Amino Acid Pools in Herbaceous Plants at the Wintering Stage and at the Beginning of Growth

Free amino acids in 40 herbaceous perennial plants were analyzedunder natural conditions. From the major amino acid contentat the wintering stage, the pools were separated into the followingfive types: 1) a group which accumulated arginine (20 plantsout of 40); 2) a group which accumulated arginine and proline(9 plants); 3) a group which accumulated glutamate and glutamine(3 plants); 4) a group which accumulated asparagine (4 plants);and 5) a group which accumulated proline (4 plants). Changesin the amino acid pools in the plants occurred under snow duringwintering for about five months. Particularly, asparagine wasno longer the major amino acid in the group which had accumulatedit in fall. There was a tendency for the glutamine content toincrease, suggesting that NH₃ is utilized for the synthesisof the amide. Also, the relative concentrations of almost allthe free amino acids increased several-fold, which was indicativeof the occurrence of biosynthetic processes of general aminoacids during wintering. As the mobile fractions of stored nitrogen,the amino acids appeared to contribute to the initial stageof rapid growth in early spring. (Received August 4, 1986; Accepted November 17, 1986)     

Xylem Transport and Storage of Amino Acids by S.W. Australian Mistletoes and their Hosts

Amino acid composition of xylem (tracheal) sap and ethanolicextracts of shoots of mistletoes (Amyema spp. and Lysiana casuarinae)and their hosts were compared, using material collected in theirnative habitats. Data indicated that certain host xylem soluteswere transferred directly to the parasite xylem, while otherswere either not absorbed or were metabolized prior to transfer.Certain solutes were major constituents of parasite xylem, butundetected or only in trace amount in the host. Shoot aminoacid pools of parasites differed markedly from those of hosts.The mistletoe, Amyema preissii, exhibited differential storageand transport of arginine when parasitizing three differentspecies, but accumulated proline on only two of these hosts.Host- specific amino acids (djenkolic acid in Acacia saligna,and tyramine in Acacia acuminata) were transported and accumulatedin relatively large amounts by the parasite, but were not detectedin other associations. Proline was the major solute of Amyemalinophyllum parasitizing Casuarina obesa, but arginine predominatedin Lysiana casuarinae on the same host. However, when L. csuarinaeparasitized A. linophyllum, in turn parasitic on C. obesa, theLysiana accumulated equal amounts of proline and arginine andmore asparagine than when directly on the Casuarina. Xylem feedingof ¹⁵N-labelled aspartic acid or ¹³N-(amide labelled) asparagineto cut shoots or whole haustoria-bearing plants of the mistletoeA. preissii resulted in 68–73% of the ¹⁵N of aspartateand 24–30% of that of asparagine appearing in ethanol-solubleshoot amino compounds other than the fed solute. ¹⁵N labellingpatterns of detached shoots were not noticeably different fromthat of whole plants suggesting that the haustorium had relativelylittle effect on processing incoming solutes. Alanine, glutamine,and arginine were principal recipients of ¹⁵N from aspartate,alanine and glutamine in the case of fed asparagine. It is estimatedthat 24% of the carbon requirements for dry matter accumulationin Amyema linophyllm were met by intake of xylem sap solutesfrom its host Casuarina obesa. Key words: Amino acids, xylem transport, mistletoes, host: parasite relations, N metabolism     

Salinity Stress and the Content of Proline in Roots of Pisum sativum and Tamarix tetragyna

Amino acid composition of the free amino acid pool and the TCA-insolubleprotein fraction were investigated in root tips of pea and Tamarixtetragyna plants grown at various levels of NaCl salinity. Salinitystress induced an increase of proline content, mainly in thefree amino acid pool in both plants, and of proline or hydroxyprolinecontent in the protein. Externally-supplied proline was absorbedand incorporated into protein, by pea roots, more effectivelythan by Tamarix roots. Salinity stress, apparently, stimulatedthe metabolism of externally-supplied labelled proline. Pearoots have a very large pool of free glutamic acid; however,70 per cent of the ¹⁴C from externally-supplied ¹⁴C-U-glutamicacid was released as CO₂. Very small amounts of it were incorporatedinto protein. No measurable amount of radioactivity could bedetected in any one of the individual amino acids, either ofprotein hydrolysate or the free amino acid pool. Proline very effectively counteracted the inhibitory effectof NaCl on pea seed germination and root growth. A similar effectbut to a lesser degree was achieved with phenylalanine and asparticacid. The feasibility of proline being a cytoplasmic osmoticumis discussed.     

Effect of low temperature on amino Acid metabolism in wintering poplar: arginine-glutamine relationships

Analyses of free amino acids in poplar (Populus gelrica) were carried out throughout a year to see the effect of low temperature on a system regulating amino acid metabolism in the tree. The results indicated that during the wintering phase arginine was the major amino acid both in bark and xylem, particularly in xylem, and that at the time of budding and growing glutamine and glutamate became dominant. Changes in the relative levels of glutamine (plus glutamate) and arginine to the total amino acids of the α-ketoglutarate family indicated the presence of a regulatory system annually controlling the synthesis between glutamine (plus glutamate) and arginine. The system appeared to be governed and sensitized by low temperatures. Neither a transition of the synthesis from arginine to glutamine (plus glutamate) nor budding occurred in the poplars which spent the winter months in a greenhouse.     

Interactive effects of salinity, nitrogen and sulphur on the organic solutes in Spartina alterniflora leaf blades

Glycinebetaine, proline, asparagine, sucrose, glucose, and dimethylsulphoniopropionate(DMSP) were the major organic solutes in Spartina alternifloraleaf blades. To investigate the physiological role(s) of thesesolutes, the effects of salinity, nitrogen, and sulphur treatmentson leaf blade solute levels were examined. Glycinebetaine wasthe major organic solute accumulated in leaf blades grown at500 mol m^–3 NaCl, although asparagine and proline alsoaccumulated when the supply of nitrogen was sufficient. Thesesolutes may play a role in osmotic adjustment. In contrast,DMSP levels either did not change or were reduced in responseto the 500 mol m^–3 NaCl treatment. Furthermore, elevatednitrogen supply decreased leaf blade DMSP levels, which wasopposite to the response of glycinebetaine, proline, and asparagine.A 1000-fold increase in external sulphate concentration hadno effect on the leaf blade levels of DMSP, glycinebetaine,proline, or asparagine. These findings suggest that the majorphysiological role of DMSP in S. alterniflora leaf blades isnot for osmotic adjustment, even under conditions of nitrogendeficit and excess sulphur. Instead, DMSP which was presentat 45—130 µmol g^–1 dry weight, may play arole as a constitutive organic osmoticum. Key words: Spartina alterniflora, dimethylsulphoniopropionate, glycinebetaine, nitrogen, salinity     

Proline Metabolism and Transport in Maize Seedlings at Low Water Potential

The growing zone of maize seedling primary roots accumulatesproline at low water potential. Endosperm removal and excisionof root tips rapidly decreased the proline pool and greatlyreduced proline accumulation in root tips at low water potential.Proline accumulation was not restored by exogenous amino acids.Labelling root tips with [¹⁴C]glutamate and [¹⁴C]proline showedthat the rate of proline utilization (oxidation and proteinsynthesis) exceeded the rate of biosynthesis by five-fold athigh and low water potentials. This explains the reduction inthe proline pool following root and endosperm excision and theinability to accumulate proline at low water potential. Theendosperm is therefore the source of the proline that accumulatesin the root tips of intact seedlings. Proline constituted 10% of the amino acids released from the endosperm. [¹⁴C]Prolinewas transported from the scutellum to other parts of the seedlingand reached the highest concentration in the root tip. Less[¹⁴C]proline was transported at low water potential but becauseof the lower rate of protein synthesis and oxidation, more accumulatedas proline in the root tip. Despite the low biosynthesis capacityof the roots, the extent of proline accumulation in relationto water potential is precisely controlled by transport andutilization rate.     

Proline Accumulation in Water-stressed Barley Leaves in Relation to Translocation and the Nitrogen Budget

Mobilization of N from leaves of barley (Hordeum vulgare L.) during water stress, and the role of proline as a mobilized species, were examined in plants at the three-leaf stage. The plants responded to water stress by withdrawing about 25% of the total reduced N from the leaf blades via phloem translocation. Most of this N loss was during the first 2 days while translocation of ¹⁴C-photosynthate out of the stressed blade still remained active. Free proline accumulation in the blade was initially slow, and became more rapid during the 2nd day of stress. Although a major free amino acid, proline accounted for only about 5% of the total N (soluble + insoluble) retained in severely stressed blades. When the translocation pathway in water-stressed leaves was interrupted just below the blade by a heat girdle, a cold jacket, or by blade excision, N loss from the blade was prevented and proline began to accumulate rapidly on 1st day of stress. Little free proline accumulated in the blades until after the ability to translocate was lost. Proline was, however, probably not a major species of N translocated during stress, because proline N accumulation in heat-girdled stressed leaves was five times slower than the rate of total N export from intact blades.     

Salt Tolerance in Simmondsia chinensis: Water Balance and Accumulation of Chloride, Sodium and Proline Under Low and High Salinity

The response of jojoba [Simmondsia chinensis (Link) Schneid]plants to salinity was studied in solution culture. At concentrationsof 0, 100, 200 and 600 m-mol l^–1 NaCI it was found thatjojoba plants have high tolerance to NaCl. The growth of theseplants was not affected by salinity. They accumulated largeamounts of Cl^–, Na⁺ and proline. These amounts decreasedrapidly in plants transferred back to control medium. Potassiumcontent decreased in NaCl-treated plants and tended to increaserapidly to the control level in plants transferred to controlmedium. The effect of salinity on water balance was not appreciable.As suggested for other xerophytic species, it could be assumedthat the high tolerance of jojoba to salinity plays an importantrole in its ability to endure periods of drought. The role ofproline during or after stress remains an open question. Simmondsia chinensis (Link) Schneid, jojoba, salt tolerance, sodium accumulation, chloride ion accumulation, proline accumulation, xerophytism, drought tolerance     

Sources of Proline-nitrogen in Water-stressed Soybean (Glycine max L.) I. Protein Metabolism and Proline Accumulation

Water stress was imposed upon soybean plants (Glycine max L.)grown in a greenhouse by withholding irrigation for 10 daysafter 5 weeks of growth, and the changes under stress in thelevels of free amino acids, free ammonia and protein were determinedin detail. With a decrease in the leaf water potential, theprotein content gradually decreased, whereas the free ammoniacontent was relatively constant. Water stress induced an increasein the levels of free amino acids normally present in proteinsuch as isoleucine, leucine, valine, phenylalanine, glutamineand histidine, indicating that protein hydrolysis occurs understress. Proline accumulated only under severe stress (below–1.5 MPa) and attained 0.86% of the dry weight on day10 (–2.6 MPa). Asparagine also accumulated only undersevere stress (below –2.0 MPa). The concentration of glutamicacid, alanine, aspartic acid, serine, glycine and arginine remainedvirtually unchanged during the stress period. Total proline(protein-bound+free) first decreased during mild to moderatestress, and then increased over that of the well-irrigated controlplants at severe stress due to a remarkable accumulation offree proline. These findings indicate that some de novo synthesisof proline occurs under severe stress and that the nitrogensource for this proline synthesis may be protein. (Received July 4, 1981; Accepted September 11, 1981)     

海马齿对无机汞的耐性和吸附积累

报道了海马齿(Sesuvium portulacastrum)对重金属汞的耐性和吸附特性。 在10 μmol·L^-1汞胁迫时, 海马齿中脯氨酸含量明显低于对照; 丙二醛(MDA)含量、根的电解质外渗率(Electrolyte leakage rate, ELR)无明显变化; 叶绿素含量增加; 植物生长良好, 形态、生长速率、鲜重和根的长度与对照无区别, 且有新的须根形成。结果表明: 低浓度汞对海马齿的生长发育起着促进作用。海马齿能大量吸附积累汞离子, 主要积累在根组织中。当培养液中汞浓度为50 μmol·L^-1时, 海马齿根中汞含量最高可达到33.9 μg·g^-1DW, 是相同处理下地上部分的70倍。培养液中汞浓度为10 μmol·L^-1时, 植物并未受到伤害, 且能快速生长, 此时根部的汞含量可达到12.02 μg·g^-1 DW。由此可见, 海马齿植物表现为很强的耐汞和吸收汞特性。     

Phosphate Regulation of Nitrate Assimilation in Soybean

It is known that phosphorus deficiency results in alterationsin the assimilation of nitrogen. An experiment was conductedto investigate mechanisms involved in altered ¹⁵NO^–₃ uptake,endogenous ¹⁵N translocation, and amino acid accumulation insoybean (Glycine max L. Merrill, cv. Ransom) plants deprivedof an external phosphorus supply for 20 d in solution culture.Phosphorus deprivation led to decreased rates of ¹⁵NO^–₃uptake and increased accumulation of absorbed ¹⁵N in the root.Both effects became more pronounced with time. Asparagine, theprimary transport amino acid in soybean, accumulated in largeexcess in roots and stems. In roots of phosphorus-deprived plants,concentrations of ATP and inorganic phosphate declined rapidly,but dry weight accumulation was similar to or above that ofthe control even after 20 d of treatment. Arginine accumulationin leaves was greatly enhanced, even though ¹⁵N partitioninginto the insoluble reduced-N fraction of leaves was unaffected.The results suggest that decreases in NO^–₃ uptake in lowphosphorus plants could be caused by feedback control factorsand by limited ATP availability. The decline in endogenous Ntransport from the root to the shoot may be associated withchanges in membrane properties, which also result in paralleleffects on hydraulic conductance and the upward flow of waterthrough the plant. Key words: Phosphorus stress, nitrate uptake, nitrate translocation, arginine     

Changes in Proline Content Accompanying the Uptake of Zinc and Copper by Lemna minor

Lemna minor L. grown in Hoagland solution containing zinc (10ppm) or copper (5 ppm) for 4 d accumulated high levels of thesemetals. Zinc and Cu accumulation in the test plants was accompaniedby a specific pattern of change in proline content. The lattershowed a steep rise during early stages (peak at 12-24 h) followedby a gradual decline until 96 h of treatment. In a dose-responsestudy, lower metal concentrations induced a sharp rise in prolinelevel with a maximum value at 5 ppm, which declined when theconcentration was further enhanced. There was a correspondencebetween the level of proline and total free amino acids in metaltreated plants. The possibility of proline involvement in tolerancemechanisms to heavy metals has been discussed.Copyright 1993,1999 Academic Press Heavy metals, zinc, copper, proline, total free amino acids, Lemna minor     

Changes in the Contents of Free and Protein Amino Acids in Tobacco Seeds and Placentae during Maturation

Changes in the levels of protein and free amino acids in theseeds and placentae of Nicotiana tabacum were studied duringseed development. Seed maturation was completed 24 days afteranthesis. During maturation, protein rapidly accumulated inthe seeds between the 6th and 18th day, along with an appreciablecompositional change in the protein amino acids as the proportionsof glutamic acid and arginine increased. The amount of freeamino acids in the seeds gradually decreased throughout maturation.The major free amino acid on the 6th day after anthesis wasglutamine, which then drastically decreased between the 6thand 12th day with increases of glutamic acid, proline, arginineand alanine. The latter amino acids decreased thereafter untilthe 24th day. On the other hand, the amount and composition of the proteinsin the placentae did not change significantly throughout seedmaturation. In the early stage of development, the major freeamino acids in the placentae were glutamine, asparagine andglutamic acid, while in the later stage asparagine was mostabundant. (Received March 12, 1982; Accepted August 16, 1982)     

Combined effects of CO2 enrichment,diurnal light levels and water stress on foliar metabolites of potato plants grown in naturally sunlit controlled environment chambers

Experiments were conducted in outdoor, naturally sunlit, soil–plant–atmosphere research (SPAR) chambers using plants grown in pots. Drought treatments were imposed on potato plants (Solanum tuberosum cv. Kennebec) beginning 10 days after tuber initiation. A total of 23 out of 37 foliar metabolites were affected by drought when measured 11 days after initiating water stress treatments. Compounds that accumulated in response to drought were hexoses, polyols, branched chain amino acids (BCAAs) and aromatic amino acids, such as proline. Conversely, leaf starch, alanine, aspartate and several organic acids involved in respiratory metabolism decreased with drought. Depending upon harvest date, a maximum of 12 and 17 foliar metabolites also responded to either CO₂ enrichment or diurnal treatments, respectively. In addition, about 20% of the measured metabolites in potato leaflets were simultaneously affected by drought, CO₂ enrichment and diurnal factors combined. This group contained BCAAs, hexoses, leaf starch and malate. Polyols and proline accumulated in response to water stress but did not vary diurnally. Water stress also amplified diurnal variations of hexoses and starch in comparison to control samples. Consequently, specific drought responsive metabolites in potato leaflets were dramatically affected by daily changes of photosynthetic carbon metabolism.     

Osmotically Induced Proline Accumulation in <Emphasis Type="Italic">Lotus Corniculatus</Emphasis> Leaves is Affected by Light and Nitrogen Source

Proline accumulation in osmotically stressed leaves of Lotus corniculatus was stimulated by increasing light intensity (photon fluence density, PFD). Treatment with propanil limited proline accumulation in response to light and osmotic stress, indicating a dependence of proline synthesis on photosynthetic NADPH. Drought stress induced proline accumulation in L. corniculatus both in nitrate-fed plant (NFP) and ammonium-fed plants (AFP), although higher proline concentration was observed in AFP than in NFP after 24 h of drought stress. Changes in proline accumulation induced by drought stress in plants grown under different nitrogen regimes could not be explained by changes of either total protein or amino acids, consistent with specifically altered regulation of proline synthesis. Under control conditions, alanine, aspartate and glutamate were the predominant amino acids in NFP; conversely, in AFP, arginine and ornithine were the predominant amino acids. Only the NFP regime showed changes in the concentrations of specific amino acids under drought stress a decrease in alanine, aspartate and glutamate and increased gama-aminobutyric acid. In AFP and especially NFP, proline accumulation under osmotic stress was associated with increased ornithine amino transferase activity. An increase of both activity and protein of ferredoxin-dependent glutamate synthase was observed in osmotic-stressed NFP; inversely both decreased in drought-stressed AFP. PFD and nitrogen source are therefore shown to be regulators of proline accumulation in L. corniculatus osmotically stressed plants.     

不同生育期水分胁迫对水稻根叶渗透调节物质变化的影响

 干旱是限制水稻(Oryza sativa)作物产量的主要生态因子之一,渗透调节是作物适应干旱逆境的生理机制之一。在人为控制水分的盆栽条件下, 对水稻生长的分蘖期、幼穗分化期、抽穗期、结实期分别进行水分胁迫,研究水稻根系及叶片渗透调节物质的变化规律。结果表明, 不同生育期 干旱胁迫后叶片水势均显著下降,根系和叶片的有机渗透调节物质如可溶性糖、游离氨基酸、脯氨酸和无机渗透调节物质包括K⁺、Mg²⁺等含量 均大幅度上升,而且幼穗分化期和抽穗期这两个对水分胁迫最敏感的时期上升幅度最大,其中又以有机渗透调节物质变化最显著。不同生育期渗 透调节大小的顺序为：抽穗期>幼穗分化期>结实期>分蘖期,反映了不同生育时期渗透调节能力的差异。同时幼穗分化期和抽穗期水分胁迫结束 后再复水后根系和叶片的有机渗透调节物质含量仍长期明显高于对照,而无机离子则变化规律比较复杂,有的升高有的则降低。叶片的渗透调 节能力大于根系,无论是叶片或根系都是K⁺对渗透调节的贡献最大;其次是Ca²⁺, 6 种渗透调节物质含量大小排列顺序为K⁺ > Ca²⁺ >可溶性糖 > Mg²⁺ > 游离氨基酸 > 脯氨酸。     

Exogenous ornithine is an effective precursor and the δ-ornithine amino transferase pathway contributes to proline accumulation under high N recycling in salt-stressed cashew leaves

The role of the δ-ornithine amino transferase (OAT) pathway in proline synthesis is still controversial and was assessed in leaves of cashew plants subjected to salinity. The activities of enzymes and the concentrations of metabolites involved in proline synthesis were examined in parallel with the capacity of exogenous ornithine and glutamate to induce proline accumulation. Proline accumulation was best correlated with OAT activity, which increased 4-fold and was paralleled by NADH oxidation coupled to the activities of OAT and Δ¹-pyrroline-5-carboxylate reductase (P5CR), demonstrating the potential of proline synthesis via OAT/P5C. Overall, the activities of GS, GOGAT and aminating GDH remained practically unchanged under salinity. The activity of P5CR did not respond to NaCl whereas Δ¹-pyrroline-5-carboxylate dehydrogenase was sharply repressed by salinity. We suggest that if the export of P5C from the mitochondria to the cytosol is possible, its subsequent conversion to proline by P5CR may be important. In a time-course experiment, proline accumulation was associated with disturbances in amino acid metabolism as indicated by large increases in the concentrations of ammonia, free amino acids, glutamine, arginine and ornithine. Conversely, glutamate concentrations increased moderately and only within the first 24 h. Exogenous feeding of ornithine as a precursor was very effective in inducing proline accumulation in intact plants and leaf discs, in which proline concentrations were several times higher than glutamate-fed or salt-treated plants. Our data suggest that proline accumulation might be a consequence of salt-induced increase in N recycling, resulting in increased levels of ornithine and other metabolites involved with proline synthesis and OAT activity. Under these metabolic circumstances the OAT pathway might contribute significantly to proline accumulation in salt-stressed cashew leaves.     

Preferential Accumulation by Mesophyll Cells at Low and by Veins at High Exogenous Amino Acid and Sugar Concentrations in Commelina benghalensis L. Leaves

The contribution to solute uptake by mesophyll cells and veinsin leaf discs, was assessed through a study of uptake in relationto concentration for ¹⁴C-labelled substrates (sucrose, glucose,arginine, proline, valine and -aminoisobutyric acid) using isolatedmesophyll cells and stripped leaf discs of Commelina benghalensisL. Uptake per unit fresh weight was higher in mesophyll cellsthan in discs at low substrate concentrations (lower than about0·5 mol m^–3). At higher concentrations, uptakeby discs exceeded that by mesophyll cells except for glucoseuptake which was higher in mesophyll cells over the whole concentrationrange. The profiles of uptake versus concentration displayedbiphasic kinetics in mesophyll cells and discs. Comparison ofthe uptake characteristics obtained by iterative fitting confirmedthat the high-affinity systems of uptake prevail in the mesophyllcells, whereas the low-affinity systems are dominant in theveins. The results provide good evidence that, supplementaryto direct vein loading, a pathway via the mesophyll contributesstrongly to the photosynthate loading by veins in stripped discs. Key words: Commelina benghalensis L., amino acid uptake, mesophyll, minor veins, phloem loading, sugar uptake     

Regulation of proline accumulation in Arabidopsis thaliana (L.) Heynh during development and in response to desiccation

Significant differences were observed in the amount and proportion of free amino acids in different organs of Arabidopsis thaliana (L.) Heynh, ecotype Columbia. The most notable were found for proline, which formed 17–26% of the total free amino acid concentration in reproductive tissues (floret and seed), but only 1–3% of the total free amino acid concentration in vegetative tissues (rosette leaf and root). Proline accumulation was associated with tissues that had relatively low water contents. Tissues which displayed high water contents, such as rosette leaves, contained low levels of proline. A significant increase in the levels of proline accumulation occurred in plants subjected to experimentally induced low water potentials as compared to unstressed plants. For instance, an 8–10-fold increase in proline was observed in the presence of 120 mmol kg^?1 NaCl or KCl, and a 20-fold increase was stimulated by 60 mmol kg^?1 PEG. However, in addition to the accumulation of proline, massive accumulation of Na⁺, K⁺ and Cl^? ions occurred in tissues of plants stressed with salt. No significant differences were observed in mineral ions in plants stressed with PEG. Isotope tracer experiments with ¹⁴C compounds established that glutamate, ornithine and arginine are precursors of the proline biosynthesis induced by PEG in response to low water potentials in Arabidopsis thaliana. We conclude that the accumulation of proline in response to PEG occurs through increased biosynthesis.     

Arginine and ornithine kinetics in severely burned patients: increased rate of arginine disposal

Arginine serves multiple roles in the pathophysiological response to burn injury. Our previous studies in burn patients demonstrated a limited net rate of arginine de novo synthesis despite a significantly increased arginine turnover (flux), suggesting that this amino acid is a conditionally indispensable amino acid after major burns. This study used [15N2-guanidino-5,5-2H2]arginine and [5-13C]ornithine as tracers to assess the rate of arginine disposal via its conversion to and subsequent oxidation of ornithine; [5,5-2H2]proline and [5,5,5-2H3]leucine were also used to assess proline and protein kinetics. Nine severely burned patients were studied during a protein-free fast ("basal" or fast) and total parenteral nutrition (TPN) feedings. Compared with values from healthy volunteers, burn injury significantly increased 1) fluxes of arginine, ornithine, leucine, and proline; 2) arginine-to-ornithine conversion; 3) ornithine oxidation; and 4) arginine oxidation. TPN increased arginine-to-ornithine conversion and proportionally increased irreversible arginine oxidation. The elevated arginine oxidation, with limited net de novo synthesis from its immediate precursors, further implies that arginine is a conditionally indispensable amino acid in severely burned patients receiving TPN.