Fluorometric microbore amino acid analyzer: the construction of an inexpensive, highly sensitive instrument using o-phthalaldehyde as a detection agent

Vicious cycle involving Na+ channels, glutamate release, and NMDA receptors mediates delayed neurodegeneration through nitric oxide formation

The mechanisms by which neurons die after cerebral ischemia and related conditions in vivo are unclear, but they are thought to involve voltage-dependent Na+ channels, glutamate receptors, and nitric oxide (NO) formation because selective inhibition of each provides neuroprotection. It is not known precisely what their roles are, nor whether they interact within a single cascade or in parallel pathways. These questions were investigated using an in vitro primary cell culture model in which striatal neurons undergo a gradual and delayed neurodegeneration after a brief (5 min) challenge with the glutamate receptor agonist NMDA. Unexpectedly, NO was generated continuously by the cultures for up to 16 hr after the NMDA exposure. Neuronal death followed the same general time course except that its start was delayed by approximately 4 hr. Application of the NO synthase inhibitor nitroarginine after, but not during, the NMDA exposure inhibited NO formation and protected against delayed neuronal death. Blockade of NMDA receptors or of voltage-sensitive Na+ channels [with tetrodotoxin (TTX)] during the postexposure period also inhibited both NO formation and cell death. The NMDA exposure resulted in a selective accumulation of glutamate in the culture medium during the period preceding cell death. This glutamate release could be inhibited by NMDA antagonism or by TTX, but not by nitroarginine. These data suggest that Na+ channels, glutamate receptors, and NO operate interdependently and sequentially to cause neurodegeneration. At the core of the mechanism is a vicious cycle in which NMDA receptor stimulation causes activation of TTX-sensitive Na+ channels, leading to glutamate release and further NMDA receptor stimulation. The output of the cycle is an enduring production of NO from neuronal sources, and this is responsible for delayed neuronal death. The same neurons, however, could be induced to undergo more rapid NMDA receptor-dependent death that required neither TTX-sensitive Na+ channels nor NO.

Insulin-like growth factor I inhibits degradation and improves retention of protein in hindlimb muscle of lambs

We infused recombinant human insulin-like growth factor I (IGF-I) for 4 h at 12.3 micrograms.h-1.kg live weight-1 directly into the left femoral artery and measured the rates of synthesis, degradation, and gain of protein by the treated and contralateral limbs of well-fed (n = 8), feed-restricted (n = 10), and fasted (n = 9) castrated male lambs. Reducing feed intake decreased net protein gain of hindlimb muscle, reduced hindlimb glucose uptake, and lowered arterial plasma concentrations of IGF-I, insulin, glucose, phenylalanine, tyrosine, and isoleucine. The effect of nutrition on IGF-binding proteins (IGFBP) was generally small; IGFBP-2 was more abundant in fasted lambs. Infusion of IGF-I into the left femoral artery increased plasma levels of IGF-I 2- to 4-fold in the left femoral vein and by 1.5- to 3-fold in the artery and right femoral vein. In the treated limb, IGF-I reduced protein degradation, increased protein gain, and increased glucose uptake without altering blood flow or oxygen uptake, regardless of feed intake. Systemically, IGF-I reduced plasma insulin, phenylalanine, tyrosine, isoleucine, and leucine in all nutrition groups. Plasma IGFBP-3 was increased by 4 h of IGF-I treatment in fasted but not in fed lambs. In fed but not fasted lambs, IGF-I increased blood glucose concentration. Effects of IGF-I on protein metabolism in the contralateral limb were affected by nutrition, generally more so in fasted than in unrestricted fed lambs.

Aspartate analysis in formulations using a new enzyme sensor

A biosensor has been developed for the purpose of directly analysing aspartate in pharmaceutical formulations and aspartame in sweeteners. This biosensor consists of an ammonia-sensitive gas-diffusion electrode and the enzyme L-aspartase immobilized by means of polyazetidine on a dialysis membrane.

Determination of enzyme activity by high-performance liquid chromatography

The application of high-performance liquid chromatography (HPLC) in the study of enzymatic reactions is reviewed. The rationale for using HPLC is given and whether the components of the reaction mixture should be derivatized prior to or after HPLC. An alphabetical list of enzymes assayed by HPLC is given. Substrate and product are included as well the derivatization reagent, detection method and biological matrix. Specific examples of these assays in a complex biological matrix viz. faeces are given. Future prospects are the detection of new enzymes using synthetic substrates and implementation of mass spectrometry to elucidate enzyme specificities.

Measurement of endogenous amino acid losses in poultry

1. Ileal endogenous amino acid losses were determined in broiler chickens and in cannulated cross-bred layer strain cockerels using either a nitrogen-free diet, regression analysis or a 48 h fast. 2. Endogenous amino acid flows to the ileum in fasted cockerels were significantly lower than those obtained both by feeding the nitrogen-free diet, and from regression analysis in either broilers or cockerels. Regression analysis gave the highest flows. 3. The apparent digestibility coefficients of amino acids in a diet containing 200 g/kg crude protein were lower in broilers (0.84) than in cockerels (0.88). When corrected, by regression analysis, for the contribution of endogenous amino acids, the true digestibility coefficients became 0.90 and 0.92 respectively.

Both the outer mitochondrial membrane and the microsomal forms of cytochrome b5 reductase contain covalently bound myristic acid. Quantitative analysis on the polyvinylidene difluoride-immobilized proteins

NADH-cytochrome b5 reductase is known to be located on two distinct membranes, i.e. endoplasmic reticulum and outer mitochondrial membranes. The endoplasmic-reticulum-associated form of the enzyme contains myristic acid in an amide linkage to its N-terminal glycine [Ozols, Carr & Strittmatter (1984) J. Biol. Chem. 259, 13349-13354]. To investigate whether the dual subcellular localization of the reductase corresponds to a difference in fatty acylation, the enzyme was purified from well-characterized rat liver microsomal and mitochondrial fractions and analysed by a new quantitative analytical procedure. The purified reductases were run on SDS/polyacrylamide gels and blotted on to polyvinylidene difluoride membranes. The reductase-containing bands were treated with hydroxylamine, and amide-linked fatty acids were then detached by acid hydrolysis. The detached fatty acids were extracted, derivatized and analysed as phenylacyl esters by reverse-phase h.p.l.c., and the protein content of the samples was determined by amino acid analysis of the acid hydrolysates. Myristic acid was found in both the microsomal and mitochondrial reductases in a molar ratio of 1:1 with protein. These results demonstrate for the first time the presence of a myristylated protein on outer mitochondrial membranes, and show that the microsomal and mitochondrial reductases are also identical in their fatty acylation.

Pharmacological studies on lamotrigine, a novel potential antiepileptic drug: II. Neurochemical studies on the mechanism of action

Lamotrigine (LTG) [3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazine] is a novel anticonvulsant chemically unrelated to current antiepileptic drugs and with a pharmacological profile similar to that of phenytoin. The effect of LTG has been compared with that of phenytoin, on the release of endogenous amino acids and radiolabelled acetylcholine evoked by veratrine or potassium, from slices of rat cerebral cortex in vitro. Both veratrine and potassium evoked a marked release of glutamate and gamma-aminobutyric acid (GABA), with a more moderate release of aspartate. LTG inhibited veratrine-evoked release of glutamate and aspartate, with ED50 values of 21 microM for both amino acids, but LTG was less potent in the inhibition of GABA release (ED50 = 44 microM). At concentrations up to 300 microM, LTG had no effect on potassium-evoked amino acid release or on spontaneous release. Also, LTG was some five times less potent in the inhibition of veratrine-evoked [3H]acetylcholine release (ED50 = 100 microM) than in glutamate or aspartate release. The total lack of effect of LTG on potassium-evoked release and the potent effect on veratrine-evoked release (at concentrations found in rat brain after anticonvulsant doses) strongly suggest that LTG acts at voltage-sensitive sodium channels to stabilise neuronal membranes and inhibit transmitter release, principally glutamate. The role of glutamate in the aetiology of epilepsy is discussed.

An automated fluorescence assay for subnanogram quantities of protein in the presence of interfering material

An automated assay for measuring nanogram and subnanogram quantities of protein in microliter samples was developed with the fluorometric reagent omicron-phthaldialdehyde/mercaptoethanol. Low molecular weight interfering substances were separated within the analysis by gel filtration. The technique allowed measurement of biological fluids without any sample pretreatment. The method presented proved to be linear within the range 1.2 ng to 1.4 microgram with a standard deviation of +/- 4.2%. The minimum detectable protein concentration was 1 microgram/liter. Special care was taken to prevent any determination errors caused by losses of protein during adsorption to surfaces. The simple analytical apparatus constructed can be used for field studies and ran several weeks when continuously used for seawater analysis aboard ship.

Changes in cortical amino acids during electrical kindling in rats

The effect of kindling rats by electrical stimulation of the frontal cortex for approx. 3 months on the concentrations of amino acids (taurine, aspartate, glutamate, glutamine and GABA) in the cerebral cortex has been studied, as well as the release of endogenous amino acids from kindled slices of brain in vitro. In these kindled rats, killed 1 week after the last shock, there were no changes in any concentrations of amino acids. However, when cortical slices, prepared from either the control or kindled rats, were stimulated in vitro by exposure to veratrine, more endogenous glutamate and aspartate were released from the kindled tissue than from the control. The neurotransmitter amino acids, glutamate and aspartate, may be involved in the permanent effects of electrical kindling.

A method for quantitating nanogram amounts of soluble protein using the principle of silver binding

A highly sensitive and quantitative assay for measuring protein in solution based on the capacity of protein to bind silver is described. In this procedure, protein samples are first treated with glutaraldehyde and then exposed to ammoniacal silver. After 10 min, the reaction is terminated by the addition of sodium thiosulfate and the optical density measured at 420 nm. The useful range of the assay for the majority of standard proteins tested lies between 15 and 2000 ng. This represents a 100-fold increase in sensitivity over the Coomassie brilliant blue dye-binding procedure. There is little or no interference from carbohydrates, nonionic detergents, or ethanol, and pretreatment of protein samples with Bio-Gel P-2 to remove salts, thiol agents, EDTA, and sodium dodecyl sulfate makes this procedure compatible with most commonly used buffers. The cost in terms of silver utilization is nominal with a typical assay involving 10 samples tested in triplicate amounting to less than $0.02 U. S.

Protein determination with trinitrobenzene sulfonate: a method relatively independent of amino acid composition

Conditions are described for precise quantitative measurement of microgram protein samples by spectrophotometric determination of the trinitrobenzene derivatives of amino acids in hydrolysates. The mean molar absorbances of individual amino acids were measured and the effective molar absorbance for use in protein measurements of 1.9 X 10(4) A M-1 cm-1 has been determined. From measurements using the trinitrobenzene sulfonate and fluorescamine reagents, and the published data on the o-phthaldialdehyde method, the molar absorption coefficients and the relative fluorescent yields are compared for the amino acids derivatives found in protein hydrolysates. The coefficients of variation for the trinitrobenzene derivatives are less than that for either the fluorescamine or the o-phthaldialdehyde derivatives. The color yields for five soluble proteins were also compared using the Lowry, Bradford, and trinitrobenzene sulfonate reagents. The results show that the described trinitrobenzene sulfonate method is more sensitive and produces a threefold smaller variation in absorbance per milligram protein than either the Lowry or the Bradford methods.

Determination of beta-galactosidase activity in the intestinal tract of mice by ion-exchange high-performance liquid chromatography using epsilon-N-1-(1-deoxylactulosyl)-L-lysine as substrate

epsilon-N-1-(1-Deoxylactulosyl)-L-lysine was synthesized and used as a substrate to assay beta-galactosidase activity. epsilon-N-1-(1-Deoxylactulosyl)-L-lysine and its degradation product epsilon-N-1-(1-deoxyfructosyl)-L-lysine were detected by high-voltage paper electrophoresis and ion-exchange high-performance liquid chromatography. The beta-galactosidase activity in different parts of the intestinal tract of germ-free and control mice was determined and compared with a beta-galactosidase activity which degrades lactose at pH 8.5 and 5.0 and which corresponded with bacterial and host enzymatic activities, respectively.

beta-Hydroxyaspartic acid in vitamin K-dependent protein C

Previous work has shown that the light chain of protein C, an anticoagulant plasma protein, contains an unusual amino acid [Fernlund, P. & Stenflo, J. (1982) J. Biol. Chem. 257, 12170-12179]. To determine the structure of this amino acid a heptapeptide, CMCys-Ile-X-Gly-Leu-Gly-Gly (residues 69-75 in the light chain), was isolated from enzymatic digests of the light chain. According to automatic Edman sequence analysis, 1H NMR spectroscopy, and mass spectrometry the heptapeptide had beta-hydroxyaspartic acid in its third position, which corresponds to position 71 in the light chain of protein C. Analysis of acid and aminopeptidase M hydrolysates of the heptapeptide showed the beta-hydroxyaspartic acid to be the erythro form. Acid hydrolysis of protein C released approximately equal to 1 mol of beta-hydroxyaspartic acid per mol of protein. The function of this amino acid, which, to the best of our knowledge, has not been found previously in proteins, is unknown.

Amperometric detection of amino acids in high-performance liquid chromatography with a copper electrode

A copper electrode has been used as an amperometric detector for amino acids in high-performance liquid chromatography. The amino acids are separated in a reversed-phase system, using silica-based and polymer-type column materials. Neutral or alkaline buffer solutions of phosphate and carbonate can be used as mobile phases. Borate buffers are less suitable. The detection method is characterized by a high linear dynamic range, good reproducibility, the absence of electrode poisoning and a sensitivity comparable to that of UV absorption methods after derivatization of the amino acids. Detection limits with conventional-scale columns are in the range 10-100 pmoles. A reduction in the flow-rate in the flow-through cell improves the sensitivity for amino acids that give relatively low signals, such as proline. Therefore, the use of microbore columns is especially advantageous for these compounds. The absolute detection limits decrease by about one order of magnitude on changing to a miniaturized system.

A rapid microdetermination of phosphoserine, phosphothreonine, and phosphotyrosine in proteins by automatic cation exchange on a conventional amino acid analyzer

A cation-exchange chromatographic method for the separation and determination of phosphoserine, phosphothreonine, and phosphotyrosine in proteins after partial acid hydrolysis is described. The short column (0.6 X 8 cm) of an automatic amino acid analyzer was used and elution was carried out isocratically with 10 mM trifluoroacetic acid. The method is highly sensitive and each of the three O-phosphoamino acids can be accurately determined down to the 50-pmol level. Higher sensitivity may be obtained by the use of [32P]phosphate-labeled proteins. A correction factor for the decomposition of phosphoserine or phosphothreonine during acid hydrolysis can be deduced from the amount of inorganic phosphate recovered at the column void volume. The method is sensitive enough to be used for 32P-labeled proteins isolated by two-dimensional gel electrophoresis.

Isoenzyme specific inhibition of the reactivation of in vitro dissociated lactic dehydrogenase isoenzymes by two different peptides isolated from human liver

The catalytic activity of the LDH-isoenzymes depends on their tetrameric structure. Low pH or other denaturants leads to dissociation into monomers and to the loss of the specific activity. After removal of the denaturing conditions reassociation and reactivation occur spontaneously. Neither NADH nor NAD+ shows a significant effect on the reactivation. We have isolated two different peptides which isoenzyme specifically inhibit the reactivation of dissociated LDH. Inhibition was abolished by treating with proteases. Additionally, NAD+ and NADH were found to be antagonists of the inhibitors. The heart-type enzyme-inhibitor system is especially susceptible for NADH whereas NAD+ affects the inhibition only slightly. The muscle-type system shows the opposite behavior, e.g., the completely inhibited system can be fully reactivated by NAD+ but not by NADH. These findings together with first kinetic studies suggest a possible specific regulatory function of these peptides.

Amino acid analysis at the picomole level. Application to the C-terminal sequence analysis of polypeptides

Amino acids labelled with dimethylaminoazobenzenesulphonyl chloride can be separated by reversed-phase high-pressure liquid chromatography and detected in the visible region (436 nm). All 19 naturally occurring amino acids can be separated on a Zorbax ODS column by employing two different gradient systems consisting of an acetonitrile/aqueous buffer mixture. As little as 2--5 pmol of an individual dimethylaminoazobenzenesulphonyl-amino acid can be quantitatively analysed with reliability, and only 10--30 ng of the dimethylaminoazobenzenesulphonylated protein hydrolysate is needed for each complete amino acid analysis. This new technique is as sensitive as any of the current amino acid analysis methods involving ion-exchange separation plus fluorescence detection, and is technically much simpler. By the combination of this sensitive amino acid-analysing technique with carboxypeptidase, we have been able to determine the C-terminal sequence of polypeptides at the picomole level.

Peptides isolated from human liver with specific inhibitory effects on reassociation/reactivation of in vitro dissociated lactic dehydrogenase (LDH-M4 and -H4) isozymes

Two different peptides have been purified from human liver, similar to those previously reported (Schoenenberger, G.A., and Wacker, W.E.C. (1966) Biochemistry 5, 1375--1379) to be present in human urine, which may serve as metabolic regulators of lactate dehydrogenase (EC 1.1.1.27) isoenzymes (LDH-M4 = muscle type; LDH-H4 = heart type). By trichloroacetic acid precipitation, ultrafiltration, Sephadex G-25 and Bio-Gel P-2 columns, affinity chromatography on immobilized LDH-isozymes and HPLC two peptides which differed with respect to molecular weight, retention on the affinity columns and amino acid composition were isolated. No effect was observed when native, tetrameric lactate dehydrogenase was incubated with these peptides. However, when lactate dehydrogenase was dissociated to monomers at low pH and allowed to reassociate by adjusting the pH to 7.5 complete inhibition of the reactivation occurred when the inhibitors were incubated together with respective reassociating monomeric isozymes. The two peptides showed no cross-specificity, i.e. each peptide exhibited inhibitory activity only on one of the two isozymes LDH-M4 or LDH-H4. From the amino acid analyses, gel filtrations and PAGE + SDS, molecular weights of 1800 for the M4 and approximately 2700 for the H4 inhibitor were calculated. An apparent Ki of approximately 3 X 10(-5) mM for the H4 and approximately 7 X 10(-5) mM for the H4 inhibitor was estimated. The interaction of the inhibitors with the enzyme system showed strong cooperativity with Hill coefficients of 2.9 (LDH-M4-specific) and 2.4 (LDH-H4-specific). Mathematical modelling of the reassociation and reactivation of lactate dehydrogenase and its specific inhibition by the peptides led to the conclusion that the peptides react with monomers, dimers or a transition state during the tetramerisation process. kappa 1 for the dimerisation step of M4 = 2.0 X 10(5) M-1 . S-1 and of H4 = 8.2 X 10(4) M-1 . S-1; kappa 2 for the tetramerisation step of M4 = 2.8 X 10(5) M-1 . S-1 and of H4 = 1.2 X 10(5) . M-1 S-1, were calculated, the second step still being the faster one (Rudolf, R. and Jaenicke, R. (1976) Eur. J Biochem. 63, 409--417).