Methionine metabolism in apple tissue: implication of s-adenosylmethionine as an intermediate in the conversion of methionine to ethylene

Induction by fungal elicitor of S-adenosyl-L-methionine synthetase and S-adenosyl-L-homocysteine hydrolase mRNAs in cultured cells and leaves of Petroselinum crispum

Treatment of cultured parsley (Petroselinum crispum) cells with fungal elicitor rapidly activates transcription of many genes encoding specific steps in pathogen defense-related pathways. We report evidence that three cDNAs corresponding to such genes represent two key enzymes of the activated methyl cycle. Two cDNAs are derived from distinct members of the S-adenosyl-L-methionine synthetase gene family, based on extensive similarity of the deduced polypeptides with authentic enzymes from Arabidopsis thaliana, rat, yeast, and Escherichia coli. The third cDNA exhibits large similarity with a functionally related gene, encoding S-adenosyl-L-homocysteine hydrolase, from rat and a slime mold. Marked differences in the mRNA levels occurred in different organs of parsley plants. Elicitor treatment strongly induced both mRNAs in cultured cells as well as intact leaves and led to marked increases in S-adenosyl-L-homocysteine hydrolase enzyme activity. These results suggest a close metabolic link between pathogen defense and an increased turnover of activated methyl groups.

Metabolism of Benzyladenine is Impaired in a Mutant of Arabidopsis thaliana Lacking Adenine Phosphoribosyltransferase Activity

Formation of the riboside-5'-monophosphate is a general feature of the metabolism of cytokinins in plants. As part of a study of the biological significance of the nucleotide form of cytokinins, we analyzed a mutant of Arabidopsis thaliana deficient in adenine phosphoribosyltransferase (APRT) activity for its ability to metabolize N(6)-benzyladenine (BA). Formation of N(6)-benzyladenosine-5'-monophosphate (BAMP) was assayed in vivo, by feeding tritiated BA to wild-type and mutant plantlets, and in crude plantlet extracts. Metabolites were separated by high performance liquid chromatography and quantitated by on-line liquid scintillation spectrometry. BA was rapidly absorbed by A. thaliana plantlets and primarily converted to BAMP and to BA 7- and 9-glucosides. BA was also rapidly absorbed by APRT-deficient plantlets, but its conversion to BAMP was strongly reduced. Formation of BAMP from N(6)-benzyladenosine was not affected in the mutant plantlets. In vitro conversion of BA to its nucleoside-5'-monophosphate was detected in crude extracts of wild-type plantlets, but not in extracts of APRT-deficient plantlets. Therefore, results of both assays indicate that APRT-deficient tissue does not convert BA to BAMP to a significant extent. Further, nondenaturing isoelectric focusing analysis of APRT activity in leaf extracts indicated that the enzyme activities which metabolize adenine and BA into their corresponding riboside-5'-monophosphate in extracts of wild-type plantlets have the same apparent isoelectric point. These activities were not detected in extracts prepared from APRT-deficient plantlets. Thus, these results demonstrate that APRT is the main enzyme which converts BA to its nucleotide form in young A. thaliana plants and that the ribophosphorylation of BA is not a prerequisite of its absorption by the plantlets.

Enzymes of ethylene biosynthesis

The properties of enzymes involved in ethylene biosynthesis are reviewed and progress toward the purification of these enzymes is described. The enzyme whose activity usually limits ethylene biosynthesis is 1-aminocyclopropane-1-carboxylate (ACC) synthase. Even though its level in plants is extremely low, it has now been purified from several sources. The enzyme that converts ACC to ethylene does not survive homogenization, apparently because it is membrane-bound and because its activity requires membrane integrity. Properties of this enzyme have been elucidated in vivo and in vacuolar preparations which possess the capacity to convert ACC to ethylene.

Enhanced ethylene emissions from red and norway spruce exposed to acidic mists

Acidic cloudwater is believed to cause needle injury and to decrease winter hardiness in conifers. During simulations of these adverse conditions, rates of ethylene emissions from and levels of 1-aminocyclopropane-1-carboxylic acid (ACC) in both red and Norway spruce needles increased as a result of treatment with acidic mists but amounts of 1-malonyl(amino)cyclopropane-1-carboxylic acid remained unchanged. However, release of significant quantities of ethylene by another mechanism independent of ACC was also detected from brown needles. Application of exogenous plant growth regulators such as auxin, kinetin, abscisic acid and gibberellic acid (each 0.1 millimolar) had no obvious effects on the rates of basal or stress ethylene production from Norway spruce needles. The kinetics of ethylene formation by acidic mist-stressed needles suggest that there is no active inhibitive mechanism in spruce to prevent stress ethylene being released once ACC has been formed.

Metabolism of 5-methylthioribose to methionine

During ethylene biosynthesis, the H(3)CS- group of S-adenosylmethionine is released as 5'-methylthioadenosine, which is recycled to methionine via 5-methylthioribose (MTR). In mungbean hypocotyls and cell-free extracts of avocado, [(14)C]MTR was converted into labeled methionine via 2-keto-4-methylthiobutyric acid (KMB) and 2-hydroxy-4-methylthiobutyric acid (HMB), as intermediates. Incubation of [ribose-U-(14)C]MTR with avocado extract resulted in the production of [(14)C]formate, indicating the conversion of MTR to KMB involves a loss of formate, presumably from C-1 of MTR. Tracer studies showed that KMB was converted readily in vivo and in vitro to methionine, while HMB was converted much more slowly. The conversion of KMB to methionine by dialyzed avocado extract requires an amino donor. Among several potential donors examined, l-glutamine was the most efficient. Anaerobiosis inhibited only partially the oxidation of MTR to formate, KMB/HMB, and methionine by avocado extract. The role of O(2) in the conversion of MTR to methionine is discussed.

5'-Methylthioadenosine Nucleosidase and 5-Methylthioribose Kinase Activities and Ethylene Production during Tomato Fruit Development and Ripening

5'-Methylthioadenosine (MTA) nucleosidase and 5-methylthioribose (MTR) kinase activities were measured in crude extracts of tomato fruits (Lycopersicon esculentum Mill cv Rutgers) during fruit development and ripening. The highest activity of MTA nucleosidase (1.2 nanomoles per milligram protein per minute) was observed in small green fruits. The activity decreased during ripening; at the overripe stage only 6.5% of the peak activity remained. MTR kinase activity was low at the small green stage and increased thereafter until it reached peak activity at the breaker stage (0.7 nanomoles per milligram protein per minute) followed by a sharp decline at the later stages of fruit ripening. 1-Amino-cyclopropane-1-carboxylic acid (ACC) levels peaked at the red stage, while ethylene reached its highest level at the light-red stage. Several analogs of MTA and MTR were tested as both enzyme and ethylene inhibitors. Of the MTA analogs examined for their ability to inhibit MTA nucleosidase, 5'-chloroformycin reduced enzyme activity 89%, whereas 5'-chloroadenosine, 5'-isobutylthioadenosine, 5'-isopropylthioadenosine, and 5'-ethylthioadenosine inhibited the reaction with MTA by about 40%. 5'-Chloroformycin and 5'-chloroadenosine inhibited ethylene production over a period of 24 hours by about 64 and 42%, respectively. Other analogs of MTA were not effective inhibitors of ethylene production, whereas aminoethoxyvinylglycine showed a 34% inhibition over the same period of time. Of the MTR analogs tested, 5-isobutylthioribose was the most effective inhibitor of both MTR-kinase (41%) and ethylene production (35%).

Quantitative analysis of pathways of methionine metabolism and their regulation in lemna

Individual rates of metabolism of the sulfur, methyl, and 4-carbon moieties of methionine were estimated in Lemna paucicostata Hegelm. 6746 growing under standard conditions, and used to quantitate pathways of methionine metabolism. Synthesis of S-adenosylmethionine (AdoMet) is the major pathway for methionine metabolism, with over 4 times as much methionine metabolized by this route as accumulates in protein. More than 90% of AdoMet is used for transmethylation. Methyl groups of choline, phosphatidylcholine, and phosphorylcholine are major end products of this pathway. Flux through methylthio recycling is about one-third the amount of methionine accumulating in protein. Spermidine synthesis accounts for at least 60% of the flux through methylthio recycling. The results obtained here, together with those reported for methionine-supplemented plants (Giovanelli, Mudd, Datko 1981 Biochem Biophys Res Commun 100: 831-839), indicate that methionine supplementation reduced methylneogenesis by no more than the small amount expected from the reduced entry of sulfate sulfur into methionine (Giovanelli, Mudd, Datko, 1985 Plant Physiol 77: 450-455). Methionine supplementation had no significant effect on transmethylation or methylthio recycling. The combined data provide the first comprehensive estimates of the quantitative relationships of major pathways for methionine metabolism and their control in plants.

Dicyclohexylamine-induced shift of biosynthesis from spermidine to spermine in plant protoplasts

An improved analytical method, based on high pressure liquid chromatography, has been developed for the simultaneous determination of the polyamines and S-adenosyl-containing compounds in extracts of plant protoplasts. The method involves simple procedures for sample preparation and permits quantification of 1 picomole or less for all the compounds. This method has been used to study the effects of dicyclohexylamine, an inhibitor of plant spermidine synthase (Sindhu, R. K., S. S. Cohen 1984 Plant Physiol 74: 645-649), on biosynthesis of polyamines and 1-aminocyclopropane-1-carboxylate in protoplasts derived from Chinese cabbage leaves. Dicyclohexylamine effectively inhibits spermidine synthase in vivo. Inhibition of the synthesis of spermidine by dicyclohexylamine resulted in a stimulation of spermine synthesis, without significant effect on the synthesis of 1-aminocyclopropane-1-carboxylate. Decarboxylated S-adenosylmethionine is present in control Chinese cabbage protoplasts at approximately 10(-18) moles per cell, and dicyclohexylamine caused an increase of this metabolite of up to 10-fold in a 4-hour period. The increase in decarboxylated S-adenosylmethionine permitted an increased synthesis of spermine. These findings suggest that the availability of decarboxylated S-adenosylmethionine may be rate-limiting for the synthesis of spermine in plant protoplasts.

A radioisotope assay for 1-aminocyclopropane-1-carboxylic acid synthase: S-adenosylhomocysteine analogs as inhibitors of the enzyme involved in plant senescence

A simple and rapid radioisotopic assay for 1-aminocyclopropane-1-carboxylic acid (ACC) synthase was developed, an enzyme involved in the biosynthesis of the plant hormone ethylene. The assay utilizes an AG50W-X4(NH+4) column which separates S-adenosyl-L-[carboxyl-14C]methionine (AdoMet) from the product [14C]ACC, since the latter is not bound to the resin while [14C]AdoMet is. As opposed to other assays, this procedure measures ACC directly and does not require further conversion to ethylene. When an enzyme preparation from ripe tomato fruits (Lycopersicon esculentum Mill). was assayed, an I50 of 2.5 +/- 0.8 microM for sinefungin and a Km of 27 +/- 2 microM for AdoMet were obtained; these values were in good agreement with previous determinations made with a gas chromatographic assay. When other nucleosides were tested as inhibitors, the following order of decreasing activity was found: sinefungin greater than S-adenosylhomocysteine (AdoHcy) greater than AdoHcy sulfoxide greater than S-n-butyladenosine greater than 3-deaza-adenosylhomocysteine greater than S-isobutyladenosine greater than S-isobutyl-1-deazaadenosine. In contrast, S-isobutyl-3-deazaadenosine, S-isobutyl-7-deazaadenosine, 3-deazaadenosine, and adenosine were not inhibitory.

Thigmomorphogenesis: on the mechanical properties of mechanically perturbed bean plants

The mechanical properties of control and mechanically perturbed (MP) bean stems (Phaseolus vulgaris L., cv. Cherokee wax) were compared. The rubbed plants were greatly hardened against mechanical rupture by previous MP. This hardening was due to a dramatic increase in the flexibility of the stems, but not in their stiffness. The MP-plants were able to bend more than 90 degree without breaking, whereas the control plants broke after just slight bending. A comparison with other work reveals that different species utilize different tactics for achieving similar resistance to rupture due to mechanical stress.

Intermediates in the recycling of 5-methylthioribose to methionine in fruits

The recycling of 5-methylthioribose (MTR) to methionine in avocado (Persea americana Mill, cv Hass) and tomato (Lycopersicum esculentum Mill, cv unknown) was examined. [(14)CH(3)]MTR was not metabolized in cell free extract from avocado fruit. Either [(14)CH(3)]MTR plus ATP or [(14)CH(3)]5-methylthioribose-1-phosphate (MTR-1-P) alone, however, were metabolized to two new products by these extracts. MTR kinase activity has previously been detected in these fruit extracts. These data indicate that MTR must be converted to MTR-1-P by MTR kinase before further metabolism can occur. The products of MTR-1-P metabolism were tentatively identified as alpha-keto-gamma-methylthiobutyric acid (alpha-KMB) and alpha-hydroxy-gamma-methylthiobutyric acid (alpha-HMB) by chromatography in several solvent systems. [(35)S]alpha-KMB was found to be further metabolized to methionine and alpha-HMB by these extracts, whereas alpha-HMB was not. However, alpha-HMB inhibited the conversion of alpha-KMB to methionine. Both [U-(14)C]alpha-KMB and [U-(14)C]methionine, but not [U-(14)C]alpha-HMB, were converted to ethylene in tomato pericarp tissue. In addition, aminoethoxyvinylglycine inhibited the conversion of alpha-KMB to ethylene. These data suggest that the recycling pathway leading to ethylene is MTR --> MTR-1-P --> alpha-KMB --> methionine --> S-adenosylmethionine --> 1-aminocyclopropane-1-carboxylic acid --> ethylene.

Intracellular localization of aspartate kinase and the enzymes of threonine and methionine biosynthesis in green leaves

The intracellular localization of several aspartate pathway enzymes has been studied in pea (Pisum sativum cv Feltham First) and barley (Hordeum vulgare cv Julia) leaves. Protoplast lysates were fractionated by differential or sucrose density gradient centrifugation, in media optimized for each enzyme. The results show that aspartate kinase, homoserine kinase, threonine synthase, and cystathionine gamma-synthase are confined to the chloroplast. Cystathionine beta-lyase appears to be present in several fractions, though more than 50% of the total activity is associated with the chloroplasts. In contrast, neither methionine synthase nor methionine adenosyl-transferase were significantly associated with chloroplasts, and only a small proportion of the methionine synthase was associated with the mitochondrial fraction. Methionine adenosyltransferase, and hence S-adenosylmethionine synthesis, is not found in any organelle fraction. The conclusion is that whereas threonine, like lysine, is synthesized only in the chloroplast, the last step in methionine biosynthesis occurs largely in the cytoplasm.

Plant 5-methylthioribose kinase: properties of the partially purified enzyme from yellow lupin (lupinus luteus L.) seeds

Activity of 5-methylthioribose kinase, the enzyme which catalyzes the ATP-dependent formation of 1-phospho-5-methylthioribose, has been revealed in the extracts from various higher plant species. Almost 2,000-fold-purified enzyme has been obtained from yellow lupin (Lupinus luteus L. cv Topaz) seed extract. Molecular weight of the native enzyme is 70,000 as judged by gel filtration. The lupin 5-methylthioribose kinase exhibits a strict requirement for divalent metal ions. Among the ions tested, only Mg(2+) and Mn(2+) acted as cofactors. The curve of kinase initial velocity versus pH reaches plateau at pH 10 to 10.5. The K(m) values calculated for 5-methylthioribose and ATP are 4.3 and 8.3 micromolar, respectively.Among nucleoside triphosphates tested as potential phosphate donors, only dATP could substitute in the reaction for ATP. 5-Isobutylthioribose, an analog of 5-methylthioribose, proved to be the gamma-ATP-phosphate acceptor, too. The compound inhibits competitively synthesis of 1-phospho-5-methylthioribose (K(i) = 1.4 micromolar). Lupin 5-methylthioribose kinase is completely and irreversibly inhibited by the antisulfhydryl reagent, p-hydroxymercuribenzoate. As in bacteria (Ferro, Barrett, Shapiro 1978 J Biol Chem 253: 6021-6025), the enzyme may be involved in a new, alternative pathway of methionine synthesis in plant tissues.

In vivo metabolism of 5'-methylthioadenosine in lemna

Evidence is presented that Lemna converts 5'-methylthioadenosine (MTA) to methionine. The methylthio moiety and four of the ribose carbons of the nucleoside contribute the methylthio and the four-carbon moieties of methionine. Plants grown in the presence of inhibitors which block methionine biosynthesis convert MTA to methionine at a rate sufficient to sustain normal growth (at least 4.4 nanomoles per colony per doubling with a molar yield of at least 65%). The pathway for conversion is shown to be constitutive in plants grown in standard medium and to function at a rate sufficient to dispose of MTA arising as a result of polyamine synthesis, and to explain the observed rate (1.4 nanomoles per colony per doubling) of preferential recycling of methionine sulfur (Giovanelli, Mudd, Datko 1981 Biochem Biophys Res Commun 100: 831-839). Rapid entry of methionine methyl into S-adenosylmethionine and phosphorylcholine was observed for plants grown in standard medium. Adenine generated during this cycle is efficiently salvaged into ADP and ATP.Conversion of MTA to methionine completes the steps in methionine thiomethyl recycling (Giovanelli, Mudd, Datko 1981 Biochem Biophys Res Commun 100: 831-839) in which the sulfur of methionine is retained while the four-carbon moiety is not. The findings further show that the four-carbon moiety of methionine can be derived via the ribose moiety of MTA in addition to the established route from O-phosphohomoserine via transsulfuration. Previous observations (Giovanelli, Mudd, Datko 1980 Biochemistry of Plants pp 453-505) can now be interpreted as establishing that exogenous methionine down-regulates its own net synthesis via the transsulfuration pathway.

Metabolism of 5'-methylthioadenosine in Aspergillus nidulans. An alternative pathway for methionine synthesis via utilization of the nucleoside methylthio group

Experiments in which 5'-methylthioadenosine was used as a culture supplement for methionine-requiring mutants of Aspergillus nidulans with various enzymatic lesions indicated that the methylthio group derived from the nucleoside can be recycled to methionine. The results strongly suggest that methionine may be synthesized in the reaction catalyzed by homocysteine synthase (EC 4.2.99.10) in which O-acetylhomoserine is an acceptor of the methylthio group. The first step on the salvage pathway of the methylthio group is, in Aspergillus nidulans, phosphorolytic cleavage of 5'-methylthioadenosine to adenine and 5-methylthioribose 1-phosphate catalyzed by a specific phosphorylase.

Recycling of 5'-methylthioadenosine-ribose carbon atoms into methionine in tomato tissue in relation to ethylene production

The ribose moiety of 5'-methylthioadenosine (MTA) is metabolized to form the four-carbon unit (2-aminobutyrate) of methionine in tomato tissue (Lycopersicon esculentum Mill., cv. Pik Red). When [U-(14)C-adenosine] MTA was administered to tomato tissue slices, label was recovered in 5-methylthioribose (MTR), methionine, 1-aminocyclopropane-1-carboxylic acid (ACC), C(2)H(4) and other unidentified compounds. However, when [U-(14)C-ribose]MTR was administered, radioactivities were recovered in methionine, ACC and C(2)H(4), but not MTA. This suggests that C(2)H(4) formed in tomato pericarp tissue may be derived from the ribose portion of MTA via MTR, methionine and ACC. The conversion of MTR to methionine is not inhibited by aminoethoxyvinylglycine (AVG), but is O(2) dependent. These data present a new salvage pathway for methionine biosynthesis which may be important in relation to polyamine and ethylene biosynthesis in tomato tissue.

Polyamines inhibit biosynthesis of ethylene in higher plant tissue and fruit protoplasts

Ethylene production in apple fruit and protoplasts and in leaf tissue was inhibited by spermidine or spermine. These polyamines, as well as putrescine, inhibited auxin-induced ethylene production and the conversion of methionine and 1-aminocyclopropane-1-carboxylic acid to ethylene. Polyamines were more effective as inhibitors of ethylene synthesis at the early, rather than at the late, stages of fruit ripening. Ca(2+) in the incubation medium reduced the inhibitory effect caused by the amines. A possible mode of action by which polyamines inhibit ethylene production is discussed.

5'-Methylthioadenosine nucleosidase. Purification and characterization of the enzyme from Lupinus luteus seeds

5'-Methylthioadenosine nucleosidase (EC 3.2.2.9), the enzyme which catalyzes hydrolytic cleavage of 5'-methylthioadenosine with the formation of adenine and 5'-methylthioribose, has been purified to homogeneity from Lupinus luteus seeds. The nucleosidase has a native molecular weight of 62 000 and consists of two identical subunits, as judged by gel filtration and dodecylsulfate/polyacrylamide gel electrophoresis. The nucleosidase exhibits highest specificity towards the natural substrate with a Km of 4.1 X 10(-7) M for 5'-methylthioadenosine. It does not cleave adenine from S-adenosylhomocysteine. Among the synthetic analogs of 5'-methylthioadenosine tested, eleven compounds appear to be able to substitute as substrates. Furthermore, the enzyme can liberate hypoxanthinine from six inosyl (deaminated) derivatives obtained by enzymatic deamination of 5'-methylthioadenosine and its synthetic analogs. The Km for 5'-methylthioinosine is 55 microM, and the maximal velocity about 50-times lower than for 5'-methylthioadenosine. The reaction catalyzed by the nucleosidase can be inhibited by adenine (Ki = 11 microM), 3-deazaadenine (Ki = 19 microM), and 9-erythro(2-hydroxyl-3-nonyl)adenine (ki = 37 microM).

Some Characteristics of the System Converting 1-Aminocyclopropane-1-carboxylic Acid to Ethylene

The rate of C(2)H(4) production in plant tissue appears to be limited by the level of endogenous 1-aminocyclopropane-1-carboxylic acid (ACC). Exogenous ACC stimulated C(2)H(4) production considerably in plant tissues, but this required 10 to 100 times the endogenous concentrations of ACC before significant increases in C(2)H(4) production were observed. This was partially due to poor penetration of ACC into the tissues. Conversion of ACC to C(2)H(4) was inhibited by free radical scavengers, reducing agents, and copper chelators, but not by inhibitors of pyridoxal phosphate-mediated reactions. The system for converting ACC to C(2)H(4) may be membrane-associated, for it did not survive treatment with surface-active agents and cold or osmotic shock reduced the capacity of the system to convert ACC to C(2)H(4). The reaction rate was sensitive to temperatures above 29 and below 12 C, which suggests that the system may be associated with membrane-bound lipoproteins. The data presented support the possibility that the conversion of exogenous ACC to C(2)H(4) proceeds via the natural physiological pathway.

Effect of 1-Aminocyclopropane-1-Carboxylic Acid on the Production of Ethylene in Senescing Flowers of Ipomoea tricolor Cav

Application of 1-aminocyclopropane-1-carboxylic acid (ACC) to rib segments excised from flowers of Ipomoea tricolor Cav. resulted in the formation of C(2)H(4) in greater quantities than produced under natural conditions. The ability of ACC to enhance C(2)H(4) production was independent of the physiological age of the tissue and its capacity to synthesize C(2)H(4) without applied ACC. When ACC was fed to rib segments that had been treated with [(14)C]methionine, incorporation of radioactivity into C(2)H(4) was reduced by 80%. Aminoethoxyvinylglycine and aminooxyacetic acid inhibited C(2)H(4) production in rib segments of I. tricolor but had no effect on ACC-enhanced C(2)H(4) production. Protoplasts obtained from flower tissue of I. tricolor did not form C(2)H(4), even when incubated with methionine or selenomethionine. They produced C(2)H(4) upon incubation with ACC, however. ACC-dependent C(2)H(4) production in protoplasts was inhibited by n-propyl gallate, AgCl, CoCl(2), KCN, Na(2)S, and NaN(3). ACC-dependent C(2)H(4) synthesis in rib segments and protoplasts was dependent on O(2), the K(m) for O(2) being 1.0 to 1.4% (v/v). These results confirm the following pathway for C(2)H(4) biosynthesis in I. tricolor. methionine [selenomethionine] --> S-adenosylmethionine [selenoadenosylmethionine] --> ACC --> C(2)H(4).

Inhibition of ethylene production by 2,4-dinitrophenol and high temperature

2,4-Dinitrophenol (DNP) and high temperature (35 to 40 C) are known to inhibit C(2)H(4) production in various plant tissues. The present study was made to determine the step in the C(2)H(4) biosynthetic pathway (methionine --> S-adenosylmethionine [SAM] --> 1-aminocyclopropane-1-carboxylic acid [ACC] --> C(2)H(4)) at which these treatments exert their inhibitory effect. In mung bean hypocotyls the dose-inhibition curves for the effect of DNP on auxin-dependent C(2)H(4) production (in which auxin exerts its effect by stimulating the conversion of SAM to ACC) and on ACC-dependent C(2)H(4) production (in which ACC is directly utilized as precursor) were similar. It was concluded, therefore, that DNP at low concentrations (below 50 micromolar) exerted its effect primarily on the conversion of ACC to C(2)H(4), a step which is common to both systems. This view was further substantiated by quantitative analysis of the intermediates in the biosynthetic sequence. DNP exerted little influence on the content of SAM but caused a significant increase in the ACC content and marked inhibition in C(2)H(4) production, indicating that the conversion of ACC to C(2)H(4) is the crossover point. At higher concentrations (above 100 micromolar), DNP inhibited the conversion of methionine to ACC and to C(2)H(4), and this effect could be attributed to the inhibition of SAM synthesis.The optimal temperature for maximal C(2)H(4) production by apple tissue and mung bean hypocotyl is about 30 C. An increase in temperature to 35 C caused an accumulation of endogenous ACC, whereas C(2)H(4) production was greatly reduced. These results suggest that the conversion of ACC to C(2)H(4) is highly vulnerable to high temperature inhibition.

Formation of ethylene from methionine. Reactivity of radiolytically produced oxygen radicals and effect of substrate activation

Ethylene was determined by gas chromatography after reaction of radiolytically produced OH and O2- radicals with methionine, methionine + pyridoxal phosphate and S-adenosyl-methionine (SAM). Both oxygen radicals, alone or in combination, liberate ethylene from methionine and methionine/pyridoxal phosphate. From SAM ethylene was primarily produced by the combined attack of OH nad H2O2 or O2-.

Biosynthesis of ethylene from methionine. Isolation of the putative intermediate 4-methylthio-2-oxobutanoate from culture fluids of bacteria and fungi

Methods are described for identifying the 2,4-dinitrophenylhydrazones of 4-methylthio-2-oxobutanoate by means of t.l.c., n.m.r. and mass spectroscopy. By using these methods 4-methylthio-2-oxobutanoate, a putative intermediate in the biosynthesis of ethylene from methionine, has been identified in culture fluids of Aeromonas hydrophila B12E and a coryneform bacterium D7F grown in the presence of methionine. Relative to 4-methylthio-2-oxobutanoate, the yield of 3-(methylthio)propanal (methional) from the same cultures was less than 1%. Because 4-[2H]methylthio-2-oxobutanoate was obtained from cultures grown on [Me-2H]methionine, the 4-methylthio-2-oxobutanoate must be derived from methionine. By means of t.l.c. alone, 4-methylthio-2-oxobutanoate was identified in the culture fluids of a range of bacteria, the yeast Saccharomyces cerevisiae and the fungus Penicillium digitatum. A photochemical assay developed for 4-methylthio-2-oxobutanoate shows it to be a product of the metabolism of methionine by Escherichia, Pseudomonas, Bacillus, Acinetobacter, Aeromonas, Rhizobium and Corynebacterium species.

Studies of ethylene-forming system in rat liver extract

Evidence of enzymatic formation of ethylene from methionine by rat liver extracts is presented. The ethylene production is closely associated with growth of the animal. The conversion of L-methionine to ehtylene is oxygen dependent. Substrate analogue studies show that the ethylene-forming system is structurally specific and requires in the center of the molecule alpha-CH2-CH2- with one end attached to an unencumbered sulfur atom from a thioether moiety and the other end attached to a carboxyl group. Sylfhydryl agents are found to be very effective inhibitors of the ethylene-forming system. The finding of alpha-keto-4-methylthiobutyric acid to be a more efficient precursor of ethylene production suggests the possibility that alpha-keto-4-methylthiobutyric acid may be an intermediate in the biosynthesis of ethylene from methionine in mammalian tissues.

Methionine metabolism and ethylene formation in etiolated pea stem sections

Stem sections of etiolated pea seedlings (Pisum sativum L. cv. Alaska) were incubated overnight on tracer amounts of l-[U-(14)C]methionine and, on the following morning, on 0.1 millimolar indoleacetic acid to induce ethylene formation. Following the overnight incubation, over 70% of the radioactivity in the soluble fraction was shown to be associated with S-methylmethionine (SMM). The specific radioactivity of the ethylene evolved closely paralleled that of carbon atoms 3 and 4 of methionine extracted from the tissue and was always higher than that determined for carbon atoms 3 and 4 of extracted SMM.Overnight incubation of pea stem sections on 1 millimolar methionine enhanced indoleacetic acid-induced ethylene formation by 5 to 10%. Under the same conditions, 1 millimolar homocysteine thiolactone increased ethylene synthesis by 20 to 25%, while SMM within a concentration range of 0.1 to 10 millimolar did not influence ethylene production. When unlabeled methionine or homocysteine thiolactone was applied to stem sections which had been incubated overnight in l-[U-(14)C]methionine, the specific radioactivity of the ethylene evolved was considerably lowered. Application of unlabeled SMM reduced the specific radioactivity of ethylene only slightly.

Interactions of Methionine and Selenomethionine with Methionine Adenosyltransferase and Ethylene-generating Systems

Since selenomethionine appears to be a better precursor of ethylene in senescing flower tissue of Ipomoea tricolor and in indole acetic acid-treated pea stem sections than is methionine (Konze JR, N Schilling, H Kende 1978 Plant Physiol 62: 397-401), we compared the effectiveness of selenomethionine and methionine to participate in reactions which may be connected to ethylene biosynthesis. Evidence is presented that selenomethionine is also a better substrate of methionine adenosyltransferase (ATP: methionine S-adenosyltransferase, EC 2.5.1.6) from I. tricolor, the V(max) for selenomethionine being twice as high as that for methionine. The affinity of the enzyme is higher for methionine than for selenomethionine, however. Methionine added to flower tissue together with selenomethionine inhibits the enhancement of ethylene synthesis by the seleno analog. Likewise, methionine reduces the high, selenomethionine-dependent reaction rates of methionine adenosyltransferase from I. tricolor flower tissue. On the other hand, selenomethionine is less effective as an ethylene precursor than is methionine in model systems involving oxidation by free radicals. It was concluded that activation of methionine by methionine adenosyltransferase and formation of S-adenosylmethionine are more likely to be involved in ethylene biosynthesis than is oxidation of methionine by free radicals.

Assay for and enzymatic formation of an ethylene precursor, 1-aminocyclopropane-1-carboxylic acid

A simple and sensitive chemical assay was developed for 1-aminocyclopropane-1-carboxylic acid (ACC), a precursor of ethylene. The assay is based on the liberation of ethylene from ACC at pH 11.5 in the presence of pyridoxal phosphate, MnCl2 and H2O2. This assay was used to detect ACC in extracts of tomato fruits (Lycopersicon esculentum Mill.) and to measure the activity of a soluble enzyme from tomato fruit that converted S-adenosylmethionine (SAM) to ACC. The enzyme had a Km of 13 μM for SAM, and conversion of SAM to ACC was competitively and reversibly inhibited by aminoethoxyvinylglycine (AVG), an analog of rhizobitoxine. The Ki value for AVG was 0.2 μM. The level of the ACC-forming enzyme activity was positively correlated with the content of ACC and the rate of ethylene formation in wild-type tomatoes of different developmental stages. Mature fruits of the rin (non-ripening) mutant of tomato, which only produce low levels of ethylene, contained much lower levels of ACC and of the ACC-forming enzyme activity than wild-type tomato fruits of comparable age.

The regulation of methionine biosynthesis and metabolism in plants and bacteria

The amino acids biosynthetically derived from asparate including methionine are all essential in the diet of monogastric animals. Most of this requirement is met by plant foods. The methionine biosynthetic pathways in plants and bacteria are outlined and compared. Regulation in bacterial systems is by a combination of repression and feedback inhibition whereas in plants repression is unimportant. Several enzymes in the branched pathway to methionine in plants are regulated by feedback inhibition; others are yet to be investigated. In plants may amino acid biosynthetic enzymes are localized in plastids and this is also likely for methionine biosynthesis. Methionine occupies an important position in cellular metabolism where the processes of one-carbon transfer via S-adenosylmethionine, protein synthesis, protein initiation and ethylene synthesis are interlocked. Attempts to increase the levels of free methionine have been made by selecting for plant mutants resistant to lysine plus threonine. One dominant mutation causes elevation of free amino acid levels in vegetative tissues but also has undesirable side-effects. The potential of such approaches is discussed.

Ethylene formation from 1-aminocyclopropane-1-carboxylic acid in homogenates of etiolated pea seedlings

Homogenates of etiolated pea (Pisum sativum L.) shoots formed ethylene upon incubation with 1-aminocyclopropane-1-carboxylic acid (ACC). In-vitro ethylene formation was not dependent upon prior treatment of the tissue with indole-3-acetic acid. When homogenates were passed through a Sephadex column, the excluded, high-molecular-weight fraction lost much of its ethylene-synthesizing capacity. This activity was largely restored when a heat-stable, low-molecular-weight factor, which was retarded on the Sephadex column, was added back to the high-molecular-weight fraction. The ethylene-synthesizing system appeared to be associated, at least in part, with the particulate fraction of the pea homogenate. Like ethylene synthesis in vivo, cell-free ethylene formation from ACC was oxygen dependent and inhibited by ethylenediamine tetraacetic acid, n-propyl gallate, cyanide, azide, CoCl3, and incubation at 40°C. It was also inhibited by catalase. In-vitro ethylene synthesis could only be saturated at very high ACC concentrations, if at all. Ethylene production in pea homogenates, and perhaps also in intact tissue, may be the result of the action of an enzyme that needs a heat-stable cofactor and has a very low affinity for its substrate, ACC, or it may be the result of a chemical reaction between ACC and the product of an enzyme reaction. Homogenates of etiolated pea shoots also formed ethylene with 2-keto-4-mercaptomethyl butyrate (KMB) as substrate. However, the mechanism by which KMB is converted to ethylene appears to be different from that by which ACC is converted.

Enhancement of ethylene formation by selenoamino acids

Selenomethionine and selenoethionine enhanced ethylene production in senescing flower tissue of Ipomoea tricolor Cav. and in auxin-treated pea (Pisum sativum L.) stem sections. This enhancement was fully inhibited by the aminoethoxy analog of rhizobitoxine. Methionine did not have a comparable promotive effect, and ethionine partly inhibited ethylene production. When [(14)C]methionine was applied to flower or pea stem tissue followed by treatment with unlabeled selenomethionine or selenoethionine, the specific radioactivity of the ethylene evolved was considerably reduced. The dilution of the specific radioactivity of ethylene by selenomethionine, and in pea stem sections also by selenoethionine, was greater than the dilution by nonradioactive methionine at the same concentration. These results indicate that both selenoamino acids serve as precursors of ethylene and that they are converted to ethylene more efficiently than is methionine.