Measurement of IL-21 in human serum and plasma using ultrasensitive MSD S-PLEX® and Quanterix SiMoA methodologies

IL-21 is a pleiotropic cytokine that plays a key role in modulating inflammatory responses, including the promotion of autoimmune diseases. Several groups have quantitated circulating levels of IL-21 in plasma and serum samples using various commercial ELISAs. We determined, however, that the most commonly used commercial assays in published literature were not specific or sensitive enough to detect levels of IL-21 in heparin plasma or serum from healthy human individuals. This finding prompted an effort to develop more specific and sensitive methods to quantitate IL-21 in complex biological matrices using proprietary anti-IL-21 antibodies with the Quanterix SiMoA platform and the Meso Scale Discovery (MSD) S-PLEX® format. Assays developed on both technology platforms were characterized in heparin plasma and serum using spike recoveries across a range of concentrations. Each method was able to detect sub-pg/mL levels of IL-21 (predicted Limit of Detection [LOD] of approximately 1.0 fg/mL for both the Quanterix SiMoA and MSD S-PLEX® platforms) which is 200-500 times lower than current commercial assays. Additionally we demonstrated that rheumatoid factor did not interfere with measuring IL-21 in the Quanterix SiMoA assay. Results obtained with the two new ultrasensitive assays showed a strong correlation (r = 0.9428; p < .0001). Additionally, IL-21 levels were significantly increased in samples from patients with Systemic Lupus Erythematosus (mean+/- SD: n = 14, 202.64 +/- 111.47 fg/mL, p = .0001 for Quanterix SiMoA and 275.4 +/- 174.66 fg/mL p = .0001 for MSD S-PLEX®) as well as in samples from patients with Sjögren's Syndrome (mean+/- SD: n = 11, 122.18 +/- 84.50 fg/mL, p = .0029 for Quanterix SiMoA and 183.64 +/- 153.00 fg/mL, p = .0082 for MSD S-PLEX®) when compared to healthy donors (mean+/- SD: n = 11, 38.1 +/- 27.8 fg/mL for Quanterix SiMoA and 58.1 +/- 30.7 fg/mL for MSD S-PLEX®). These ultrasensitive assays, for the first time, allow for the accurate quantitation of human IL-21 in heparin plasma and serum. In addition, these experiments also provide a direct comparison of the MSD S-PLEX® format and Quanterix SiMoA platform technologies, which may have broader implications to future application of these methods to evaluate low abundance proteins in complex biological matrices.

Development of a Whole Cell High Throughput Screen for Muscarinic Receptor Agonists

We have developed a high throughput screen using intact cells to identify muscarinic receptor agonists. Chinese hamster ovary cells stably transfected with the Ml muscarinic acetylcholine receptor (CHO-Ml) were pre-labeled with [3HJ-arachidonic acid (AA). Stimulation of muscarinic receptors with known muscarinic agonists resulted in release of AA from the cells into the culture medium. The released [3H]-AA in this assay was counted using both standard scintillation methods and Luma Plates. Because muscarinic antagonists do not cause release of AA, only agonists are identified. A follow-up screen using a competitive antagonist was used to confirm agonist properties of active compounds. This screen was relatively simple, reproducible, and compatible with many organic solvents and natural products growth media. Thus, it may be useful for the discovery of muscarinic agonists from natural product broths or synthetic compounds.

Herbicide resistance caused by spontaneous mutation of the cytoskeletal protein tubulin

The dinitroaniline herbicides (such as trifluralin and oryzalin) have been developed for the selective control of weeds in arable crops. However, prolonged use of these chemicals has resulted in the selection of resistant biotypes of goosegrass, a major weed. These herbicides bind to the plant tubulin protein but not to mammalian tubulin. Here we show that the major alpha-tubulin gene of the resistant biotype has three base changes within the coding sequence. These base changes swap cytosine and thymine, most likely as the result of the spontaneous deamination of methylated cytosine. One of these base changes causes an amino-acid change in the protein: normal threonine at position 239 is changed to isoleucine. This position is close to the site of interaction between tubulin dimers in the microtubule protofilament. We show that the mutated gene is the cause of the herbicide resistance by using it to transform maize and confer resistance to dinitroaniline herbicides. Our results provide a molecular explanation for the resistance of goosegrass to dinitroanaline herbicides, a phenomenon that has arisen, and been selected for, as a result of repeated exposure to this class of herbicide.

Monoclonal antibodies as surrogate receptors in a high throughput screen for compounds that enhance insulin sensitivity

Monoclonal antibodies (MoAbs) were made to a known insulin sensitivity enhancer (ISE) compound, CS-045. The MoAbs were characterized with respect to binding other known thiazolidinedione ISE compounds using a CS-045 labeled with b-phycoerythrin in a competitive particle concentration fluorescence immunoassay (PCFIA). By comparing the rank order of IC50 values for each compound to its respective potency as an ISE, one MoAb (13E3) was selected for further characterization. This MoAb was also used as a surrogate receptor in a high throughput screen to identify novel compounds that compete for binding to CS-045. Some of the hits were found to have efficacy in reducing blood glucose. Subsequently, another group reported that several compounds with the core thiazolidinedione structure of the ISE compounds bound with high affinity to peroxisome proliferator-activating receptors (PPAR). Therefore, we used the MoAb assay to test these and other compounds that are known to bind to PPARgamma and noted crossreactivity with some of the compounds.

Competitive particle concentration fluorescence immunoassays for measuring anti-diabetic drug levels in mouse plasma

Two competitive particle concentration fluorescence immunoassays were developed to measure blood levels of analogs of anti-diabetic drugs being tested in diabetic mice. Ligands that contained the active pharmacophores were conjugated to PPD for immunization and to beta-phycoerythrin for use as a tracer in the immunoassays. Approximately 90% of 262 compounds assayed were detectable at less than 120 nM in plasma which was well below the estimated therapeutic level of 1 microM for lowering blood glucose. These data were used to define the bioavailability of test compounds and assist in decisions of constructing active analogs. Of additional interest, we noted crossreactivity of one monoclonal antibody for 3 different compound classes that are all known to bind with varying affinities to peroxisome proliferator-activated receptors.

Characterization of the safener-induced glutathione S-transferase isoform II from maize

The safener-induced maize (Zea mays L.) glutathione S-transferase, GST II (EC 2.5.1.18) and another predominant isoform, GST I, were purified from extracts of maize roots treated with the safeners R-25788 (N,N-diallyl-2-dichloroacetamide) or R-29148 (3-dichloroacetyl-2,2,5-trimethyl-1,3-oxazolidone). The isoforms GST I and GST II are respectively a homodimer of 29-kDa (GST-29) subunits and a heterodimer of 29- and 27-kDa (GST-27) subunits, while GST I is twice as active with 1-chloro-2,4-dinitrobenzene as GST II, GST II is about seven times more active against the herbicide, alachlor. Western blotting using antisera raised against GST-29 and GST-27 showed that GST-29 is present throughout the maize plant prior to safener treatment. In contrast, GST-27 is only present in roots of untreated plants but is induced in all the major aerial organs of maize after root-drenching with safener. The amino-acid sequences of proteolytic fragments of GST-27 show that it is related to GST-29 and identical to the 27-kDa subunit of GST IV.

Cloning and characterization of maize herbicide safener-induced cDNAs encoding subunits of glutathione S-transferase isoforms I, II and IV

Several GSTs have been characterised in maize. GST I is a homodimer of 29 kDa subunits, GST II a hetrodimer of 27 kDa and 29 kDa subunits and GST IV a homodimer of 27 kDa subunits. We report the isolation and characterization of a herbicide-safener inducible cDNA clone, GST-27. Based on partial amino acid sequence, GST-27 encodes the 27 kDa subunit present in both glutathione S-transferase isoforms GST II and IV. Northern blotting was used to compare the expression patterns of GST-27 with that of GST-29. Transcripts corresponding to GST-27 were found to be constitutively expressed in RNA isolated from the root, but no expression was detected in RNA isolated from aerial parts of the plant. The application of herbicide safener caused a dramatic increase in the expression of GST-27 in all aerial plant parts tested. GST-29 was found to be constitutively expressed in RNA isolated from a number of maize tissues. The basal level of GST-29 expression showed a minimal increase upon herbicide safener treatment. Although a range of hormonal, environmental and physiological stimuli failed to elevate GST-27 levels, some increase in GST-27 mRNA was observed in the late stages of leaf senescence and after treatments resulting in phytotoxic effects.

Monoclonal antibody to mouse lipopolysaccharide receptor protects mice against the lethal effects of endotoxin

Specific endotoxic lipopolysaccharide (LPS) binding sites on the cell membranes of murine lymphocytes and macrophages that may serve as functional receptors for LPS have recently been identified using photoactivatable cross-linking LPS derivatives. A monoclonal antibody (Mab 5D3) with specificity for this 80-kDa protein has also been generated and characterized. The capacity of MAb 5D3 to protect mice against the lethal effects of endotoxin was investigated. Pretreatment of CF1 mice with as little as 15 micrograms of MAb 5D3 provided virtually complete protection against a dose of endotoxin 10-fold greater than that required to kill all mice in an untreated control group using the galactosamine sensitization model. Significant protection was also afforded normal mice given MAb 5D3 relative to saline. Several lines of evidence suggest that MAb 5D3-mediated protection is due to the agonist properties of this antibody rather than a receptor blockade mechanism.

Generation and characterization of hamster-mouse hybridomas secreting monoclonal antibodies with specificity for lipopolysaccharide receptor

Experiments are described for the partial purification of the 80-kDa LPS binding protein expressed on macrophages and lymphocytes. This partially purified Ag was used to immunize adult Armenian hamsters and splenocytes from immunized animals were fused with murine myeloma cell lines. Hybridoma cell culture supernatants containing mAb were screened by ELISA for positive binding to the immunizing Ag, murine splenocytes and the murine 70Z/3 pre B cell and for an absence of binding to sheep E. Positive clones were further screened for reciprocal competitive binding with LPS on spleen cells and ability to modulate B lymphocyte mitogenic activity. Two hybridoma cell lines secreting IgM monoclonals, termed mAb3D7 and mAb5D3, were identified that satisfied all of the selection criteria. These hybridoma cell lines were subcloned and expanded. Binding of one (mAb3D7) was abrogated by treatment of Ag with mild periodate; binding of the second (mAb5D3) was destroyed by digestion of Ag with proteinase K. Binding specificity for mAb5D3 has been confirmed by ELISA using highly purified 80-kDa protein. These mAb have been of value in establishing that the 80-kDa LPS binding protein previously identified may serve as a specific functional receptor for LPS.

Induction of macrophage-mediated tumor cytotoxicity by a hamster monoclonal antibody with specificity for lipopolysaccharide receptor

Experiments have been carried out to assess the immunostimulatory activity of a hamster IgM mAb (mAb5D3) with specificity for an 80-kDa LPS-binding protein expressed on murine macrophages and monocytes. The addition of mAb5D3 to cultures of murine bone marrow-derived macrophages activated these cells to become tumoricidal for mastocytoma cells in vitro. The activity of mAb5D3 was enhanced in the presence of IFN-gamma. Neither mAb5D3 nor LPS were able to activate macrophages from the LPS-hyporesponsive C3H/HeJ mouse, although these cells responded normally to heat-killed Listeria monocytogenes. The results of several experiments establish that the observed LPS-like activity of mAb5D3 was not due to contaminating endotoxin: 1) the activity of mAb5D3 but not LPS was heat labile at 100 degrees C; 2) the activity of LPS but not mAb5D3, was inhibited by addition of polymyxin B; and 3) quantitative estimates of endotoxin contamination by Limulus amoebocyte lysate reactivity. These experiments thus demonstrate that mAb5D3 can serve as an agonist for LPS-dependent macrophage responses and, when considered with those of our companion paper showing specificity of mAb5D3 for the 80-kDa LPS-binding protein, provide strong support for the concept that the 80-kDa LPS-binding protein previously identified serves as a functional receptor for LPS on murine macrophages.

Induction of macrophage-mediated tumor cytotoxicity by a hamster monoclonal antibody with specificity for lipopolysaccharide receptor

Experiments have been carried out to assess the immunostimulatory activity of a hamster IgM mAb (mAb5D3) with specificity for an 80-kDa LPS-binding protein expressed on murine macrophages and monocytes. The addition of mAb5D3 to cultures of murine bone marrow-derived macrophages activated these cells to become tumoricidal for mastocytoma cells in vitro. The activity of mAb5D3 was enhanced in the presence of IFN-gamma. Neither mAb5D3 nor LPS were able to activate macrophages from the LPS-hyporesponsive C3H/HeJ mouse, although these cells responded normally to heat-killed Listeria monocytogenes. The results of several experiments establish that the observed LPS-like activity of mAb5D3 was not due to contaminating endotoxin: 1) the activity of mAb5D3 but not LPS was heat labile at 100 degrees C; 2) the activity of LPS but not mAb5D3, was inhibited by addition of polymyxin B; and 3) quantitative estimates of endotoxin contamination by Limulus amoebocyte lysate reactivity. These experiments thus demonstrate that mAb5D3 can serve as an agonist for LPS-dependent macrophage responses and, when considered with those of our companion paper showing specificity of mAb5D3 for the 80-kDa LPS-binding protein, provide strong support for the concept that the 80-kDa LPS-binding protein previously identified serves as a functional receptor for LPS on murine macrophages.

Generation and characterization of hamster-mouse hybridomas secreting monoclonal antibodies with specificity for lipopolysaccharide receptor

Experiments are described for the partial purification of the 80-kDa LPS binding protein expressed on macrophages and lymphocytes. This partially purified Ag was used to immunize adult Armenian hamsters and splenocytes from immunized animals were fused with murine myeloma cell lines. Hybridoma cell culture supernatants containing mAb were screened by ELISA for positive binding to the immunizing Ag, murine splenocytes and the murine 70Z/3 pre B cell and for an absence of binding to sheep E. Positive clones were further screened for reciprocal competitive binding with LPS on spleen cells and ability to modulate B lymphocyte mitogenic activity. Two hybridoma cell lines secreting IgM monoclonals, termed mAb3D7 and mAb5D3, were identified that satisfied all of the selection criteria. These hybridoma cell lines were subcloned and expanded. Binding of one (mAb3D7) was abrogated by treatment of Ag with mild periodate; binding of the second (mAb5D3) was destroyed by digestion of Ag with proteinase K. Binding specificity for mAb5D3 has been confirmed by ELISA using highly purified 80-kDa protein. These mAb have been of value in establishing that the 80-kDa LPS binding protein previously identified may serve as a specific functional receptor for LPS.

Spontaneous shift of anti-DNA antibody idiotypes in systemic lupus erythematosus

Spontaneous shift in Id expression of polyclonal anti-DNA antibodies in a patient, BS, with SLE was investigated. BS had active lupus nephritis in 1982 and developed central nervous system lupus in 1986 without evidence of active nephritis. Two rabbit polyclonal anti-Id (BS-82 and BS-86 R-anti-Id) were raised against affinity-purified anti-DNA antibodies prepared from 1982 serum (BS-82) and 1986 serum (BS-86), respectively. In addition, murine monoclonal anti-Id was prepared against BS-82 Id. Direct binding assays showed that all three anti-Id had preferential binding to the immunizing anti-DNA antibodies (the homologous Id) and poor binding to anti-DNA antibodies prepared from the different dated sample of BS. This was confirmed by inhibition assays of binding of anti-Id to the homologous Id by various Id. Moreover, inhibition assays of binding of various Id to DNA by the R-anti-Id showed that the R-anti-Id was the most effective inhibitor for the homologous Id. Testing for Id expression in serial (1982 to 1986) serum samples of BS with the R-anti-Id as probes showed that BS-82 Id declined and was undetectable after October, 1984, whereas BS-86 Id was first detectable in July, 1985, and increased by June, 1986. These results clearly demonstrate spontaneous shifts in Id expression of human anti-DNA antibodies. The phenomenon of Id shift should be considered in any future strategy for the diagnosis and therapy of human autoimmune disease by anti-Id.

Spontaneous shift of anti-DNA antibody idiotypes in systemic lupus erythematosus

Spontaneous shift in Id expression of polyclonal anti-DNA antibodies in a patient, BS, with SLE was investigated. BS had active lupus nephritis in 1982 and developed central nervous system lupus in 1986 without evidence of active nephritis. Two rabbit polyclonal anti-Id (BS-82 and BS-86 R-anti-Id) were raised against affinity-purified anti-DNA antibodies prepared from 1982 serum (BS-82) and 1986 serum (BS-86), respectively. In addition, murine monoclonal anti-Id was prepared against BS-82 Id. Direct binding assays showed that all three anti-Id had preferential binding to the immunizing anti-DNA antibodies (the homologous Id) and poor binding to anti-DNA antibodies prepared from the different dated sample of BS. This was confirmed by inhibition assays of binding of anti-Id to the homologous Id by various Id. Moreover, inhibition assays of binding of various Id to DNA by the R-anti-Id showed that the R-anti-Id was the most effective inhibitor for the homologous Id. Testing for Id expression in serial (1982 to 1986) serum samples of BS with the R-anti-Id as probes showed that BS-82 Id declined and was undetectable after October, 1984, whereas BS-86 Id was first detectable in July, 1985, and increased by June, 1986. These results clearly demonstrate spontaneous shifts in Id expression of human anti-DNA antibodies. The phenomenon of Id shift should be considered in any future strategy for the diagnosis and therapy of human autoimmune disease by anti-Id.

Barley mutants lacking chloroplast glutamine synthetase-biochemical and genetic analysis

Eight mutants of barley (Hordeum vulgare cv Maris Mink) lacking the chloroplast isozyme of glutamine synthetase (EC 6.3.1.2.) were isolated by their inability to grow under photorespiratory conditions. The cytoplasmic isozyme of glutamine synthetase was present in the leaves of all the mutants, with activities comparable to the wild-type (10-12 nanokatals per gram fresh weight). The mutant plants developed normally and were fully fertile under conditions that minimize photorespiration. In 1% O(2) the rate of CO(2) fixation in leaves of one of the mutants, RPr 83/32, was the same as the wild-type, but in air this rate declined to 60% of the wild-type after 30 minutes. During this time the ammonia concentration in leaves of the mutant rose from 1 to 50 micromoles per gram fresh weight. Such ammonia accumulation in air was found in all the mutant lines. In back-crosses with the parent line, F(1) plants were viable in air. In the F(2) generation, nonviability in air and the lack of chloroplast glutamine synthetase co-segregated, in both the lines tested. These two lines and four others proved to be allelic; we designate them gln 2a-f. The characteristics of these mutants conclusively demonstrate the major role of chloroplast glutamine synthetase in photorespiration and its associated nitrogen recycling.

Detection of infectious pancreatic necrosis virus using an inhibition enzyme-linked immunosorbent assay

An inhibition enzyme-linked immunosorbent assay was used to detect infectious pancreatic necrosis (IPN) virus. In this assay the presence of virus was determined by measuring the decrease in titer of a known antiserum after incubation with a sample suspected to contain virus. The titer of the antiserum was measured with an indirect enzyme-linked assay. Compared to the double antibody sandwich method this assay required fewer reagents (only one anti-IPN serum was required). This assay was also sensitive enough to detect virus at levels of 1 X 10(2) TCID 50/ml. of purified virus and was able to detect virus in samples obtained in the field.

Biochemical characterisation of Nicotiana plumbaginifolia auxotrophs that require branched-chain amino acids

The biochemical lesions in six amino acid-requiring auxotrophic lines of Nicotiana plumbaginifolia have been investigated, by means of feeding experiments with [(14)C] and unlabelled substrates, and enzyme analysis. Three of the lines require isoleucine for growth, are unable to synthesise 2-oxobutyrate in vivo and have no detectable threonine dehydratase (E.C.4.2.1.16) in vitro. The other three lines require (isoleucine + valine), accumulate [(14)C] dihydroxymethylvalerate when fed [(14)C]-L-threonine, and contain no detectable dihydroxyacid dehydratase (E.C.4.2.1.9).

Altered feedback sensitivity of acetohydroxyacid synthase from valine-resistant mutants of tobacco (Nicotiana tabacum L.)

Acetohydroxyacid synthase (EC 4.1.3.18) has been extracted from leaves of three valine-resistant (Val(r)) tobacco (Nicotiana tabacum) mutants, and compared with the enzyme from the wild-type. The enzyme from all three mutants is appreciably less sensitive to inhibition by leucine and valine than the wild-type. Two of the mutants, Val(r)-1 and Val(r)-6, have very similar enzymes, which under all conditions are inhibited by less than half that found for the wild-type. The other mutant, Val(r)-7, has an enzyme that only displays appreciably different characteristics from the wild-type at high pyruvate or inhibitor concentrations. Enzyme from Val(r)-7 also has a higher apparent Km for pyruvate, threefold greater than the value determined for the wild-type and the other mutants. The sulphonylurea herbicides strongly inhibit the enzyme from all the lines, though the concentrations required for half-maximal inhibition of enzyme from Val(r)-1 and Val(r)-6 are higher than for Val(r)-7 or the wildtype. No evidence has been found for multiple isoforms of acetohydroxyacid synthase, and it is suggested that the valine-resistance of these mutant lines is the result of two different mutations affecting a single enzyme, possibly involving different subunits.

Regulation of Aspartate Kinase Isoenzymes in Barley Mutants Resistant to Lysine plus Threonine : Construction and Analysis of Combinations of the Lt1a, Lt1b, and Lt2 Mutant Genes

Two homozygous mutant lines of barley (Hordeum vulgare L.) R3202 (Lt1b/Lt1b) and R3004 (Lt2/Lt2), are resistant to lysine plus threonine. They contain aspartate kinase isoenzymes with lost or decreased feedback sensitivity to lysine in either isoenzyme AKII (R3202) or isoenzyme AKIII (R3004). A homozygous double mutant line (Lt1b/Lt1b, Lt2/Lt2) has now been constructed that grows vigorously on 8 millimolar lysine, 8 millimolar threonine, and 1 millimolar arginine. Both AKII and AKIII from the double mutant have altered lysine sensitivities, identical to those previously observed in R3202 and R3004, respectively. Aspartate kinase activity in extracts of leaves, roots, and the maturing endosperm of the double mutant was much less sensitive to lysine inhibition than the enzyme in comparable extracts of the parent cv Bomi, suggesting that aspartate kinase is expressed in a similar manner in different tissues of barley.A further mutant, R2501, resistant to lysine plus threonine has now given rise to a homozygous line (Lt1a/Lt1a), which had previously not been possible. AKII isolated from the homozygous line was completely insensitive to 10 millimolar lysine; however, the combined action of 10 millimolar lysine and 0.8 millimolar S-adenosylmethionine inhibited it by 60%, demonstrating the retention of some of the regulatory characteristics of the wild type enzyme.

Lysine transport in two barley mutants with altered uptake of basic amino acids in the root

Amino acid uptake was examined in two barley (Hordeum vulgare L.) mutants R906 and R4402 which had been selected as resistant to the lysine analog S-(2-aminoethyl)-cysteine. The mutants were found to be allelic by crossing and examination of F(1) and F(2) progeny. The mutant genes were designated aec1a and aec1b, respectively. The uptake of the basic amino acids lysine, arginine, and ornithine from 50 micromolar solutions was strongly decreased in roots of the mutants, whereas uptake of neutral and acidic amino acids was unaffected. The pattern of uptake of lysine over the range 10(-7) to 10(-2) molar was consistent with there being, principally, two uptake systems operating for basic amino acids in roots and that a low-concentration, high-affinity system is reduced or lacking in the mutants. The residual transport activity in the mutants had a different relative affinity for lysine and arginine to the wild-type system. Uptake of lysine by leaf slices was unimpaired in the mutants suggesting that the leaf uptake system is unaffected by the aec1 gene.

Feedback-insensitive aspartate kinase isoenzymes in barley mutants resistant to lysine plus threonine

The regulatory properties of aspartate kinase (EC 2.7.2.4) and homoserine dehydrogenase (EC 1.1.1.3) in two barley (Hordeum vulgare L.) mutants resistant to growth inhibition by lysine plus threonine, Rothamsted (R) 3004 and R3202, were compared with those in the normal, sensitive parent line cv. Bomi. Three forms of aspartate kinase (AKI, AKII, AKIII) were chromatographically separated and were considered to represent at least three independently regulated isoenzymes. Aspartate kinase I was inhibited by threonine; AKII and AKIII by lysine or lysine plus S-adenosylmethionine. The characteristics of AKI were unchanged in the mutants. Aspartate kinase II and AKIII from Bomi were both inhibited by lysine and by lysine plus S-adenosylmethionine. Aspartate kinase II from mutant R3202 was altered in its properties such that it was insensitive to lysine or lysine plus S-adenosylmethionine; AKII from mutant R3004 did not differ in its properties from AKII of Bomi. The concentration of lysine required to give half maximal inhibition of AKIII from R3004 was ten times that required for AKIII of Bomi; AKIII from R3202 did not differ from that of Bomi in this regard. There was no change in the properties of homoserine dehydrogenase of the mutants as compared with that of Bomi. We conclude that the lt1 and lt2 loci code for structural genes for lysine- and lysine plus S-adenosylmethionine-sensitive aspartate kinase isoenzymes. The mutant genes Lt1b and Lt2 in R3202 and R3004 respectively code for feedback-desensitized isoenzymes. The presence of one of these is sufficient to allow the synthesis of methionine to overcome the growth inhibition by lysine plus threonine.

Chromosome variation in protoplast-derived potato plants

Chromosomes have been studied in protoplast-derived potato plants of the tetraploid cultivars Maris Bard and Fortyfold. A high degree of aneuploidy was found amongst the regenerants of both cultivars but the nature of the chromosome variation differed. The Maris Bard regenerants were characterised by high chromosome numbers, a wide range of aneuploidy (46-92) and a low percentage of plants with the normal chromosome number (2n = 48), whereas a much higher proportion of the Fortyfold regenerants had 48 chromosomes and the variants were within a more limited aneuploid range. In both cultivars chromosome variation was found between calluses, within calluses and even within shoot cultures. The origin of the chromosome variation and the differences found between the two cultivars are discussed.

Threonine accumulation in the seeds of a barley mutant with an altered aspartate kinase

Barley (Hordeum vulgare L.) mutants altered in the regulation of synthesis of aspartate-derived amino acids were sought by screening embryos for growth on a medium containing lysine plus threonine. One mutant, Rothamsted 2501, was selected with good growth. From the segregation of resistance in the following generations, it was concluded that the resistance was conferred by a dominant gene, Lt1. No homozygous Lt1/Lt1 fertile plants have been recovered. Partially purified aspartate kinase preparations from resistant and sensitive plants were separated on DEAE-cellulose chromatography into three peaks of activity (I, II, III) and the feedback regulatory properties of these peaks determined. These peaks are considered to be three isozymic forms of aspartate kinase, one predominantly sensitive to threonine and two sensitive to lysine or lysine plus S-adenosyl methionine. The feedback characteristics of one of the peaks of aspartate kinase activity from resistant plants were changed such that lysine was half-maximally inhibitory at 10 rather than 0.4 mM. Increases in te concentrations of the free pools of threonine (4x) and methionine (2x) were measured in young plants grown on a basal medium. Threonine in the soluble fraction of mature seeds from resistant plants was increased from 0.8 to 9.6% of the total threonine content. The total content of both threonine and methionine of the seeds was increased by 6% compared with grain of similar nitrogen content.

Variation amongst protoplast-derived potato plants (Solatium tuberosum cv. 'Maris Bard')

Plants were obtained from protoplasts of shoot cultures of potato (Solarium tuberosum L. cv. 'Maris Bard') and from in situ calluses upon plants of cv. 'Majestic'. None of the protoplast-derived plants resembled each other in all of ten morphological characteristics scored and only one resembled the parental 'Maris Bard' type. As there were a number of plants regenerated from each of ten protoplast-derived calluses it is concluded that variation arose after protoplast isolation during the cell culture phase. Plants regenerated from in situ calluses of cv. 'Majestic' were quite uniform. Reported cases of variation and uniformity from cultured potato tissues are discussed. It is concluded that the variation is not a consequence of using protoplasts and that the expression or induction of variation is controllable.

Proline accumulation in a barley mutant resistant to trans-4-hydroxy-L-proline

Five proline analogues were tested for inhibition of the growth of mature barley (Hordeum vulgare L.) embryos in sterile culture. Inhibition by all analogues was relieved by proline. Inhibition by trans-4-hydroxy-L-proline was relieved by low amounts of proline. Twenty thousand mature embryos were dissected from M2 seeds after sodium azide mutagenesis. Four plants (Rothamsted 5201, 6102, 6901, 6902) were selected with good growth on 4 mM trans-4-hydroxyproline. Properties of mutant R5201 were studied in detail. Selfed progeny of R5201 were all resistant to trans-4-hydroxyproline and also to L-thiazolidine-4-carboxylic acid and trans-3-hydroxy-L-proline but not L-azetidine-2-carboxylic acid. The content of soluble proline in progeny of R5201 was higher in leaves by a factor of up to six-fold. Proline content was measured in the soluble fraction of the terminal 20 mm of 4 d old plants subjected to severe water stress in 40% w/v polyethylene glycol. Leaves of the mutant contained more proline initially and accumulated proline morer rapidly than the parental leaves. As mutant leaves were larger and lost water more rapidly the greater increase in proline may have been caused by more severe water stress. Resistance to trans-4-hydroxyproline in R5201 was due to a single partially dominant nuclear gene.

Lysine metabolism in a barley mutant resistant to S(2-aminoethyl)cysteine

Lysine and S(2-aminoethyl)cysteine (AEC) metabolism were investigated in normal barley (Hordeum vulgare L. cv. Bomi) and a hemozygous recessive AEC-resistant mutant (R906). Feedback regulation of lysine and threonine synthesis from [(14)C] acetate was unimpaired in plants of the mutant 3 d after germination. Seeds of Bomi and R906 contained similar total amounts of lysine, threonine, methionine and isoleucine. Concentrations of these amino acids in the soluble fraction of plants grown 6 d without AEC were also similar. The concentration of AEC in R906 plants was less than in the parent variety when both were grown in the presence of 0.25 mM AEC for 6 d. The uptake of [(3)H]AEC and [(3)H]lysine by roots of R906 was, respectively, 33% and 32% of that by Bomi roots whereas the uptake of these compounds into the scutellum was the same in both the mutant and its parent. The uptake of [(3)H]leucine and its incorporation into proteins was also the same in Bomi and R906 plants. These results suggest that a transport system specific for lysine and AEC but not leucine is altered or lost in roots of the mutant R906. AEC is incorporated into protein and this could be the reason for inhibition of growth rather than action as a false-feedback inhibitor of lysine biosynthesis.

Amino Acid levels in carrot cell suspension culture: no correlation with aspartate kinase isoenzyme levels

Free and protein amino acids were analyzed during the growth cycle of a suspension culture of carrot (Daucus carota L.) in which there is a 10-fold increase in a lysine-sensitive isoenzyme of aspartate kinase during the early part of the growth cycle. There is little change in either total amino acids or in the amino acids derived from aspartate between 3 and 24 days of culture. It is estimated that the demand for net synthesis of aspartate-derived amino acids is decreased as the growth rate declines and that there is no correlation between the amount of extractable lysine-sensitive aspartate kinase and the amounts of amino acids being synthesized.

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.

Isolation of a recessive barley mutant resistant to S-(2-aminoethyl)L-cysteine

S-(2-aminoethyl)L-cysteine (AEC) inhibits the growth of mature barley (Hordeum vulgare L vars. 'Bomi' and 'Maris Mink') embryos grown on sterile medium. This inhibition is relieved by lysine and, to a lesser extent, arginine and ornithine. In order to try and select plants which accumulate lysine, 8200 M2 embryos of sodium azide mutagenised barley were screened for growth in the presence of 0.25 mM AEC. One line, R906 was selected for further characterisation. Progeny of the originally selected plant after selfing were all resistant to AEC. In a reciprocal cross with a sensitive barley the resistant trait was inherited as a single recessive nuclear gene which we designate aec-1.

The effect of aspartate-derived amino acids (lysine, threonine, methionine) on the growth of excised embryos of wheat and barley

Excised wheat (Triticum aestivum L. var. Maris Freeman) and barley (Hordeum vulgare L. var. Maris Mink) embryos were grown on medium containing both nitrate and ammonium ions. Addition of lysine (1 mM) plus threonine (1 mM) caused a synergistic inhibition of growth measured by length of first leaf or dry weight. The inhibition was specifically relieved by methionine, homocysteine and homoserine. Threonine at 0.2-0.3 mM caused half-maximal inhibition of growth at all lysine concentrations whereas lysine increased the synergistic inhibition up to 3 mM. The inhibition is explained by a model in which lysine acts as a feedback inhibitor of aspartate kinase and threonine of homoserine dehydrogenase. This is compatible with published studies of the enzymes involved. The implications of these findings for using lysine plus threonine as a selection system for lysine-overproducing cereals are discussed.

Aspartate kinase and the synthesis of aspartate-derived amino acids in wheat

Aspartate kinase (EC 2.7.2.4.) has been purified from 7 day etiolated wheat (Triticum aestivum L. var. Maris Freeman) seedlings and from embryos imbibed for 8 h. The enzyme was 50% inhibited by 0.25 mM lysine. In this study wheat aspartate kinase was not inhibited by threonine alone or cooperatively with lysine; these results contrast with those published previously. In vivo regulation of the synthesis of aspartate-derived amino acids was examined by feeding [(14)C]acetate and [(35)S]sulphate to 2-3 day germinating wheat embryos in culture in the presence of exogenous amino acids. Lysine (1 mM) inhibited lysine synthesis by 86%. Threonine (1 mM) inhibited threonine synthesis by 79%. Lysine (1 mM) plus threonine (1 mM) inhibited threonine synthesis by 97%. Methionine synthesis was relatively unaffected by these amino acids, suggesting that there are important regulatory sites other than aspartate kinase and homoserine dehydrogenase. [(35)S]sulphate incorporation into methionine was inhibited 50% by lysine (2 mM) plus threonine (2 mM) correlating with the reported 50% inhibition of growth by these amino acids in this system. The synergistic inhibition of growth, methionine synthesis and threonine synthesis by lysine plus threonine is discussed in terms of lysine inhibition of aspartate kinase and threonine inhibition of homoserine dehydrogenase.

A deficiency of hypoxanthine phosphoribosyltransferase in a sycamore callus resistant to azaguanine

Sycamore (Acer pseudoplatanus L.) calluses resistant to azaguanine were isolated after mutagenesis with nitrosoguanidine. One callus SNC61 was highly variable in its growth on 10 μg/ml azaguanine and it was concluded that its resistance was not due to a stable alteration in the callus. Another callus SNE31 was isolated after 4.5 months culture in 0.5 or 1 μg/ml azaguanine which had a 10-fold increase in resistance to azaguanine. This was shown to be accompanied by a reduction to 50% of the normal level of hypoxanthine phosphoribosyltransferase (E.C.2.4.2.8).