Tbe enzymic basis for the flow of .ltrgeu from arginie to e during the first 3 days of gIlnaon has bed In extracts from cotton (Goss_ypm hnm) cotyledons. Eviene that a synthetase reguates n accuma t (for tsrt to the axis) Is Further, eviene that the bWuk of the passed from one generatio to the next In dicots through a cycle Invdvi the fIn seq ee r arg storae protein -arginine -* asparaIe s discussed.Data on the pathway of ammonia assimilation in nodules of leguminous roots and green leaves have been accumulating during the past several years, two review articles appearing recently (19,20). In green leaves, ammonia from nitrate assimilation serves as a nitrogen source for amino acid synthesis, whereas in root nodules, ammonia produced by nitrogen fixation serves as a nitrogen source. In both cases, much ofthe carbon skeletons for amino acid synthesis presumably comes as a result of photosynthesis. The situation is likely to be different in dicot seeds, such as legumes and cotton, that do not depend on external nitrogen sources. The storage proteins and starch or lipid satisfy their nutritional needs at least in the early days of germination. In cotton, amino acids and carbon skeletons released by the hydrolysis of the stored material appear to go through active interconversions as indicated by the lack of correlation between the amino acid composition of the storage proteins (13) and that of the free amino acid content of germinating cotyledons (4). The predominant nitrogen storage unit in the cotton storage protein is arginine which contains at least 25% of the total stored nitrogen (13), whereas most of the nitrogen transported from cotyledons to the growing axis is by way of asparagine (4). It seems that a major pathway of nitrogen metabolism in germinating cotyledons lies in the transfer of nitrogen from ARG' to ASN. Furthermore, we have observed that cotyledons germinated in actinomycin D (Act D) do not accumulate ASN to the extent that untreated cotyledons do, but rather accumulate massive amounts of glutamine (4). This suggests that nitrogen for the build-up of ASN may pass through glutamine and that the enzyme(s) responsible for synthesis of ASN at the expense of glutamine may be derived from mRNA newly synthesized in germination.Against this background we have examined the levels during germination ofactivities in crude extracts ofcotyledons ofenzymes 'Abbreviations: Act D: actinomycin D; GDH: glutamate dehydrogenase; GS: glutamine synthetase; GOGAT: glutamate synthase; AS: asparagine synthetase; ASN: asparagine; ARG: arginine.likely to be involved in the transfer of nitrogen from ARG to ASN, and have examined the effects of Act D and cycloheximide on some of these activity levels.Arginase (EC 3.5.3.1; L-argimne amidinohydrolase; reaction 1) and urease (EC 3.5.1.5; urea amidohydrolase; reaction 2) activities were assayed to establish the possible pathway for the release of ammonia from arginine of the storage protein. purchased from Morrow Bros., Farmington, Georgia, were germinated in the dar...
The characteristics of nitrate-nitrate reductase and benzyladenine-nitrate reductase were compared by four methods but no differences could be detected: (a) Both enzymes sedimented with the same velocity during sucrose density gradient centrifugation. (b) Their distribution among fractions obtained by differential precipitation with (NH4)2SO4 was identical. (c) The elution profile of nitrate-nitrate reductase and benzyladenine-nitrate reductase after chromatography on diethylaminoethyl Sephadex A-25 columns showed no significant difference. (d) On polyacrylamide gel, the electrophoretic migration of the two enzymes was also identical.NR' in excised embryos of Agrostemma githago can be induced by incubating them in media containing either nitrate or a cytokinin (1-3). The physiological significance of the hormonal induction of NR, as opposed to substrate induction, is not known at this point. Accumulating evidence indicates, however, that cytokinins are acting in a manner different from nitrate in inducing NR. BA loses its effectiveness as an inducer of NR after 64 hr of seed germination, whereas the effect of nitrate is not time-limited (4). The level of NR decreases upon
When excised embryos of Agrostemnna githago were incubated with nitrate, the activities of both nitrate reductase and nitrite reductase were enhanced. By contrast, benzyladenine induced nitrate reductase only. Our data suggest that nitrate affected nitrite reductase activity directly, without first being reduced to nitrite. When the endogenous nitrite production was increased by raising the level of nitrate reductase through simultaneous treatment with nitrate and benzyladenine, the activity of nitrite reductase was not higher than in embryos treated with nitrate alone. On the other hand, tungstate given together with nitrate drastically inhibited the development of nitrate reductase activity without reducing the enhancement of nitrite reductase activitv. Nitrite enhanced nitrite reductase activity, though less efficiently than nitrate.mogenates, obtained according to Kende and Shen (6), were used as enzyme sources. In some experiments, the fractions precipitated at 50% and 50 to 75% saturation of (NH4)2SO0were used as sources of NR and NiR, respectively. Each precipitate was redissolved in a small volume of 0.1 M potassium phosphate buffer, pH 7.5, containing 1 mm EDTA, 1 mM L-Cysteine, and 1 bLM FAD.Enzyme Assays. NADH-NR was assayed according to Wray and Filner (9), and NiR according to Kelker and Filner (4) with minor modifications as described below. The reaction mixture was prepared by mixing 20 ,wmoles of potassium phosphate buffer (pH 7.5), 0.7 ,umole of methyl viologen chloride, 0.8 ,umole of KNO2, and the enzyme preparation with distilled water to give a final volume of 0.5 ml. The reaction was started by adding 0.1 ml Na2S20 (100 mm in 0.29 M NaHCO,) and was carried out at 30 C for 45 min. The conditions were sufficiently anaerobic so that no N2 atmosphere was required.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.