Quantification aspects of constant pressure (ultra) high pressure liquid chromatography using mass-sensitive detectors with a nebulizing interface

The present contribution investigates the quantitation aspects of mass-sensitive detectors with nebulizing interface (ESI-MSD, ELSD, CAD) in the constant pressure gradient elution mode. In this operation mode, the pressure is controlled and maintained at a set value and the liquid flow rate will vary according to the inverse mobile phase viscosity. As the pressure is continuously kept at the allowable maximum during the entire gradient run, the average liquid flow rate is higher compared to that in the conventional constant flow rate operation mode, thus shortening the analysis time. The following three mass-sensitive detectors were investigated: mass spectrometry detector (MS), evaporative light scattering detector (ELSD) and charged aerosol detector (CAD) and a wide variety of samples (phenones, polyaromatic hydrocarbons, wine, cocoa butter) has been considered. It was found that the nebulizing efficiency of the LC-interfaces of the three detectors under consideration changes with the increasing liquid flow rate. For the MS, the increasing flow rate leads to a lower peak area whereas for the ELSD the peak area increases compared to the constant flow rate mode. The peak area obtained with a CAD is rather insensitive to the liquid flow rate. The reproducibility of the peak area remains similar in both modes, although variation in system permeability compromises the 'long-term' reproducibility. This problem can however be overcome by running a flow rate program with an optimized flow rate and composition profile obtained from the constant pressure mode. In this case, the quantification remains reproducibile, despite any occuring variations of the system permeability. Furthermore, the same fragmentation pattern (MS) has been found in the constant pressure mode compared to the customary constant flow rate mode.

On-column sample enrichment for the high-sensitivity sheath-flow CE-MS analysis of peptides

A sample enrichment technique to increase sensitivity in capillary electrophoresis-mass spectrometry (CE-MS) is described. Peptides or glycopeptides are retained and concentrated on a short (3-5-mm) reversed-phase (C18) packed-bed situated in the fused-silica separation capillary and are subsequently released for electrophoretic separation by injection of an organic elutant. The concentration limits of detection are in the high picomolar range with a sheath-flow CE-MS interface.

Practical evaluation of the influence of excessive sample concentration on the estimation of dissociation constants with affinity capillary electrophoresis

A practical approach for the evaluation of binding constants with affinity capillary electrophoresis (ACE) is presented using the different linear and nonlinear regression methods. The influence of the sample concentration on the different obtained curves is depicted and it is shown that the different representations should always be compared. The well-known strong molecular interaction between the macrocyclic antibiotic vancomycin and a (D)-Ala-(D)-Ala terminating peptide is used as model in this study.

Considerations concerning interaction characterization of oligopeptide mixtures with vancomycin using affinity capillary electrophoresis-electrospray mass spectrometry

In the past few years affinity capillary electrophoresis (ACE) has proven to be a powerful tool to study molecular interactions. In ACE the change in electrophoretic mobility between a free and a complexed ligand with a receptor dissolved in the background electrolyte is observed. It provides an accurate way to calculate binding or dissociation constants and, when coupled to mass spectrometry, it forms a promising method to analyze solution-based combinatorial libraries. We report a model study on the macrocyclic antibiotic vancomycin using a 36-component library of tetrapeptides of the type 9-fluorenylmethoxycarbonyl (Fmoc)-L-Asp-L-Asp-D-Xaa-D-Xaa. The mass spectrometry conditions were optimized by fine-tuning the background electrolyte and sheath flow composition to achieve optimal sensitivity in the negative ionization mode. Different types of capillaries were also evaluated on their potential to screen combinatorial libraries. The library components that show the strongest interaction were identified. The dissociation constants of a mixture of six compounds with a broad affinity range were simultaneously established by Scatchard analysis on ACE-MS.

Comparative studies of the inhibitory action of some legume seeds, potato tubers, and bran against human and bovine proteinases

Inhibition of human trypsin and chymotrypsin by proteinase inhibitors of plant origin was studied using the juice from the small intestine, without separation of the two enzymes, and synthetic amide substrates (BAPA, GLUPHEPA). The results were compared with the inhibition of the corresponding bovine enzymes. Extracts and preparations from legume seeds (13 Papilionoideae, 2 Caesalpinioideae, 3 Mimosoideae), potato tubes and bran were used as inhibitors; 9 of the seed extracts studied inhibited human chymotrypsin more and human trypsin less than the bovine enzymes. In similar tests 5 seed extracts inhibited the two human enzymes more than the bovine enzymes, whilst only one extract inhibited the two human enzymes less than the two bovine enzymes, and another inhibited human trypsin more and human chymotrypsin less than the bovine enzymes. In particular, human chymotrypsin was between two and twelve times as strongly inhibited as the bovine enzyme by some two thirds of the species studied, sometimes exhibiting chymotrypsin-inhibitory activities usually observed only with trypsin. Inhibitor preparations from the above legumes and two non-leguminous plant foods exhibited similar results.

Characterization of the flavoenzyme enoyl reductase of fatty acid synthetase from yeast

Enoyl reductase in the fatty acid synthetase from brewer's yeast, a flavoenzyme function, has been used as a specific probe for one partial activity of the multi-functional enzyme. The enzyme has an absorption maximum at 460 nm with epsilon = 18600 M-1 cm-1 and A280 = 1.37 mg-1 ml. The circular dichroism spectrum shows negative peaks at 373 and 466 nm. The fluorescence maximum is at 540 nm. The apoenzyme has an absorption maximum at 279 nm and shows fluorescence at 345 nm. The association constant for the FMN is 4 X 10(7) M-1. The redox potential was determined as Eh = --0.193 V. The reductase is characterized as a 'true' transhydrogenase as no flavin free radical can be obtained by photochemical or chemical reduction or oxidation, i.e. it only functions via two-electron steps. An interpretation of the hydrophobic nature of the flavin binding site based on spectral data is presented.

New experiments of biotin enzymes

The objects of structural studies on biotin-enzymes were acetyl CoA-carboxylase and pyruvate carboxylase of Saccharomyces cerevisiae and beta-methylcrotonyl CoA-carboxylase and acetyl CoA-carboxylase of Achromobacter IV S. It was found that these enzymes can be arranged in three groups. In the first group, as represented by acetyl CoA-carboxylase of Achromobacter, the active enzyme could be resolved in three types of functional components: (1) the biotin-carboxyl carrier protein, (2) the biotin carboxylase, and (3) the carboxyl transferase. In the second group, as represented by beta-methylcrotonyl CoA-carboxylase from Achromobacter only two types of polypeptides are present. The one carries the biotin carboxylase activity together with the biotin-carboxyl-carrier protein, the other one carries the carboxyl transferase activity. In this third group, as represented by the two enzymes of yeast, all three catalytic functions are incorporated in one multifunctional polypeptide chain. The evolution of the different enzymes is discussed. The animal tissues acetyl CoA-carboxylase is under metabolic control, as known from previous studies. It thus has to be expected that the levels of malonyl CoA in livers of rats in all states of depressed fatty acid synthesis are much lower than under normal conditions because the carboxylation of acetyl CoA is strongly reduced and cannot keep pace with the consumption of malonyl CoA by fatty acid synthetase. A new highly sensitive assay method for malonyl CoA was developed which uses tritiated NADPH and measures the incorporation of radioactivity into the fatty acids formed from malonyl CoA in the presence of purified fatty acid synthetase. The application of this method to liver extracts showed that the level of malonyl CoA which amounts to about 7 nmoles per gram of wet liver drops to less than 10% within a starvation period of 24 hr and even further if the starvation period is extended to 48 hr. A low malonyl CoA concentration is also found in the alloxan diabetic animals and in animals being fed a fatty diet after starvation. On the other hand, feeding a carbohydrate rich diet leads to malonyl CoA levels surpassing the levels found after feeding a balanced diet. These observations reconfirm the concept that fatty acid synthesis is principally regulated by the carboxylation of acetyl CoA.

Identity of malonyl and palmitoyl transferase of fatty acid synthetase from yeast. Functional interrelationships between the acyl transferases

Functional interrelationships between the acyl transferases of yeast fatty acid synthetase were investigated. In binding assays with synthetase modified by 5,5'-dithiobis(2-nitrobenzoic acid), 4--5 malonyl transferase entities per multienzyme complex molecule could be titrated. In the presence of palmitoyl-CoA these malonyl transferases were found inaccessible to malonyl-CoA, whereas the acetyl transferases were reactive towards acetyl-CoA. Between four and five palmitoyl transferase entities per synthetase equivalent were found reactive towards palmitoyl-CoA, the palmitoyl binding being inhibited by malonyl-CoA. Following palmitoyl binding the acetyl transferases were found towards acetyl-CoA. Substrate model assays were consistent with these data. It is concluded that malonyl and palmitoyl transferases are closely coupled enzyme components of the multienzyme complex which are fairly independent of the acetyl transferase entities. The molecular basis for the observed coupling will be given in the following paper.

Identity of malonyl and palmitoyl transferase of fatty acid synthetase from yeast. 2. A comparison of active-site peptides

Active-site peptides of malonyl and palmitoyl transferase from yeast fatty acid synthetase were isolated and sequenced to try to prove the hypothesis [J. Ayling, R. Pirson & F. Lynen (1979) Biochemistry 11, 526--533] that both enzymes are identical. For this purpose synthetase modified with 5,5'-dithiobis(2-nitrobenzoic acid) was labelled with either [14C]malonyl or [14C]palmitoyl residues followed by proteolytic digestion of the labelled protein. [14C]Malonyl-peptides were isolated by conventional purification procedures; their structures were determined by a combination of methods. [14C]Palmitoyl-peptide material was purified by high-performance liquid chromatography and the structure determined by solid-phase Edman degradation and other analytical methods. Serine was identified as the acyl acceptor group in both transferases. Comparison of the sequence data available shows that the sequence around the acyl acceptor group in both cases is identical. This proves the identity of malonyl and palmitoyl transferase.

Studies on the multi-enzyme complex of yeast fatty-acid synthetase. Reversible dissociation and isolation of two polypeptide chains

1. The multi-enzyme complex of fatty acid synthetase, Mr 2300,000, was dissociated by acylation with dimethyl maleic anhydride under conditions which lead to an acylation of about 30% of the epsilon amino groups of lysine. The complete dissociation into the subunits alpha and beta is demonstrated by analytical ultracentrifugation as well as disc gel electrophoresis. 2. This dissociation is reversible. Hydrolysis of the resulting protein dicarboxylic acid monoamides under mildly acidic conditions leads to the unmodified subunits, which can be reconstituted to form a complex displaying about 60% of the original activity. 3. The subunits were isolated by sucrose-density-gradient centrifugation and studied for the different partial enzyme activities involved in long-chain fatty acid synthesis: malonyl, palmitoyl and acetyl transferase, enoyl reductase and dehydratase were shown to be exclusive functions of the beta chains of the complex, confirming a pentafunctional role of this subunit.

Distribution of yeast fatty acid synthetase subunits: three-dimensional model of the enzyme

Rabbit and goat antibodies against the isolated alpha and beta subunits of yeast fatty acid aynthetase were raised and characterized. The purified IgG fractions were studied as to their capability to precipitate their antigens and the holoenzyme and to inhibit the partial reactions involved in overall fatty acyl-CoA synthesis. The specificity of the antibodies was investigated by immunodiffusion and by immunotitration. Native enzyme was crosslinked with each of the antibodies, and dimeric and oligomeric groups of IgG-crosslinked fatty acid synthetase molecules were isolated by sucrose density gradient centrifugation. Electron microscopic investigation of the crosslinked material as well as other data led us to suggest a three-dimensional model of yeast fatty acid synthetase.

The palmityl binding sites of fatty acid synthetase from yeast

Fatty acid synthetase was covalently labelled with [14C]palmitic acid from [14C]palmityl-CoA. Tryptic and peptic digestion of the [14C]palmityl enzyme resulted in the formation of radioactive palmityl peptides carrying the long-chain acyl residue both in oxygen-ester and thio-ester linkage. The lipophilic palmityl peptides were purified by column and thin-layer chromatography using organic lolvent systems. Peptides arising from the acyl carrier protein, the condensing enzyme and the palmityl transferase were identified and characterized. The amino acid sequence of a 4'-phosphopant-etheine-containing peptide was established. It comprises 13 residues and shows a high degree of homology with the acyl carrier protein from Escherichia coli. A heptapeptide and an octapeptide from the palmityl transferase active site were partially sequenced. The identical amino acid composition of palmityl transferase and malonyl transferase core peptides is briefly discussed.

Reaction of yeast fatty acid synthetase with iodoacetamide. 3. Malonyl-coenzyme A decarboxylase as product of the reaction of fatty acid synthetase with iodoacetamide

Yeast fatty acid synthetase possesses very low malonyl-CoA decarboxylase activity. Treatment with iodoacetamide, while abolishing synthetase activity, induces a strong malonyl decarboxylase activity which, in turn, can be inhibited by N-ethylmaleimide. Kinetic analysis shows that the emergence of the decarboxylase activity is synchronized to the disappearance of the fatty-acid-synthesizing activity and thus, is due to carboxamidomethylation of the peripheral SH-groups of the multienzyme complex. Strong decarboxylase activity was also found after treatment of the synthetase with methylmalonyl-CoA. A hypothetical scheme is proposed which explains the origination of the decarboxylase activity as a consequence of conformational changes of the condensing enzyme component which happen when the peripheral SH-group is acylated or alkylated.

6-Methylsalicylic acid synthetase from Penicillium patulum. Some catalytic properties of the enzyme and its relation to fatty acid synthetase

1. The specificity of 6-methylsalicylic acid synthetase with respect to the priming substrate was studied. If acetyl-CoA was replaced by propionyl-CoA 6-ethylsalicylic acid was synthesized. The rate of this synthesis was about 13% that of 6-methylsalicylic acid synthesis in presence of acetyl-CoA. 2. 6-Methylsalicylic acid synthetase contains an acetyl transferase activity as demonstrated by the transfer of acetyl residues from acetyl-CoA to pantetheine. This transferase also catalyses propionyl transfer but only with an about 13 times slower rate. 3. Analogous to fatty acid synthetase, treatment with iodoacetamide converts 6-methylsalicylic acid synthetase into a malonyl-CoA decarboxylase. Under certain conditons the iodoacetamide-treated enzyme could catalyse 6-methylsalicylic acid formation from malonyl CoA and NADPH without the external addition of acetyl CoA.

Investigations of the structure of 3-methylcrotonyl-CoA carboxylase from Achromobacter

It was shown by gel electrophoresis in sodium dodecylsulphate solution that 3-methylcrotonyl-CoA carboxylase from Achromobacter IVS is composed of two different subunits with molecular weights of about 78000 and 96000, respectively. The biotin is bound to the heavier subunit. It was previously found that 3-methylcrotonyl-CoA carboxylase contains four biotin molecules per complex. A complex composed of four of each subunit would thus have a molecular weight of about 700000. This is compatible with the molecular weight of 760000 determined earlier by analytical ultracentrifugation. Both subunits were isolated preparatively. As the subunits, unlike the complex, are very sensitive to oxygen, special precautions had to be taken during isolation. The biotin-containing subunit was isolated by chromatography on DEAE-cellulose in 5 M urea. It no longer catalyzed the overall reaction, yet could still carboxylate free biotin. The biotin-free subunit was separated after dissociation of the enzyme by three-days' dialysis at pH 9.8 under nitrogen. On chromatography over a Sepharose-bound avidin column, the biotin-subunit was fixed and the biotin-free subunit was eluted unretarded. The latter subunit showed no enzymic activity. After the addition of the biotin-containing subunit, overall activity was regenerated. The speed of reassociation is very much enhanced by 3-methylcrotonyl-CoA. It was shown by reassociation experiments under different conditions that probably an initial complex, AxBy is formed, possessing a binding site for 3-methylcrotonyl-CoA. Upon the binding of this substrate the conformation may be changed to a form favourable for reconstitution. Finally, the structures of biotin enzymes from different sources are compared. In the course of evolution there is a tendency toward integration of the different constituent proteins into only one polypeptide chain.

Patulin biosynthesis: the metabolism of m-hydroxybenzyl alcohol and m-hydroxybenzaldehyde by particulate preparations from Penicillium patulum

The ring hydroxylation of m-hydroxybenzyl alcohol to gentisyl alcohol by a particulate preparation from Penicillium patulum has been characterised. The activity was shown to be closely associated with, but not necessarily identical to, m-cresol 2-hydroxylase activity of the 105 000 X g microsomal fraction. As with both the m-cresol hydroxylases of this system, m-hydroxybenzyl alcohol hydroxylase required oxygen and NADPH for activity. A Km value for m-hydroxybenzyl alcohol of 15 muM was measured. Inhibition of the hydroxylase activity and its reversal by light, as well as the action of cytochrome c, KCN and other effectors suggested a mixed-function oxidase reaction of the cytochrome P-450, NADPH-cytochrome reductase type. m-Hydroxybenzaldehyde was not ring hydroxylated by any preparation from P. patulum. Apart from the previously described conversion to m-hydroxybenzyl alcohol by a predominantly soluble dehydrogenase, m-hydroxybenzaldehyde was metabolized to m-hydroxybenzoic acid by a particulate fraction. This activity required NADPH. It was concluded that the main biosynthetic pathway to patulin must be through m-hydroxybenzyl alcohol, gentisyl alcohol and gentisaldehyde.

[Mechanism for the condensation reaction of fatty-acid biosynthesis (author's transl)]

The spontaneous hydrogen-deuterium exchange of the methylene group of malonyl-thioesters was investigated by nuclear-magnetic-resonance (NMR) spectroscopy using the model compound S-malonyl-N-acetylcysteamine. The half life of the methylene proteins is 12 to 16 min in 0.1 M K-phosphate buffer at pH 6.5 to 7.0 at 25 degrees C, the conditions of maximal activity of fatty acid synthetase from yeast. Proton catalysis was used for the quick preparation of deuterium- and tritium-labeled malonylthioesters. Compared with malonyl-CoA, dideutero-malonyl-CoA had no primary isotope effect on the reaction velocity of the yeast enzyme catalysed fatty acid synthesis, in which the rate limiting step is the condensation reaction. Although deuterium oxide had a solvent isotope effect, there was no difference in reaction velocities between malonyl CoA and dideuteromalonyl CoA in deuterium oxide. The condensation reaction was investiaged separately from the overall fatty acid synthesis using beta-ketoacyl-acyl-carrier-protein (ACP) synthetase (condensing enzyme) of Escherichia coli. The condensation reaction with deuteromalonyl-ACP had no kinetic isotope effect, in agreement with the observations on the overall reaction. However, in this case no solvent isotope effect was observed with 2H2O. When the condensation reaction was carried out in the presence of tritiated water, there was no incorporation of label into the reaction product acetoacetyl-thioester, excluding proton exchange with the solvent. The results exclude a mechanism for the condensation reaction involving a malonyl carbanion and its acylation as intermediates in the sense of an organic-chemical malonic ester synthesis, and they indicate that the condensation reaction follows a concerted mechanism: The formation of the new carbon-carbon bond is coupled with the cleavage of the carboxyl bond of the malonyl group.