Partial sequence of rat prothrombin and the activity of two related pentapeptides as substrates for the vitamin K-dependent carboxylase system

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.

Viral transformation increases vitamin K-dependent gamma-carboxylation of glutamate

Mammalian cells contain a microsomal vitamin K-dependent carboxylase activity which catalyzes the gamma-carboxylation of glutamate. While most cells have a limited ability to fully gamma-carboxylate proteins, it has been suggested that the ability of transformed cells to perform this complex post-translational modification may play a role in tumor biology. In this study, we examined the effect of transformation by adenovirus oncogenes on the ability of cells to efficiently gamma-carboxylate a vitamin K-dependent protein. Several morphologically transformed BHK-21 cell lines (BHK-Ad) were isolated following the chromosomal integration of the viral oncogenes E1A/E1B from human adenovirus type 12 (Ad12). The lines were capable of growing in soft agar and low serum and produced functional E1A as determined by promoter activation studies. Using a vector for the expression of the vitamin K-dependent recombinant human protein C (HPC), a regulator of the clotting cascade, Ad-transformed and nontransformed lines secreting rHPC were generated. The rHPC from the transformed and nontransformed cell lines displayed identical serine protease activities, and there were no apparent differences in the proteolytic processing of the proteins, although a minor difference in the proportion of each HPC glycoform was observed. However, the functional anticoagulant activity, which depends on the gamma-carboxyglutamic acid (Gla) content, was approximately 70% higher in the Ad-transformed lines. Approximately 90% of the rHPC from the Ad-transformed lines exhibited a calcium-dependent (high Gla) elution profile on anion-exchange resin, compared to only 15 to 26% from the nontransformed cell clones. By analyzing endogenous microsomal carboxylase, we determined that enzyme activity increased approximately 50% following transformation. Overall, our data demonstrate that transformation can increase the potential of a cell to efficiently gamma-carboxylate a protein and lend support to the suggested involvement of this post-translational modification in tumor cell function. Further, our results demonstrate a potential means of altering cells to enable full modification of vitamin K-dependent factors for structure/function studies and potentially for therapeutic use.

Cephalosporin-induced alteration in hepatic glutathione redox state. A potential mechanism for inhibition of hepatic reduction of vitamin K1,2,3-epoxide in the rat

Hypoprothrombinemia is a serious adverse effect of antimicrobial therapy that occurs after administration of some second- and third-generation cephalosporins which contain the methyltetrazole-thiol (MTT) group. Previous studies have shown that in vitro MTT directly inhibits microsomal gamma-carboxylation of a synthetic pentapeptide. Since MTT is a thiocarbamide, a type of compound that can increase oxidation of glutathione, the present studies were carried out to determine whether alterations in hepatic glutathione redox state might interfere with vitamin K metabolism. Dose-related increases in biliary efflux and hepatic concentration of oxidized glutathione (GSSG) occurred after intravenous administration of MTT or MTT-containing antibiotics to rats. This finding suggested that these compounds could alter the hepatic glutathione redox state in vivo. Microsomal reduction of vitamin K epoxide occurred in the presence of 100 microM dithiothreitol (DTT), but was inhibited by preincubation with GSSG at concentrations as low as 10 microM. At higher concentrations of DTT (1.0 mM) inhibition by GSSG persisted, but higher concentrations were required, suggesting that the thiol/disulfide ratio, rather than the absolute concentration of GSSG was important. By contrast, GSSG did not effect microsomal gamma-carboxylation of a pentapeptide, using either vitamin K1 or its hydroquinone as a cofactor. These findings suggest a novel mechanism for the hypoprothrombinemia occurring after administration of MTT-containing antibiotics.

The vitamin K dependent reaction

The vitamin K's are 2-methyl-1,4-naphthoquinones. The vitamin is required for the post-translational gamma-carboxylation of glutamyl residues in precursor polypeptides. The vitamin K step in this carboxylation, however, requires not the quinone but the hydroquinone plus oxygen. Thus, the vitamin K-dependent step is a "mixed function" oxidation requiring a reducing compound plus molecular oxygen to provide a form of oxidant (e.g., a free radical, a hydroperoxide) capable of abstracting a particular, slightly labile hydrogen from a glutamyl residue, leaving this position free to accept a carbon dioxide molecule. This oxidation appears similar to that of other mixed function oxidants such as cytochrome P450 plus oxygen, ascorbic acid (with traces of ferrous iron) plus oxygen, ferrous iron plus oxygen, and a number of other systems which function in a wide variety of oxidation. Inhibition by spin-trapping agents suggests a free radical step in the vitamin K hydroquinone-dependent reaction, similar to other mixed function oxidations.

Modification of the vitamin K-dependent carboxylase assay

Methods are presented that describe alternative protocols for the isolation of rat liver microsomes containing the vitamin K-dependent carboxylase and the procedure in which the solubilized enzyme is assayed. The method for determining the rate of 14CO2 incorporation into low molecular weight, acid soluble substrates by the rat liver microsomal vitamin K-dependent carboxylase has been modified in order to optimize safety, accuracy and simplicity. For these studies the rat liver microsomes containing the vitamin K-dependent carboxylase were isolated by CaCl2 precipitation. These Triton X-100 solubilized microsomes were found to be equivalent to the microsomes obtained by high speed ultracentrifugation with regard to protein concentration, pentapeptide carboxylase activity, carboxylase activity, preprothrombin concentration and total carboxylatable endogenous protein substrate. This modified assay procedure requires fewer steps and pipetting transfers and is quantitatively equivalent to previously employed protocols. The described technique can be adapted for any assay where 14CO2 or H14CO3- is incorporated into non-volatile products. This newly developed assay procedure was employed to assess conditions necessary for optimal vitamin K-dependent carboxylation of the less expensive substrate, N-t-Boc-L-glutamic acid alpha-benzyl ester. The optimal conditions for the carboxylation of N-t-Boc-L-glutamic acid alpha-benzyl ester by the carboxylase were found to be 10 mM N-t-Boc-L-glutamic acid alpha-benzyl ester, 10 mM MgCl2 at 15-18 degrees C. The rate of N-t-Boc-L-glutamic acid alpha-benzyl ester carboxylation under these optimized conditions was found to be higher (1.5-fold) than the rate of carboxylation of 1 mM Phe-Leu-Glu-Glu-Ile in the presence of the cation activator, MgCl2.

Covalent modification of the solubilized rat liver vitamin K-dependent carboxylase with pyridoxal-5'-phosphate

The carboxylation of the pentapeptide substrate, Phe-Leu-Glu-Glu-Ile, by a rat microsomal vitamin K-dependent carboxylase was stimulated two- to threefold at pyridoxal-5'-P concentrations between 0.5 and 1.0 mM. This stimulation was reduced at concentrations higher than 1.0 mM. The Km for the pentapeptide was lowered twofold in the presence of 1 mM pyridoxal-5'-P. The activation by pyridoxal-5'-P is specific, as 1 mM pyridoxal, pyridoxine, pyridoxine-5'-P, pyridoxamine, pyridoxamine-5'-P, or 4-pyridoxic acid did not stimulate the pentapeptide carboxylation rate. All six analogs, as well as formaldehyde and acetaldehyde, inhibited the carboxylation reaction in a concentration-dependent manner. The activation of the carboxylase by pyridoxal-5'-P appeared to be mediated by its direct binding to the enzyme via Schiff base formation. Sodium borohydride reduction of solubilized microsomes in the presence of pyridoxal-5'-P, followed by dialysis to remove unbound material, resulted in a carboxylase preparation with a specific activity twice that of the untreated control microsomes. The derivatized enzyme was not further stimulated by added pyridoxal-5'-P. This derivatized carboxylase could be obtained in the absence of pentapeptide and divalent cations. The stimulation of the carboxylase activity by divalent cations and pyridoxal-5'-P was mediated at separate site(s) on the enzyme. Studies of the NH2-terminal pyridoxalated pentapeptide with both a normal and PLP-modified enzyme, in the presence and absence of PLP, demonstrated competition of the pentapeptide PLP moiety to a PLP site on the enzyme. It was concluded that pyridoxal-5'-P forms a covalent attachment to an epsilon-NH2 of a lysine near the active site of the carboxylase.

Vitamin K-dependent carboxylation: inhibition by a peptide containing 4-methylene glutamic acid

The peptide Boc-4-methylene Glu-Glu-Val has been synthesized and shown to be a strong inhibitor of the vitamin K-dependent carboxylation catalyzed by a detergent solubilized rat liver microsome preparation. The inhibition is apparently competitive with respect to the substrate peptide and non-competitive with respect to HCO3-.

Mechanism of the inhibition of the gamma-carboxylation of glutamic acid by N-methylthiotetrazole-containing antibiotics

Antibiotics that contain a 1-N-methyl-5-thiotetrazole (MTT) side group have been associated with hypoprothrombinemia. In a detergent-treated rat liver microsomal system, MTT inhibited the carboxylation of the gamma carbon of glutamic acid, a necessary reaction in the synthesis of four of the clotting factors. In the present work, the inhibition by MTT was found to be slow in onset, with a lag time of 15 min before significant inhibition occurred. A preincubation of MTT with the microsomes decreased the lag time and increased the extent of inhibition. Glutathione at 1 mM was found to markedly decrease the ability of MTT to inhibit this reaction. The disulfide dimer of MTT was a more potent inhibitor of the system than was MTT, with inhibition detected as low as 1 microM dimer. Disulfiram also inhibited the carboxylation system. These results indicate that the sulfhydryl group of MTT is important for the inhibitory effect of MTT and suggest that a slowly formed metabolite of MTT may be directly responsible for the observed inhibition. The inhibitory mechanism of MTT may be analogous to that of disulfiram, which would explain some pharmacologic effects in common with disulfiram. In addition, the in vitro observations presented here and a closer examination of the clinical evidence raise the possibility that MTT-containing antibiotic-induced hypoprothrombinemia may not be a vitamin K reversible phenomenon.

Kinetics of carboxylation of endogenous and exogenous substrates by the vitamin K-dependent carboxylase

The vitamin K-dependent carboxylation of the exogenous pentapeptide, Phe-Leu-Glu-Glu-Ile, and endogenous liver microsomal protein was studied in solubilized rat liver microsomes. The MnCl2 stimulation of the vitamin K-dependent pentapeptide carboxylation rate, which is conducted at subsaturating concentrations of pentapeptide, is due to the cation's ability to lower the Km of the substrate. Although there are clear kinetic differences observed between the carboxylation rates for the pentapeptide and the endogenous protein substrates, several lines of evidence suggest that the same carboxylase system is responsible for both. These points of evidence are (i) the initial velocity of endogenous protein carboxylation is lowered in the presence of 3 mM pentapeptide; (ii) the presence of endogenous microsomal protein substrate causes an initial lag in pentapeptide carboxylation; and (iii) this initial lag phase is not observed when the total endogenous substrate pool is carboxylated by a preincubation reaction prior to the addition of pentapeptide.

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.

The effects of 1-methyl-5-thiotetrazole in a rat liver vitamin K-dependent carboxylase assay

1-Methyl-5-thiotetrazole (NMTT), a metabolite of moxalactam (MoxamR), was studied for its potential inhibition of vitamin K-dependent carboxylation. The assay system utilized a detergent solubilized rat liver microsomal preparation. Vitamin K1H2 was artificially produced in situ by the NADH-dependent reduction of exogenous phylloquinone and the resultant carboxylation monitored by 14CO2 incorporation into a soluble peptide substrate. Warfarin, used as a reference inhibitor, gave results expected from the literature - 50% inhibition at a pharmacologically excessive level of 1.0 mM. Carboxylation was unaffected by 1.0 mM NMTT and was marginally (0-14%) diminished by 5.0 mM NMTT. Carboxylation was 25% diminished at 10.0 mM NMTT, a concentration far above that achieved in human testing of moxalactam. When NMTT was pre-incubated with the liver microsomal carboxylase enzyme preparation, 10.0 mM NMTT again caused merely a 25% diminution of carboxylation in the assay. These results do not support a role for NMTT as an inhibition of Vitamin K-dependent carboxylation which would produce pharmacological side effects during moxalactam therapy. During these studies it was found that dramatic consumption of NADH occurs in the presence of liver microsomal preparations (independent of vitamin K and of NMTT) and that NMTT effects on these processes may explain the small carboxylation diminution observed at 10.0 nM NMTT in the carboxylase assay.

N-methyl-thio-tetrazole inhibition of the gamma carboxylation of glutamic acid: possible mechanism for antibiotic-associated hypoprothrombinaemia

Several new antibiotics which contain an N-methyl-thio-tetrazole side chain have been associated with the development of hypoprothrombinaemia. This side chain was found to be capable of the in-vitro inhibition of a necessary reaction in the synthesis of prothrombin, the gamma carboxylation of glutamic acid. The 50% inhibitory concentration was 1.1 mmol/l. The intact antibiotics latamoxef, cephamandole, and cefoperazone, which contain the side chain, did not inhibit the reaction at concentrations up to 2 mmol/l. These results suggest that hypoprothrombinaemia associated with antibiotics containing N-methyl-thio-tetrazole side chains may be a consequence of the in-vivo degradation of these antibiotics and the subsequent inhibition of the gamma carboxylation of glutamic acid by the side chain.

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.

Post-translational carboxylation of preprothrombin

In summary, in this review on the function of vitamin K in post-translational modification of precursor proteins by carboxylation of certain glutamyl residues, I have tried to cover in particular the recent work on the reaction, the enzymes involved and the mechanisms being considered. In doing this I have also considered vitamin K, its discovery, its functional form and the possible relation of its metabolism to the carboxylation reaction. Equally the various vitamin K-dependent gla-containing proteins currently known have been described. The carboxylation of synthetic small molecule exogenous substrates and the synthesis and metabolism of the products of carboxylation are of great help in studying the reaction. Structural specificity of vitamin K analogs in vivo and in vitro has been compared and the use of various antagonists in vivo and in vitro considered in attempts to gain an understanding of the overall reaction. The reactions subsequent to carboxylation, e.g., the activation of prothrombin to thrombin via serine proteases and the related activation of the other vitamin K-dependent proteins have not been considered in this review. The review has not covered prothrombin or other vitamin K-dependent protein isolation, nor the determination of these proteins. As the vitamin K-dependent protein carboxylation story has developed over the past six years, a number of reviews have been written which help in keeping up with the various aspects of the field as it has expanded. These reviews refer to many of the papers I have had to eliminate due to space limitations. They are referenced as 469-489. The review is in no sense comprehensive and many papers have been missed or only mentioned. I have tried to concentrate on the more recent work and, thus, much of the very fine work of the 1940's on vitamin K chemistry is hardly mentioned. Some redundancy has been built into the organization of the review so that a reader can obtain a reasonable view of any one section without having to search the whole review for all possible relevant information on any particular part of the field.

Mechanism of action of vitamin K: synthesis of gamma-carboxyglutamic acid

Vitamin K (2-methyl-3-phytyl-1,4-naphthoquinone) is required for the synthesis of prothrombin, Factor VII, Factor IX, Factor X, and a number of newly discovered proteins. These plasma proteins participate in calcium-dependent phospholipid membrane interactions which are mediated through the presence of gamma-carboxyglutamyl residues in their amino-terminal region. Vitamin K is required for the postribosomal conversion of glutamyl residues in liver precursors of these proteins to gamma-carboxyglutamyl residues in the completed plasma proteins. In the absence of vitamin K, or in the presence of vitamin K antagonists, animals produce plasma forms which lack the carboxylated residue. These proteins are nonfunctional because of their lack of phospholipid interaction. The vitamin K-dependent carboxylase which carries out this reaction has been studied in rat liver microsomal preparations where it will carboxylate the endogenous precursor proteins. Low-molecular-weight glutamyl-containing peptide substrates, such as Phe-Leu-Glu-Glu-Leu, which are homologous to regions of the prothrombin precursor, will also serve as substrates for the detergent-solubilized enzyme. This enzyme has been shown to require the reduced form of the vitamin and O2 but no ATP or a biotin-containing protein for its activity. The same microsomal preparations will also convert vitamin K to its 2,3-epoxide, and it is possible that activity may be related to the role of the vitamin in driving the carboxylase reaction.