Analogs of pyridoxal or pyridoxal phosphate: relation of structure to binding with apoenzymes and to catalytic activity

Structural insights into the mechanism of internal aldimine formation and catalytic loop dynamics in an archaeal Group II decarboxylase

Formation of the internal aldimine (LLP) is the first regulatory step that activates pyridoxal 5'-phosphate (PLP) dependent enzymes. The process involves a nucleophilic attack on PLP by an active site Lys residue, followed by proton transfers resulting in a carbinolamine (CBA) intermediate that undergoes dehydration to form the aldimine. Despite a general understanding of the pathway, the structural basis of the mechanistic roles of specific residues in each of these steps is unclear. Here we determined the crystal structure of the LLP form (holo-form) of a Group II PLP-dependent decarboxylase from Methanocaldococcus jannaschii (MjDC) at 1.7 Å resolution. By comparing the crystal structure of MjDC in the LLP form with that of the pyridoxal-P (non-covalently bound aldehyde) form, we demonstrate structural evidence for a water-mediated mechanism of LLP formation. A conserved extended hydrogen-bonding network around PLP coupled to the pyridinyl nitrogen influences activation and catalysis by affecting the electronic configuration of PLP. Furthermore, the two cofactor bound forms revealed open and closed conformations of the catalytic loop (CL) in the absence of a ligand, supporting a hypothesis for a regulatory link between LLP formation and CL dynamics. The evidence suggests that activation of Group II decarboxylases involves a complex interplay of interactions between the electronic states of PLP, the active site micro-environment and CL dynamics.

Immunotoxicity of the colour additive caramel colour III; a review on complicated issues in the safety evaluation of a food additive

Food additives can be regarded as the safest constituents of our daily food. Nevertheless, complicated issues with respect to their safety evaluation do also occur. In this review paper, some of these issues are illustrated by the description and evaluation of the research on the immunotoxicity of the food additive Caramel Colour III. Caramel Colour III is commonly used as a color additive in many products for human consumption. Toxicity studies conducted in the seventies demonstrated that administration of Caramel Colour III can cause a reduction in total white blood cell counts in rats, due to reduced lymphocyte counts. Studies reviewed in this paper demonstrated several other effects of Caramel Colour III on the immune system of rodents, including disturbed immune functions and changed resistance in infection models. In addition, studies in rats demonstrated that most of the effects occur only when the animals are fed a diet low in vitamin B6. The imidazole derivative 2-acetyl-4(5)-(1,2,3,4-tetrahydroxy-butyl)-imidazole (THI) was found to be responsible for the immunotoxicity. Issues such as the mechanism of action of THI and the role of vitamin B6 are discussed. Finally, the results of a human intervention study and the observed effect levels of THI in rats are discussed in terms of safety of the use of Caramel Colour III in our daily food supply.

Catalytic function of a tyrosyl residue in tryptophanase

Tryptophanase has an essential tyrosyl residue/active site which can be modified by tetranitromethane. Pyridoxal 5'-phosphate can prevent this modification efficiently, whereas pyridoxal 5'-phosphate N-oxide cannot, indicating that the free pyridinium N is required for the interaction of the coenzyme with the tyrosyl residue, probably via a hydrogen bond. The weakened binding of the coenzyme to the modified enzyme was confirmed on gel filtration, the modified enzyme being dissociated from the coenzyme seven-fold faster than the native enzyme. Furthermore, absorption spectral analyses demonstrated that the modified enzyme can catalyze the transaldimination step, but fails to abstract the alpha-H of substrates. The tyrosyl residue, therefore, not only participates in coenzyme binding, but also contributes to alpha-H labilization.

Nutritional requirements of Plasmodium falciparum in culture. II. Effects of antimetabolites in a semi-defined medium

A semi-defined minimal medium, in which pantothenic acid is the only vitamin, was used to culture Plasmodium falciparum for the analysis of antimetabolite drugs. Analogs of riboflavin, nicotinamide, pyridoxine, and thiamin inhibited the growth of this parasite; for each drug, effects were much more pronounced after 96 h of exposure compared to 48 h. The most potent drug examined was 8-methylamino-8-desmethyl riboflavin (IC50 value approximately 1.0 X 10(-10) M at 96 h). Avidin, a protein which complexes and thus inactivates biotin, did not affect parasite viability. Other antimalarial drugs, including chloroquine and quinine derivatives and antibiotics, were equipotent in the minimal medium and in RPMI 1640. Four strains of P. falciparum showed only minor differences in sensitivity to these antimetabolites. The use of prolonged drug exposure times and a vitamin-depleted medium allowed the preliminary characterization of antimalarial antimetabolites in vitro.

Measurement of aminotransferases: Part 1. Aspartate aminotransferase

Aminotransferases are ubiquitous enzymes of mammalian cells and several are of important diagnostic use. The application of aspartate aminotransferase activity measurements in serum from individuals suffering from myocardial infarction brought about a new dimension in clinical laboratory testing in the 1950s. This review focuses on measurement techniques for aspartate aminotransferase and their application (a subsequent article will review other aminotransferases). Assay techniques measuring enzyme activity are direct spectrophotometric measurements, manometric techniques, assays using dye substances, coupled enzyme techniques, and radiometric procedures. Of these procedures, the one employing malate dehydrogenase and NADH is the most important and is covered in particular detail. The estimation of the mitochondrial isoenzyme of aspartate aminotransferase is also of clinical interest, in particular for estimating severity of disease or in specific applications (e.g., chronic alcoholism). Methods reviewed for estimation of this enzyme are electrophoresis, chromatography, differential kinetic behavior, and immunochemical separation. Determination of the enzyme protein by techniques independent of its catalytic activity are also reviewed.

Determination of pyridoxal phosphate levels in the brains of audiogenic and normal mice

The concentrations of pyridoxal-5'-phosphate in the brains of audiogenic and normal mice were measured fluorimetrically. The brain of the audiogenic mouse (DBA/2J) contains 25% more pyridoxal-5'-phosphate than the brain of a control mouse. Intraperitoneal injection of this substance causes a transient increase of its concentration in the brain, lasting a few hours. The substance thereafter is degraded to pyridoxal and pyridoxic acid.

Polyamine-pyridoxal Schiff bases in urine

Schiff bases of the diamines 1,3-diaminopropane, putrescine, and cadaverine and the polyamines spermidine and spermine with pyridoxal or pyridoxal phosphate occur in human urine, as shown by gas chromatographic/mass spectrometric selected ion-monitoring techniques. By use of synthetic standards, procedures were devised for conversion of the Schiff bases to stable derivatives amenable to gas chromatographic/mass spectrometric analysis. These procedures involve borohydride reduction of the C = N double bond, hydrolytic removal of the phosphate group, chromatographic separation from the bulk of urinary constituents, and trifluoroacetylation of polar functional groups. The levels of the polyamine-pyridoxal Schiff bases were estimated to be in the range of pmol/ml or urine.

Homocystinuria. Evidence for three distinct classes of cystathionine beta-synthase mutants in cultured fibroblasts

We have compared in vivo pyridoxine responsiveness with in vitro cystathionine beta-synthase activity in extracts of confluent fibroblasts from 14 synthase-deficient patients. Enzyme activity was measured with and without addition of its cofactor, pyridoxal-5'-phosphate, using a radioisotopic assay which detects as little as 0.25% of control activity. Six of seven lines from responsive patients had measurable activity without the added cofactor (0.6-15% of mean control). Two of these lines showed a five- and sevenfold stimulation of cystathionine beta-synthase activity with added pyridoxal-5'-phosphate; in the other four, the cofactor addition increased activity only modestly, as in controls. Two of seven lines from nonresponsive patients had measurable activity (each 3% of mean control) which increased two- and fivefold with the added cofactor. Cystathionine beta-synthase activity was undetectable in one line from a responsive patient and in five lines from nonresponsive ones. To characterize control and mutant synthase further, dissociation constants for pyridoxal-5'-phosphate were estimated and thermostability (54 degrees C) was studied in two control and five mutant lines. In one mutant, both parameters were normal; in the others, the affinity for the cofactor was reduced 3-to 11-fold and thermostability was much impaired. We conclude that at least three general classes of cystathionine beta-synthase mutants exist: those with no residual activity; those with reduced activity and normal affinity for pyridoxal-5' phosphate; and those with reduced activity and a reduced affinity for the cofactor. Pyridoxine responsiveness in vivo cannot be correlated simply with the presence or absence of residual synthase activity in vitro or with stimulation of in vitro enzyme activity by cofactor.

Pyridoxal 5'-phosphate and analogs as probes of coenzyme-protein interaction in Baccillus alvei tryptophanase

Trytophanase from Bacillus alvei was resolved from its coenzyme, pyridoxal phosphate, by treatment with cysteine followed by column chromatography. Spectrophotometric titration of apoenzyme with pyridoxal-P showed 1 mol of pyridoxal-P bound per 52,000 g of enzyme. Kinetic analysis of coenzyme binding showed hyperbolic activation curves with a Km of 1.6 muM. Pyridoxal-P was used as a natural active site probe in spectrophotometric studies to distinguish differences in the active center of holotryptophanase and reconstituted enzyme that were not apparent by other techniques. The pKa for holotryptophanase is 7.9 while the pKa for reconstituted apoenzyme is 8.4. Apotryptophanase binds 2-nor, 2'-methyl, 2'-hydroxy, 6-methyl, and N-oxide pyridoxal-P to form analog enzymes distinguishable on the basis of absorption spectra and relative activity in catalyzing both the alpha, beta-elimination and beta-replacement reactions of tryptophanase. Apoenzyme also binds pyridoxal but pyridoxal analog enzyme is not active.

Coenzymatic activity of pyridoxal 5'-sulfate and related analogues of pyridoxal 5'-phosphate

The effect of changes in the substituent at the 5'-position of pyridoxal 5'-phosphate on the coenzymatic activity of six analogues of this coenzyme was determined for three bacterial enzymes. Pyridoxal 5'-sulfate showed substantial coenzymatic activity for arginine decarboxylase and tryptophanase, but not for D-serine dehydratase; alpha(5)-pyridoxalmethylphosphonate showed substantial activity for D-serine dehydratase, but not for the other two enzymes. These results demonstrate that neither the dianionic phosphate group nor the ester oxygen of pyridoxal 5'-phosphate is a general requirement for coenzymatic activity. Minor variations in orientation and charge of the group at position 5 exert major effects on the capacity for complex formation between analogue and apoenzyme, and on the catalytic efficiency of the resulting complex, but have comparatively little effect on the substrate affinity of the analogue-apoenzyme complexes. These effects show no general pattern from enzyme to enzyme, and do not correlate with the affinity of analogue for apoenzyme under the conditions tested.

Vitamin B 6 -responsive histidine deficiency in mutants of Salmonella typhimurium

A mutant of Salmonella typhimurium LT-2 that requires either vitamin B(6) or histidine for growth was found to synthesize vitamin B(5) in amounts comparable to the parent strain, but to be deficient in imidazoleacetol phosphate transaminase (L-histidinolphosphate: 2-oxoglutarate aminotransferase, EC 2.6.1.9), an enzyme required for histidine biosynthesis. The mutant apotransaminase required a 50-fold higher concentration of pyridoxal 5'-phosphate for half-maximum activation than the corresponding wild-type enzyme; the fully activated mutant enzyme also displays a much lower maximum rate of catalysis than the enzyme from the parent strain. Such mutational changes in bacteria resemble those in certain vitamin B(6)-responsive genetic diseases in man and provide useful experimental material for the study of factors involved in coenzyme binding and enzymatic catalysis.