amidines↗
PubMed1997

Genotype determining low catechol-O-methyltransferase activity as a risk factor for obsessive-compulsive disorder.

In the present study, we address the role of the gene for catechol-O-methyltransferase (COMT), a key modulator of dopaminergic and noradrenergic neurotransmission, in the genetic predisposition to obsessive-compulsive disorder (OCD). We show that a common functional allele of this gene, which results in a 3- to 4-fold reduction in enzyme activity, is significantly associated in a recessive manner with susceptibility to OCD, particularly in males. This association is further supported by psychiatric evaluation of patients who carry microdeletions encompassing the comt gene. The mechanism underlying this sex-selective association remains to be defined and may include a sexual dimorphism in COMT activity, although close linkage with a nearby disease susceptibility locus cannot be excluded at this point.

Alleles↗
PubMed1999

Coniferyl aldehyde 5-hydroxylation and methylation direct syringyl lignin biosynthesis in angiosperms.

A central question in lignin biosynthesis is how guaiacyl intermediates are hydroxylated and methylated to the syringyl monolignol in angiosperms. To address this question, we cloned cDNAs encoding a cytochrome P450 monooxygenase (LsM88) and a caffeate O-methyltransferase (COMT) from sweetgum (Liquidambar styraciflua) xylem. Mass spectrometry-based functional analysis of LsM88 in yeast identified it as coniferyl aldehyde 5-hydroxylase (CAld5H). COMT expressed in Escherichia coli methylated 5-hydroxyconiferyl aldehyde to sinapyl aldehyde. Together, CAld5H and COMT converted coniferyl aldehyde to sinapyl aldehyde, suggesting a CAld5H/COMT-mediated pathway from guaiacyl to syringyl monolignol biosynthesis via coniferyl aldehyde that contrasts with the generally accepted route to sinapate via ferulate. Although the CAld5H/COMT enzyme system can mediate the biosynthesis of syringyl monolignol intermediates through either route, k(cat)/K(m) of CAld5H for coniferyl aldehyde was approximately 140 times greater than that for ferulate. More significantly, when coniferyl aldehyde and ferulate were present together, coniferyl aldehyde was a noncompetitive inhibitor (K(i) = 0.59 microM) of ferulate 5-hydroxylation, thereby eliminating the entire reaction sequence from ferulate to sinapate. In contrast, ferulate had no effect on coniferyl aldehyde 5-hydroxylation. 5-Hydroxylation also could not be detected for feruloyl-CoA or coniferyl alcohol. Therefore, in the presence of coniferyl aldehyde, ferulate 5-hydroxylation does not occur, and the syringyl monolignol can be synthesized only from coniferyl aldehyde. Endogenous coniferyl, 5-hydroxyconiferyl, and sinapyl aldehydes were detected, consistent with in vivo operation of the CAld5H/COMT pathway from coniferyl to sinapyl aldehydes via 5-hydroxyconiferyl aldehyde for syringyl monolignol biosynthesis.

Cloning, Molecular↗
PubMed2000

Heterogeneity within animal thioredoxin reductases. Evidence for alternative first exon splicing.

Animal thioredoxin reductases (TRs) are selenocysteine-containing flavoenzymes that utilize NADPH for reduction of thioredoxins and other protein and nonprotein substrates. Three types of mammalian TRs are known, with TR1 being a cytosolic enzyme, and TR3, a mitochondrial enzyme. Previously characterized TR1 and TR3 occurred as homodimers of 55-57-kDa subunits. We report here that TR1 isolated from mouse liver, mouse liver tumor, and a human T-cell line exhibited extensive heterogeneity as detected by electrophoretic, immunoblot, and mass spectrometry analyses. In particular, a 67-kDa band of TR1 was detected. Furthermore, a novel form of mouse TR1 cDNA encoding a 67-kDa selenoprotein subunit with an additional N-terminal sequence was identified. Subsequent homology analyses revealed three distinct isoforms of mouse and rat TR1 mRNA. These forms differed in 5' sequences that resulted from the alternative use of the first three exons but had common downstream sequences. Similarly, expression of multiple mRNA forms was observed for human TR3 and Drosophila TR. In these genes, alternative first exon splicing resulted in the formation of predicted mitochondrial and cytosolic proteins. In addition, a human TR3 gene overlapped with the gene for catechol-O-methyltransferase (COMT) on a complementary DNA strand, such that mitochondrial TR3 and membrane-bound COMT mRNAs had common first exon sequences; however, transcription start sites for predicted cytosolic TR3 and soluble COMT forms were separated by approximately 30 kilobases. Thus, this study demonstrates a remarkable heterogeneity within TRs, which, at least in part, results from evolutionary conserved genetic mechanisms employing alternative first exon splicing. Multiple transcription start sites within TR genes may be relevant to complex regulation of expression and/or organelle- and cell type-specific location of animal thioredoxin reductases.

Alternative Splicing↗
PubMed2001

Rapid conversion of tea catechins to monomethylated products by rat liver cytosolic catechol-O-methyltransferase.

1. The metabolic O-methylation of several catechol-containing tea polyphenols by rat liver cytosolic catechol-O-methyltransferase (COMT) has been studied. 2. When (-)-epicatechin was used as substrate, its O-merthylation showed dependence on incubation time, cytosolic protein concentration, incubation pH and concentration of S-adenosyl-L-methionine. The O-methylation of increasing concentrations of (-)-epicatechin followed typical Michaelis-Menten kinetics, and the apparent Km and Vmax were 51 microM and 2882 pmol mg protein(-1) min(-1), respectively, at pH 7.4, and were 17 microM and 2093 pmol mg protein(-1) min(-1), respectively, at pH 10.0. 3. Under optimized conditions for in vitro O-methylation, (-)-epicatechin, (+)-epicatechin and (-)-epigallocatechin were rapidly O-methylated by rat liver cytosol. In comparison, (-)-epicatechin gallate and (-)-epigallocatechin gallate vere O-methylated at significantly lower rates under the same reaction conditions. catalysed O-methylation of (-)-epicatechin and (-)-epigallocatechin was inhibited in a concentration-dependent manner by S-adenosyl-L-homocysteine, a demethylated product of S-adenosyl-L-methionine. The IC50 was approximately 10 microM. 5. In summary, the results showed that several catechol-containing tea polyphenols were rapidly O-methylated by rat liver cytosolic COMT. These observations raise the possibility that some of the biological effects of tea polyphenols may be exerted by their O-methylated products or may result from their potential inhibition of the COMT-catalysed O-methylation of endogenous catecholamines and catechol oestrogens.

Animals↗
PubMed2006

Polymorphisms in estrogen bioactivation, detoxification and oxidative DNA base excision repair genes and prostate cancer risk.

To date, the potential impact of hormones on prostate cancer has predominantly focused on receptor-mediated events. However, catechol estrogens, if not inactivated by catechol-O-methyltransferase (COMT), can generate large quantities of reactive oxygen species (ROS). ROS may cause a spectrum of damage including oxidative DNA base lesions, which can lead to irreversible mutation(s) if they are not repaired by base excision repair (BER) systems. hOGG1 is a key enzyme in short patch BER because it recognizes and performs initial excision of the most common form of oxidative DNA base damage, 8-hydroxyguanine (8-oxo-dG). To investigate potential non-receptor-mediated estrogen effects, we evaluated the association between COMT Val158Met and hOGG1 Ser326Cys polymorphisms and prostate cancer in a family-based case-control study (439 prostate cancer cases, 479 brother controls). We observed no noteworthy associations between these polymorphisms and prostate cancer risk in the total study population. However, among men with more aggressive prostate cancer, the hOGG1 326 Cys/Cys genotype was inversely associated with disease (OR=0.30; 95% CI=0.09-0.98). Combining the lower activity CYP1B1 432 Leu/Leu or Leu/Val genotypes (which may decrease the level of catechol estrogens and ROS generated) with the hOGG1 326 Cys/Cys genotype and the XRCC1 399 Arg/Arg or Arg/Gln genotypes (which may enhance BER) resulted in an even further reduced risk in Caucasians with more aggressive disease (OR=0.09; 95% CI=0.01-0.56). Including the high-activity COMT 158Val allele to this combination also lowered aggressive prostate cancer risk but the effect was not as strong (OR=0.20; 95% CI=0.05-0.88). The decreased risk we observed with the hOGG1 326 Cys/Cys genotype confirms an earlier report and the further reduced risk found with the CYP1B1 (432 Leu/Leu or Leu/Val)-hOGG1 (326 Cys/Cys)-XRCC1 (Arg/Arg or Arg/Gln) genotype combination may lend new insights to the importance of ROS generated from non-receptor-mediated estrogenic mechanisms in more aggressive prostate cancer.

Aged↗
PubMed2006

Involvement of hyperprolinemia in cognitive and psychiatric features of the 22q11 deletion syndrome.

Microdeletions of the 22q11 region, responsible for the velo-cardio-facial syndrome (VCFS), are associated with an increased risk for psychosis and mental retardation. Recently, it has been shown in a hyperprolinemic mouse model that an interaction between two genes localized in the hemideleted region, proline dehydrogenase (PRODH) and catechol-o-methyl-transferase (COMT), could be involved in this phenotype. Here, we further characterize in eight children the molecular basis of type I hyperprolinemia (HPI), a recessive disorder resulting from reduced activity of proline dehydrogenase (POX). We show that these patients present with mental retardation, epilepsy and, in some cases, psychiatric features. We next report that, among 92 adult or adolescent VCFS subjects, a subset of patients with severe hyperprolinemia has a phenotype distinguishable from that of other VCFS patients and reminiscent of HPI. Forward stepwise multiple regression analysis selected hyperprolinemia, psychosis and COMT genotype as independent variables influencing IQ in the whole VCFS sample. An inverse correlation between plasma proline level and IQ was found. In addition, as predicted from the mouse model, hyperprolinemic VCFS subjects bearing the Met-COMT low activity allele are at risk for psychosis (OR = 2.8, 95% CI = 1.04-7.4). Finally, from the extensive analysis of the PRODH gene coding sequence variations, it is predicted that POX residual activity in the 0-30% range results into HPI, whereas residual activity in the 30-50% range is associated either with normal plasma proline levels or with mild-to-moderate hyperprolinemia.

Adolescent↗
PubMed2001

Effect of phosphate transporter and methylation inhibitor drugs on the disposition of arsenate and arsenite in rats.

Arsenate (AsV) is biotransformed into the more toxic arsenite (AsIII) and monomethylarsonous acid (MMAsIII), but it is unknown how to decrease production of these harmful metabolites. We investigated the effects of foscarnet and fosfomycin, drugs interacting with the phosphate transporter, on biotransformation of AsV, an analog of inorganic phosphate. The effects of entacapone, an inhibitor of catechol-O-methyl transferase (COMT), and nitrous oxide, an inactivator of methylcobalamin, were also tested on the formation of MMAsIII from AsIII in order to clarify the role of COMT and methylcobalamin in biomethylation of AsIII. Arsenic in bile and urine of control and treated rats receiving AsV or AsIII was speciated by HPLC-HG-AFS. In AsV-injected rats, foscarnet, but not fosfomycin, increased the urinary excretion of AsV and decreased the biliary and urinary excretion of AsIII as well as biliary excretion of MMAsIII. In AsIII-injected rats, however, foscarnet failed to influence the excretion of AsIII and its metabolites, suggesting that this drug inhibits the hepatic uptake and renal reabsorption of AsV, thereby decreasing formation of AsIII and MMAsIII from AsV. Entacapone or nitrous oxide pretreatment slightly or not at all influenced the biliary excretion of MMAsIII and urinary excretion of dimethylarsinic acid (DMAsV) in AsIII-injected rats. In contrast, periodate-oxidized adenosine, an inhibitor of S-adenosylmethionine-dependent methyltransferases, nearly abolished appearance of methylated arsenic metabolites in bile and urine. Thus, foscarnet facilitates urinary clearance of AsV and decreases formation of toxic AsIII and MMAsIII, indicating that this drug may be used to promote elimination and counter toxification of AsV. Because entacapone and nitrous oxide influenced the excretion of MMAsIII and DMAsV negligibly, neither COMT nor methylcobalamin appears to be involved in arsenic methylation in rats.

Arsenates↗
PubMed1998

Distribution of catechol-O-methyltransferase expression in human central nervous system.

Catechol-O-methyltransferase (COMT, EC 2.1.1.6) is a ubiquitous enzyme crucial to catechol metabolism. Two isoforms exist in the human central nervous system (CNS) and they are encoded by two transcripts (1.3 and 1.5 kb) in most human tissues. Using two alpha-32P-labeled probes, we found only the 1.5 kb transcript in all 16 regions of the human CNS using commercially available Northern blots. Spinal cord had the highest and amygdala had the lowest levels of expression. The other CNS regions shared a similar level of expression. The distributions of COMT gene expression relative to whole brain between both probes were significantly correlated. Our study shows that the expression of the 1.5kb transcript is crucial for COMT activity in all regions of the human CNS.

Blotting, Northern↗
PubMed1993

The effect of catechol-O-methyl transferase inhibition by entacapone on the pharmacokinetics and metabolism of levodopa in healthy volunteers.

We studied the effect of inhibiting the enzyme catechol-O-methyltransferase (COMT) by a novel COMT inhibitor, entacapone, on the pharmacokinetics and metabolism of levodopa in 12 healthy male volunteers. Single increasing oral doses of entacapone (50-400 mg) were administered concomitantly with a single oral dose of levodopa/carbidopa (100/25 mg). The subjects were treated with carbidopa (100 mg t.i.d.) for 1 day prior to the administration of study drugs. Plasma concentrations of levodopa; its metabolites 3-O-methyldopa (3-OMD), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA); as well as carbidopa and entacapone were determined for pharmacokinetic calculations. Entacapone dose-dependently increased the area under the plasma concentration-time curve (AUC) of levodopa; the increase was 65% after the 400 mg dose of entacapone. Neither Cmax nor Tmax of levodopa was statistically significantly influenced by entacapone. Entacapone dose-dependently decreased the AUC of 3-OMD, maximally by 58%. The AUC of DOPAC was statistically significantly increased but no change in the AUC of HVA was observed after entacapone. No drug-related adverse events or hemodynamic effects were observed. The in vivo biochemical effects of entacapone indicate that it is an orally active COMT inhibitor and that it may improve the therapeutic efficacy of levodopa in Parkinson's disease.

3,4-Dihydroxyphenylacetic Acid↗
PubMedDate not supplied

The effects of different repeated doses of entacapone on the pharmacokinetics of L-Dopa and on the clinical response to L-Dopa in Parkinson's disease.

We performed a double-blind, placebo-controlled, randomized, crossover, multiple-dose study on entacapone in 25 patients with Parkinson's disease with levodopa (L-Dopa) treatment-related fluctuations. A run-in period was followed by four 2-week treatment periods during which the patients took 4 to 6 daily doses of L-Dopa concomitantly with 100, 200, or 400 mg of entacapone or with placebo. The effects were assessed at the end of each period; the inhibition of soluble catechol-O-methyltransferase (S-COMT) activity in red blood cells and the plasma concentrations of entacapone, L-Dopa, and 3-O-methyldopa (3-OMD) were measured and clinical effects assessed on an 18-hour home diary. Twenty-one patients completed the study. Entacapone decreased the COMT activity from predose level: 100 mg by 25%, 200 mg by 33%, and 400 mg by 32% (p < 0.001 vs. placebo for each dose). Correspondingly, the 3-OMD concentrations decreased by 39%, 54%, and 66% with 100-, 200-, and 400-mg doses, respectively. The elimination half-life of L-Dopa was prolonged by 23% (p < 0.05), 26% (p < 0.001), and 48% (p < 0.001), and the area under the curve of L-Dopa increased by 17% (p < 0.05), 27% (p < 0.001), and 37% (p < 0.001) with the increasing doses. Despite a significant decrease in the daily dose of L-Dopa, entacapone decreased the proportion of daily "off" time: 100 mg by 11%, 200 mg by 18%, and 400 mg by 20% compared with placebo. However, this decrease was not statistically significant for any of the doses in this small patient population. The dyskinetic "on" time did not increase with different doses of entacapone. All doses were well tolerated, and no severe adverse events were reported. The study showed that repeated dosing of entacapone inhibits the COMT activity in a dose-dependent manner and thereby reduces the loss of L-Dopa to 3-OMD. Therefore, the area under the curve of L-Dopa is increased and the patient's clinical condition improved.

Adult↗
PubMed2005

Genetic variants of myeloperoxidase and catechol-O-methyltransferase and breast cancer risk.

This nested case-control study evaluated the role of polymorphisms in the myeloperoxidase (MPO) and catechol-O-methyltransferase (COMT) genes that modulate oxidative stress in breast cancer risk in a Chinese population. Our results demonstrate that the MPO A/A genotype was associated with a reduced risk of breast cancer (odds ratio (OR) 0.64; 95% confidence interval (CI) 0.11-3.76), whereas there was no overall association of COMT genotype with breast cancer. Of note, an elevated breast cancer risk associated with the increasing numbers of high-risk genotypes of MPO and COMT genes was observed in women with a longer duration between menarche and first full-term pregnancy.

Breast Neoplasms↗
PubMed1994

New medical and surgical treatments for Parkinson's disease.

This article reviews new medical and surgical treatments for Parkinson's disease (PD). Catechol-O-methyl-transferase (COMT) inhibitors supplement the variety of antiparkinsonian drugs interacting with the dopaminergic system. Clinical studies show that COMT inhibitors prolong the action of levodopa in patients with the "wearing off" phenomenon. The atypical antipsychotic drug clozapine is the treatment of choice for the alleviation of levodopa-induced psychosis. Clozapine also has beneficial effects on tremor and levodopa-induced dyskinesias. Thus, COMT inhibitors and clozapine provide new opportunities for the treatment of patients with longstanding PD and fluctuating responses to levodopa. Experimental evidence in animals suggests that glutamate antagonists have symptomatic and neuroprotective actions in PD. At present, however, only weak antiglutamatergic drugs that have low specificity, such as memantine, amantadine, and budipine are available for clinical studies. Neurotrophic factors, in particular ciliary neurotrophic factor and glial cell line-derived neurotrophic factor, are among the most promising new approaches for neuroprotection in PD. Problems of bioavailability, however, thus far preclude their use in patients. An improved understanding of the pathophysiology of parkinsonism has led to a renaissance of stereotaxic surgery. The subthalamic nucleus is a potential new target for surgical intervention. Ventroposterior pallidotomy has been shown to improve not only rigidity and tremor, but also akinesia. The techniques for thalamic interventions have been refined by introducing chronic thalamic stimulation. Future transplantation approaches to PD will focus on the use of genetically modified cells carrying genes for dopamine-synthesizing enzymes or neurotrophic factors. Animal studies show the feasibility of in vivo gene transfer for the treatment of PD.

Animals↗
PubMed1999

Association study between high and low activity polymorphism of catechol-O-methyltransferase gene and alcoholism.

Catechol-O-methyltransferase (COMT) is a key modulator of dopaminergic and noradrenergic neurotransmission. There is a functional polymorphism of the COMT gene, Val108Met in the soluble form of the enzyme (Val158Met in the membrane-bound form). Involvement of the dopaminergic systems in alcoholism has been suggested in mice and humans. We examined associations between this polymorphism and alcoholism in 175 Japanese alcoholics and 354 age- and gender-matched Japanese controls. No significant difference in the allelic distributions in alcoholics and controls and no significant associations between antisocial behaviors in alcoholics and this polymorphism were observed. Therefore, the COMT gene is not likely to play a significant role in alcoholism.

Adult↗
PubMedDate not supplied

The controversy concerning plasma homocysteine in Parkinson disease patients treated with levodopa alone or with entacapone: effects of vitamin status.

Levodopa treatment of Parkinson disease results in hyperhomocysteinemia (HHcy) as a consequence of levodopa methylation by catechol-O-methyltransferase (COMT). Although inhibition of COMT should theoretically prevent or reduce levodopa-induced HHcy, results from several prospective studies are conflicting. Our review of these studies suggests that the ability of COMT inhibition to reduce or prevent levodopa-induced HHcy in Parkinson disease patients may be attributed to differences in the vitamin status of the study participants. In patients with low or low-normal folate levels, levodopa administration is associated with a greater increase in homocysteine and concomitant entacapone administration is associated with a greater reduction in homocysteine.

Catechol O-Methyltransferase↗
PubMed2006

Association of catecholamine-O-methyltransferase and 5-HTTLPR genotype with eating disorder-related behavior and attitudes in females with eating disorders.

OBJECTIVE: There is growing evidence, that genetic variants contribute to the pathogenesis of anorexia nervosa and bulimia nervosa. Genetic studies have revealed candidate genes, but no satisfactory associations with the disorders have been found so far. The aim of the present study was to evaluate, whether behavioral and attitudinal traits of the disorders can serve as phenotypes with a possible association with two common functional polymorphisms of the monoaminergic pathways. METHOD: Forty-five female in-patients of a specialized hospital for eating disorders were included into the study. Eating disorder symptomatology was assessed using the Eating Disorder Inventory-2. The functional catecholamine-O-methyltransferase (COMT) 158 Val-->Met polymorphism and the deletion/insertion polymorphism of the serotonin transporter promoter 5-HTTLPR were determined. RESULTS: Carriers of at least one Met-allele of the COMT gene had significantly higher total scores of the Eating Disorder Inventory-2, as well as significantly higher scores on the subscales bulimia, ineffectiveness, interoceptive awareness, maturity fears and impulse regulation. Carriers of the deletion of the 5-HTTLPR had significantly higher scores on the subscales drive for thinness and body dissatisfaction. CONCLUSION: We found associations between the COMT and the 5-HTTLPR polymorphisms and specific clinical, behavioral and attitudinal traits of eating disorders. These polymorphisms may predispose their carriers to exhibit certain symptoms of eating disorders or confer a general risk for more severe forms of these disorders.

Adolescent↗
PubMed2006

The high-activity Val allele of the catechol-O-methyltransferase gene predicts greater cognitive deterioration in patients with psychosis.

The objective of this study was to examine whether the functional genetic polymorphism Val158Met in the catechol-O-methyltransferase (COMT) gene influences cognitive deterioration in a sample of patients with psychosis under treatment with atypical antipsychotics. Eighty-seven patients with psychosis were genotyped for this polymorphism and were assessed with three Wechsler Adult Intelligence Scale (WAIS)-III subtests (Vocabulary, Information, and Digit Symbol-Coding). Performance on these three subtests was used to compute a 'cognitive deterioration index', and the effect of COMT genotype on this cognitive deterioration index was examined. A linear relationship between the number of Val alleles and the score on the cognitive deterioration index (i.e. the more Val alleles, the more cognitive deterioration) was observed. These results confirm the role of COMT genotype in the cognition of patients under treatment for psychosis, suggesting that it influences the extent of their cognitive deterioration.

Adolescent↗
PubMed2005

Water deficits affect caffeate O-methyltransferase, lignification, and related enzymes in maize leaves. A proteomic investigation.

Drought is a major abiotic stress affecting all levels of plant organization and, in particular, leaf elongation. Several experiments were designed to study the effect of water deficits on maize (Zea mays) leaves at the protein level by taking into account the reduction of leaf elongation. Proteomic analyses of growing maize leaves allowed us to show that two isoforms of caffeic acid/5-hydroxyferulic 3-O-methyltransferase (COMT) accumulated mostly at 10 to 20 cm from the leaf point of insertion and that drought resulted in a shift of this region of maximal accumulation toward basal regions. We showed that this shift was due to the combined effect of reductions in growth and in total amounts of COMT. Several other enzymes involved in lignin and/or flavonoid synthesis (caffeoyl-CoA 3-O-methyltransferase, phenylalanine ammonia lyase, methylenetetrahydrofolate reductase, and several isoforms of S-adenosyl-l-methionine synthase and methionine synthase) were highly correlated with COMT, reinforcing the hypothesis that the zone of maximal accumulation corresponds to a zone of lignification. According to the accumulation profiles of the enzymes, lignification increases in leaves of control plants when their growth decreases before reaching their final size. Lignin levels analyzed by thioacidolysis confirmed that lignin is synthesized in the region where we observed the maximal accumulation of these enzymes. Consistent with the levels of these enzymes, we found that the lignin level was lower in leaves of plants subjected to water deficit than in those of well-watered plants.

Gene Expression Regulation, Enzymologic↗