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R Azerad

Publications and source records attributed to R Azerad.

At least 37 records · Page 2Linked to original sources
PubMed1991

Synthesis of diastereoisomeric peptides incorporating cycloglutamic acids. Substrate specificity of vitamin K-dependent carboxylation.

Tripeptides Boc-X-Glu-Val where X is alpha-methyl glutamic acid or various cyclic analogues of glutamic acid, such as 1-amino-1,3-dicarboxycyclohexane (cis or trans-CHGA) or -cyclopentane (cis or trans-CPGA) have been synthesized. Methods for the selective protection, activation, and coupling of such unnatural amino acids are described. The peptides, which are potential competitive inhibitors of the vitamin K-dependent carboxylation, have been preliminarily tested with the rat liver microsomal carboxylase and found to be effective substrates of the carboxylation reaction.

Amino Acid Sequence↗
PubMed1990

Vitamin K-dependent carboxylase: effect of ammonium sulfate on substrate carboxylation and on inhibition by stereospecific substrate analogs.

(1) High concentrations of ammonium sulfate may stimulate the carboxylase activity of bovine liver microsomes about 10-fold. This effect results from an increase of the Vmax, whereas neither the apparent Km for a number of substrates nor the Ki for substrate analogs is affected. (2) The effect of ammonium sulfate was only found in substrates lacking the pro-sequence. No effect was measurable on the carboxylation of pro-PT28 and endogenous precursor proteins. (3) If the pro-fragment was added as a peptide not covalently bound to a carboxylatable substrate, the carboxylation thereof was only slightly affected and ammonium sulfate remained active as a stimulator of carboxylase activity. (4) S-MeTPT is a much stronger inhibitor of carboxylase activity than is R-MeTPT. (5) The inhibition of carboxylase by the methylated tripeptides is competitive and independent of the type of substrate. Also pro-PT28, which contains the full pro-sequence, could be inhibited completely. (6) On the other hand the carboxylation of endogenous protein precursors could only be partly inhibited by the substrate analogs: even at high concentrations of S-MeTPT a residual endogenous substrate carboxylation of about 30% was left.

Ammonium Sulfate↗
PubMed1988

Resolution of alpha-substituted amino acid enantiomers by high-performance liquid chromatography after derivatization with a chiral adduct of o-phthalaldehyde. Application to glutamic acid analogues.

The fluorescent diastereoisomeric adducts formed in a precolumn derivatization reaction of enantiomeric alpha-substituted amino acids with o-phthalaldehyde and N-acetyl-L-cysteine were separated by high-performance liquid chromatography on a conventional reversed-phase column. This method was particularly efficient for the complete analytical resolution of alpha-substituted glutamic acid analogues, such as 2-methylglutamic acid, and several cyclic analogues (1-amino-1,3-dicarboxycyclohexane, 1-amino-1,3-dicarboxy-2-cyclohexene and 1-amino-1,3-dicarboxycyclopentane). The order of elution from the column was correlated with the absolute configuration of the derivatives.

Amino Acids↗
PubMed1984

The stereochemistry of hydrogen abstraction in vitamin K-dependent carboxylation.

The stereochemistry of the hydrogen abstraction in the vitamin K-dependent carboxylation of synthetic peptides has been investigated; the carboxylation rates of various peptidic substrates containing a stereospecifically 4-monodeuterated glutamic acid residue have been compared to that of nondeuterated peptides. A significant isotope effect was found only with the substrates containing (4S)-4-deuterated glutamic acid. These data reveal that the rat liver microsomal vitamin K-dependent carboxylase acts stereospecifically in abstracting the 4-pro-S hydrogen of the glutamyl residue. The low values of the measured isotope effects indicate that the hydrogen abstraction does not constitute a limiting step in the carboxylation mechanism.

Animals↗
PubMed1984

Nature of products formed in the vitamin K-dependent carboxylation of synthetic peptides.

Vitamin K-dependent carboxylation of synthetic Phe-Leu-Glu-Glu-Val by rat liver microsomes yields a secondary product, which has been identified as Leu-Gla-Glu-Val. Similar results are obtained with other synthetic substrates. The microsomal preparation has been shown to contain an aminopeptidase activity which splits carboxylation products and substrates but is unable to hydrolyse the Leu-Gla peptide bond.

Aminopeptidases↗
PubMed1984

An enzymatic method for the determination of enantiomeric composition and absolute configuration of deuterated or tritiated succinic acid.

The distribution of hydrogen isotope between pro-R and pro-S positions of succinic acid has been determined by comparison of its isotopic content before and after incubation with isocitrate lyase. This enzyme, in the presence of glyoxylate, exchanges exclusively the pro-S protons of succinate with water (M. Sprecher, R. Berger, and D. B. Sprinson (1964) J. Biol. Chem. 239, 4268-4271). With [1-14C,2(R,S)-3H]succinate as substrate, the exchange was easily followed by the decrease of 3H/14C ratio (dried aliquots), which accounted for the high isotopic effect of this reaction. The final ratio was within +/- 5% of the theoretical one. The evolution of the exchange of deuterated succinate added with [1-14C,2(R,S)-3H]succinate acid was again followed by 3H/14C ratio. The deuterium content of [2,3-2H2]succinic acid, [2-2H2]succinic acid (derived from L-[4-2H2]glutamic acid by oxidation) and of the corresponding succinates isolated after incubation with isocitrate lyase was determined by gas chromatography-mass spectroscopy of their dimethylester under NH4+ chemical ionization. This method provides the basis for a quantitative measurement of the distribution of hydrogen isotopes in unsymmetrically 2-labeled succinate or 4-labeled glutamate.

Deuterium↗
PubMed1983

Vitamin K-dependent carboxylation. Study of the hydrogen abstraction stereochemistry with gamma-fluoroglutamic acid-containing peptides.

Two pentapeptides Phe-Leu-X-Glu-Val where X is either the L-threo-gamma-fluoroglutamic acid or the L-erythro-isomer have been synthesized and tested as substrates in the vitamin K-dependent carboxylation. Both peptides are carboxylated, but the reaction occurs exclusively on the glutamic acid of the L-threo-gamma-fluoroglutamate-containing peptide, whereas both glutamic and fluoroglutamic residues of the L-erythro-gamma-fluoroglutamate-containing peptide are carboxylated. These results reveal that the enzymatic hydrogen abstraction step is stereospecific and corresponds, in the gamma-fluoroglutamate case, to the elimination of the hydrogen equivalent to the pro-S hydrogen of glutamic acid.

Chemical Phenomena↗
PubMed1983

Vitamin K dependent carboxylation: study of diastereoisomeric gamma-methylglutamic acid containing peptidic substrates.

Two pentapeptides Phe-Leu-X-Glu-Val where X is either the L-threo-gamma-methylglutamic acid or the L-erythro isomer have been synthesized and tested as substrates in the vitamin K dependent carboxylation. The gamma-methylglutamic residue is not carboxylated and both peptides are inhibitors of the carboxylation of the reference peptide Phe-Leu-Glu-Glu-Val. The threo containing isomer has a much better affinity than the reference and is the best inhibitor of this reaction described so far.

Animals↗
PubMed1981

Effect of reduced vitamin K esters on vitamin K-dependent carboxylation.

The hypothesis of a vitamin K hydroquinone hemicarbonate as intermediate of vitamin K-dependent carboxylation of glutamic residues, has been examined, by testing several vitamin K hydroquinone esters as inhibitors of the reaction. Among the esters that have been synthetized, a monoacetate proved to be an inhibitor. Kinetic analysis shows that the inhibition is no competitive with respect to vitamin K.

Animals↗
PubMed1976

The shikimate pathway. III. 3-dehydroquinate synthetase of E. coli. Mechanistic studies by kinetic isotope effect.

The conversion of 3-deoxy D-arabino heptulosonate 7-phosphate to 3-dehydroquinate by the 3-dehydroquinate synthetase from E. coli is characterized by a low but significant kinetic isotope effect for tritium carried in position-5 of DAHP, while no isotope effect was detectable for tritium in position-4. This effect was observed at different pH nad is interpreted as a result of theintermediary of a 5-ketonic form of the substrate, formed in a preliminary non limiting step during the enzymic cyclization reaction. A tentative scheme for the 3-DHQ synthetase reaction is proposed involving five steps: oxidation by NAD+ in position-5, phsophate elimination after enolization, reduction with precedently formed NADH and cyclization by attack of the 2-carbonyl by the C-7 methylene group.

Chemical Phenomena↗