Enhancement of Ethylene Formation by Selenoamino Acids

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Application of 1-aminocyclopropane-1-carboxylic acid (ACC) to rib segments excised from flowers of Ipomoea tricolor Cav. resulted in the formation of C2H4 in greater quantities than produced under natural conditions. The ability of ACC to enhance C2H, production was independent of the physiological age of the tissue and its capacity to synthesize C2H4 without applied ACC. When ACC was fed to rib segments that had been treated with 114Clmethionine, incorporation of radioacthivty into C2H4 was reduced by 80%. Aminethoxyvinylglycine and aminooxyacetic acid inhibited C2H4 production in rib segments of L tricolor but had no effect on ACC-enhanced C2H4 production. Protoplasts obtained from flower tissue of L tricolor did not form C2H4, even when incubated with methie or selenomethionine. They produced C2H4 upon incubation with ACC, however. ACC-dependent C2H4 production in protoplasts was inhibited by n-propyl gaDlate, AgCl, CoC12, KCN, Na2S, and NaN3. ACC-depndent C2H4 synthesis in rib segments and protoplasts was dependent on 02, the Km for 02 being 1.0 to 1.4% (v/v

mentioning

confidence: 99%

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

Konze¹,

Jones²,

Böller³

et al. 1980

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“…In some cases, added methionine was found to enhance ethylene synthesis in plant tissues (4). However, methionine had little effect upon ethylene production in flower tissue of I. tricolor (6) in which ethylene synthesis was enhanced and occurred prematurely in the presence of homocysteine thiolactone (5). This treatment also advanced aging of the flower tissue.…”

mentioning

confidence: 99%

Methionine Metabolism and Ethylene Formation in Etiolated Pea Stem Sections

Schilling¹,

Kende²

1979

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Stem sections of etiolated pea seedlings (Pisum sativum L. cv. Alaska) were incubated overnight on tracer amounts of L-IU-'4Clmethionine and, on the following morning, on 0.1 mnllmlar 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 Smethylmethionine (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 milimolar methionine enhanced indoleacetic acid-induced ethylene formation by 5 to 10%. Under the same conditions, 1 mil;imoar homocysteine thiolactone increased ethylene synthesis by 20 to 25%, while SMM within a concentration range of 0.1 to 10 miilimlar did not influence ethylene production. When unlabeled methionine or homocysteine thiolactone was applied to stem sections which had been incubated overnight in L-IU-_4Clmethlonine, the specific radioactivity of the ethylene evolved was considerably lowered. Application of unlabeled SMM reduced the specific radioactivity of ethylene only slightly.Tissues of higher plants produce ethylene when subjected to stress, during ripening and senescence or after application of auxin (for a review see ref. 1). Methionine has been shown to be the precursor of ethylene in a number of higher plants (for reviews see refs. 1 and 10). Recently, S-adenosylmethionine was proposed as an intermediate in the conversion of methionine to ethylene in apple tissue (3, 9). Results of Hanson and Kende (5) indicated that ethylene production in flower tissue of Ipomoea tricolor might be dependent on methionine derived from SMM3 and homocysteine.This investigation using etiolated pea stem sections was carried out to determine: (a) whether all ethylene evolved as a result of IAA treatment is derived from carbon atoms 3 and 4 of methionine; (b) whether SMM is a metabolite of methionine; and (c) whether induction of ethylene synthesis is accompanied by formation of methionine from SMM and homocysteine as found earlier in flower tissue of Ipomoea tricolor (5).

“…Most recent studies concerned with the interactions of selenocompounds in plant systems have been carried out with the intact plant or with cultured cells (6,11,22). A few comparable in vitro plant studies have focused on the following three aspects of selenium biochemistry: (a) aminoacylation of tRNA with selenoamino acids (2,3,21,24); (b) interactions of selenomethionine with methionine adenosyltransferase (12); and (c) the translational incorporation of selenomethionine into protein (5).…”

Section: Discussionmentioning

confidence: 99%

“…One of these analogs, selenomethionine, is known to be formed by a wide variety of plants (7,9,(16)(17)(18). Although the biochemical characteristics of selenomethionine resemble those of methionine in a great many ways, the metabolic activity of selenomethionine apparently differs in certain respects; one of these differences is, in general, reaction rate (4,5,11,12). Examination of those reactions in which selenomethionine functions at a rate different than methionine could reveal some of the factors that underlie selenium toxicity.…”

Section: Methodsmentioning

confidence: 99%

Selenium Toxicity: Aminoacylation and Peptide Bond Formation with Selenomethionine

Eustice¹,

Kull²,

Shrift³

1981

Selenomethionine and methionine were compared as substrates for in vitro aminoacylation, ribosome binding, and peptide bond formation with preparations from wheat germ. Selenomethionine paralleled methionine in all steps of the translation process except peptide bond formation. Peptide bond formation with the initiating species of tRNAMet demonstrated that selenomethionyl-tRNAM't was less effective as a substrate than was methionyl-tRNAf't. Participation of selenomethionine in the initiation process of translation could be expected to reduce the overall rate of protein synthesis and might aid in explaining selenium toxicity in selenium-sensitive plants.methionine, each step enroute to incorporation of this amino acid into protein should be studied independently of the other. For this purpose, ribosomes and supernatant fractors, prepared from wheat germ, were used. Selenomethionine and methionine functioned as equivalent substrates in the aminoacylation reaction and in ribosome binding of the aminoacyl-tRNA. Peptide bond formation between puromycin and selenomethionine, in contrast, was curtailed compared to bond formation between puromycin and methionine. As an explanation for selenium toxicity at the subcellular level, it would appear that selenomethionine exerts one of its effects in wheat by limiting the rate at which the first peptide bond is formed during protein synthesis. MATERIALS AND METHODSMethionine is a key intermediate in several crucial metabolic reactions. Since methionine contains a sulfur atom, substances which disrupt sulfur metabolism, such as those that contain selenium, in all likelihood will interfere with the metabolism of this sulfur amino acid. In selenium-sensitive plants, the assimilation of selenium usually parallels that of sulfur. As a consequence, selenoanalogs of several sulfur-containing metabolites can be synthesized (17). One of these analogs, selenomethionine, is known to be formed by a wide variety of plants (7,9,(16)(17)(18). Although the biochemical characteristics of selenomethionine resemble those of methionine in a great many ways, the metabolic activity of selenomethionine apparently differs in certain respects; one of these differences is, in general, reaction rate (4,5,11,12). Examination of those reactions in which selenomethionine functions at a rate different than methionine could reveal some of the factors that underlie selenium toxicity.One vital metabolic process with which these methionine analogs might interfere is the biosynthesis of protein. Methionine, required as the initiating amino acid during the translation process, is also incorporated at internal positions of most protein chains. Alteration of initiation process of translation or with the chain elongation process could drastically affect normal functions of the cell. Previous in vitro studies with polysomes from Vigna radiata root tips have established that methionine is a slightly better substrate than is selenomethionine during chain elongation, provided that both amino acids are supplied s...