Vitamin K and gamma-carboxyglutamate biosynthesis

Production of Vitamin K by Wild-Type and Engineered Microorganisms

Vitamin K is a fat-soluble vitamin that mainly exists as phylloquinone or menaquinone in nature. Vitamin K plays an important role in blood clotting and bone health in humans. For use as a nutraceutical, vitamin K is produced by natural extraction, chemical synthesis, and microbial fermentation. Natural extraction and chemical synthesis methods for vitamin K production have limitations, such as low yield of products and environmental concerns. Microbial fermentation is a more sustainable process for industrial production of natural vitamin K than two other methods. Recent advanced genetic technology facilitates industrial production of vitamin K by increasing the yield and productivity of microbial host strains. This review covers (i) general information about vitamin K and microbial host, (ii) current titers of vitamin K produced by wild-type microorganisms, and (iii) vitamin K production by engineered microorganisms, including the details of strain engineering strategies. Finally, current limitations and future directions for microbial production of vitamin K are also discussed.

Investigation and identification of protein γ-glutamyl carboxylation sites

BACKGROUND

Carboxylation is a modification of glutamate (Glu) residues which occurs post-translation that is catalyzed by γ-glutamyl carboxylase in the lumen of the endoplasmic reticulum. Vitamin K is a critical co-factor in the post-translational conversion of Glu residues to γ-carboxyglutamate (Gla) residues. It has been shown that the process of carboxylation is involved in the blood clotting cascade, bone growth, and extraosseous calcification. However, studies in this field have been limited by the difficulty of experimentally studying substrate site specificity in γ-glutamyl carboxylation. In silico investigations have the potential for characterizing carboxylated sites before experiments are carried out.

RESULTS

Because of the importance of γ-glutamyl carboxylation in biological mechanisms, this study investigates the substrate site specificity in carboxylation sites. It considers not only the composition of amino acids that surround carboxylation sites, but also the structural characteristics of these sites, including secondary structure and solvent-accessible surface area (ASA). The explored features are used to establish a predictive model for differentiating between carboxylation sites and non-carboxylation sites. A support vector machine (SVM) is employed to establish a predictive model with various features. A five-fold cross-validation evaluation reveals that the SVM model, trained with the combined features of positional weighted matrix (PWM), amino acid composition (AAC), and ASA, yields the highest accuracy (0.892). Furthermore, an independent testing set is constructed to evaluate whether the predictive model is over-fitted to the training set.

CONCLUSIONS

Independent testing data that did not undergo the cross-validation process shows that the proposed model can differentiate between carboxylation sites and non-carboxylation sites. This investigation is the first to study carboxylation sites and to develop a system for identifying them. The proposed method is a practical means of preliminary analysis and greatly diminishes the total number of potential carboxylation sites requiring further experimental confirmation.

Carboxylator: incorporating solvent-accessible surface area for identifying protein carboxylation sites

In proteins, glutamate (Glu) residues are transformed into γ-carboxyglutamate (Gla) residues in a process called carboxylation. The process of protein carboxylation catalyzed by γ-glutamyl carboxylase is deemed to be important due to its involvement in biological processes such as blood clotting cascade and bone growth. There is an increasing interest within the scientific community to identify protein carboxylation sites. However, experimental identification of carboxylation sites via mass spectrometry-based methods is observed to be expensive, time-consuming, and labor-intensive. Thus, we were motivated to design a computational method for identifying protein carboxylation sites. This work aims to investigate the protein carboxylation by considering the composition of amino acids that surround modification sites. With the implication of a modified residue prefers to be accessible on the surface of a protein, the solvent-accessible surface area (ASA) around carboxylation sites is also investigated. Radial basis function network is then employed to build a predictive model using various features for identifying carboxylation sites. Based on a five-fold cross-validation evaluation, a predictive model trained using the combined features of amino acid sequence (AA20D), amino acid composition, and ASA, yields the highest accuracy at 0.874. Furthermore, an independent test done involving data not included in the cross-validation process indicates that in silico identification is a feasible means of preliminary analysis. Additionally, the predictive method presented in this work is implemented as Carboxylator ( http://csb.cse.yzu.edu.tw/Carboxylator/ ), a web-based tool for identifying carboxylated proteins with modification sites in order to help users in investigating γ-glutamyl carboxylation.

Comparison of intestinal absorption of vitamin K2 (menaquinone) homologues and their effects on blood coagulation in rats with hypoprothrombinaemia

To examine the physiological activities of vitamin K2 (menaquinone, MK) homologues with different numbers of isoprene units, MK with 1-14 isoprene units and menadione (MK-0) were administered to rats with hypoprothrombinaemia, and the absorption, concentration in liver and ameliorating effect of these MK on hypoprothrombinaemia were compared. Hypoprothrombinaemia was induced by giving a vitamin K-deficient diet and warfarin (0.06 mg/kg body weight) for 8 days. Before MK treatment, the MK were undetectable in plasma and liver. At 6 hr after oral MK administration (0.1 mg/kg): MK was not detected in the plasma in rats treated with MK with 1, 2, 3 or more than 12 isoprene units; the MK level in the liver was increased but blood coagulation activity was not improved in rats treated with MK with 0, 9, 10 or 11 isoprene units; the MK level in the liver was increased and hypoprothrombinaemia was slightly improved in rats treated with MK with 7 or 8 isoprene units; and the MK level in the liver was increased and hypoprothrombinaemia was markedly improved in rats treated with MK with 4, 5 or 6 isoprene units. Almost identical results were observed 3 hr after intravenous injection of MK with 4, 5 or 6 isoprene units (10 nmol/kg). These findings suggest that the number of isoprene units of MK is an important factor in its absorption and incorporation into the liver and that the ameliorating effect of MK on hypoprothrombinaemia does not parallel their concentrations in the liver.

Blood coagulation proteins and urolithiasis are linked: crystal matrix protein is the F1 activation peptide of human prothrombin

OBJECTIVES

To determine the relationship between prothrombin and crystal matrix protein (CMP). CMP is the predominant protein found in the organic matrix of calcium oxalate (CaOx) crystals generated from human urine and is a 31 kDa glycoprotein, whose N-terminal amino acid sequence shares homology with human prothrombin.

MATERIALS AND METHODS

CaOx crystallization was induced in ultrafiltered (UF) human urine containing either plasma or serum derived from the same healthy donor, by the addition of sodium oxalate. The crystals were demineralized and the resulting protein extracts analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting, using antibodies raised against human prothrombin and the C-terminus of prothrombin fragment 1 + 2 (F1 + 2).

RESULTS

Prothrombin was detected in extracts of crystals precipitated from the UF urine in the presence of plasma, while CMP was completely absent. Crystals precipitated from UF urine supplemented with serum contained relatively large amounts of F1 + 2 and a protein with the same electrophoretic mobility as CMP. Analysis of a standard preparation of F1 + 2 which also contained prothrombin fragment 1 (F1) as a minor contaminant, showed a protein with electrophoretic and staining properties comparable to CMP.

CONCLUSION

CMP is a urinary form of F1, a degradation product of prothrombin possessing the domain rich in gamma-carboxyglutamic acid, which may have undergone some molecular modification either before or after its release into the urine.

Effect of vitamin K2 (menatetrenone) on osteoclast-like cell formation in mouse bone marrow cultures

The effects of menatetrenone, a vitamin K2 homologue, on osteoclast-like cell formation in mouse bone marrow culture were investigated. After 7 days of incubation, menatetrenone at 10(-6) M, 3 x 10(-6) M and 10(-5) M dose dependently inhibited the tartrate-resistant acid phosphatase-positive multinucleated cell formation induced by 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). The addition of menatetrenone for the last 3 days of the 7-day incubation period was required to inhibit formation of multinucleated cells in response to 1,25(OH)2D3. Moreover, the addition of 1,25(OH)2D3 for the last 3 days was essential for multinucleated cell formation, and this activity was markedly inhibited by the simultaneous addition of menatetrenone. The inhibitory effects of menatetrenone on multinucleated cell formation may contribute to its ameliorative action on bone loss in vivo, and may indicate a new mechanism of vitamin K2 activity in bone metabolism.

Oral anticoagulants: pharmacodynamics, clinical indications and adverse effects

Discovered early in this century consequent to investigations of a bovine hemorrhagic disease, the oral anticoagulants inhibit a post translational modification required for various hepatically derived serine proteases to become active. These include Factors II, V, VII and X. Through their effect on Proteins S and C, the extrinsic pathway is also effected. While most of these agents exist as optically active enantiomers, with differing kinetics and pharmacologic profiles, they are generally administered in the clinic as racemic mixtures. Over the last 45 years, various studies have sought to better evaluate the role of the oral anticoagulation in both prevention and treatment of various types of thrombo-occlusive and -embolic disease. Such exercises have allowed us to better understand the pathophysiology of those diseases as well as to better characterize the pre-thrombotic state.

Des-gamma-carboxyprothrombin detection by immunoblotting after polyacrylamide gel affinoelectrophoresis in human plasmas

In the absence of vitamin K or in the presence of the vitamin K antagonists, abnormal nonfunctional forms of prothrombin circulate in the blood. A reliable and reproducible technique, derived from traditional crossed affinoimmunoelectrophoresis in presence of calcium lactate, was developed and optimized. The technique is based on nondenaturing polyacrylamide gel affinoelectrophoresis, with calcium lactate, of plasma samples, followed by immunoblotting with rabbit anti-human prothrombin serum and detection with an anti-rabbit immunoglobulin peroxidase conjugate. Depending on the plasmas, one or two bands were visualized and quantified by densitometry of the immunoblots. The technique was able to detect abnormal des-gamma-carboxylated prothrombins at concentration of 0.1 microgram/mL.

Tissue distribution and warfarin sensitivity of vitamin K epoxide reductase

The distribution of vitamin K epoxide reductase activity and its sensitivity to warfarin have been examined in whole microsomes from tissues of both control and warfarin-resistant strain rats. The distribution of activity roughly paralleled that previously shown for the vitamin K-dependent carboxylase. Activity on a per gram tissue basis was highest in kidney, adrenal, spleen, lung, testes, and epididymis at a level about 1/20th of that present in liver microsomes. Vitamin K quinone formation by microsomes from warfarin-resistant rats was approximately half that of control strain samples. In addition, hydroxy vitamin K was formed by warfarin-resistant strain microsomes to about the same extent as vitamin K quinone in all tissues. The Km values for dithiothreitol (DTT) and vitamin K epoxide were similar in all tissues (range = 0.1-0.2 mM DTT at 40 microM vitamin K epoxide, and 10-30 microM vitamin K epoxide at 2 mM DTT). The sensitivities to warfarin were similar for all control strain rat tissues (I50 = 10-20 microM at 2 mM DTT and 40 microM vitamin K epoxide) and similarly elevated for all warfarin-resistant rat tissues (I50 = 30 to greater than 80 microM). These results suggest that the identical enzyme is expressed in all tissues and that tissue specific isozymes do not occur.

Expression of human factor IX and its subfragments in Escherichia coli and generation of antibodies to the subfragments

A cDNA clone encoding the entire human blood clotting factor IX (amino acids -3 to 415 has been placed under control of transcription and translation signals from bacteriophage T7 and expressed in Escherichia coli. The full-length cDNA and 13 different subfragments (which together cover the entire coding sequence of mature factor IX plus amino acids -40 to -19 of the prepro leader sequence) have each been joined to the coding sequence for the major capsid protein of T7 after the 326th codon and expressed as fusion proteins. All of the fusion proteins were insoluble, which facilitated their purification. A goat polyclonal antiserum against human factor IX reacted to different extents with the different fusion proteins, and rabbit polyclonal antibodies raised against the purified fusion proteins recognize the factor IX molecule, as demonstrated by immunoblotting techniques. Antibodies against at least one of the fusion proteins can also inhibit the biological activity of purified factor IX in a one-stage partial thromboplastin time bioassay. We expect these fusion proteins and the antibodies against them to be useful in studying the structure and function of factor IX.

DT-diaphorase-catalyzed two-electron reduction of quinone epoxides

DT-diaphorase catalyzes the two-electron reduction of the unsubstituted quinone epoxide, 2,3-epoxy-p-benzoquinone, at expense of NAD(P)H with formation of 2-OH-p-benzohydroquinone as the reaction product. The further conversion reactions of 2-OH-p-benzohydroquinone are influenced by the presence of O2 in the medium. Under aerobic conditions, 2-OH-p-benzohydroquinone undergoes autoxidation--probably with formation of 2-OH-semiquinone intermediates--to 2-OH-p-benzoquinone. The latter product is rapidly reduced by DT-diaphorase and, thus, its accumulation can be only observed upon exhaustion of NADPH. Under anaerobic conditions, 2-OH-p-benzohydroquinone does not undergo autoxidation and its accumulation is stoichiometrically (1:1) related to the amount of NADPH oxidized and epoxide substrate reduced. DT-diaphorase also catalyzes the reduction of the disubstituted quinone epoxide, 2,3-dimethyl-2,3-epoxy-1,4-naphthoquinone. Neither the aliphatic epoxide, trans-stilbene oxide, nor the aromatic epoxide, 4,5-epoxy-benzo[a]pyrene are substrates for DT-diaphorase. The reduction of 2,3-epoxy-p-benzoquinone is also catalyzed by the one-electron transfer enzyme, NADPH-cytochrome P450 reductase at a rate similar to that found with DT-diaphorase. However, this reaction differs from that catalyzed by DT-diaphorase in the distribution of molecular products as well as in the relative contribution of nonenzymatic reactions, i.e. semiquinone disproportionation and autoxidation.

Effects of 1,25-dihydroxycholecalciferol administration on the rat renal vitamin K-dependent carboxylating system

We have shown previously that the in vitro activity of the renal vitamin K-dependent gamma-glutamyl carboxylase toward synthetic oligopeptide substrates is stimulated by administration of either parathyroid hormone (PTH) or 1,25-dihydroxycholecalciferol [1,25(OH)2D3] to rats [(1983) J. Biol. Chem. 258, 12783-12786]. Here we report that administration of 1,25(OH)2D3 to rats increases their levels of endogenous carboxylase substrate as well. Rats fed a vitamin D-deficient diet had highly elevated serum PTH levels while vitamin D-replete animals had undetectable levels. Furthermore, since PTH increases 1,25(OH)2D3 levels by stimulating renal 25-hydroxyvitamin D-1 alpha-hydroxylase, it is very likely that the stimulatory effects of PTH on the renal vitamin K-dependent carboxylating system are mediated by 1,25(OH)2D3.

Purification of a vitamin K epoxide reductase that catalyzes conversion of vitamin K 2,3-epoxide to 3-hydroxy-2-methyl-3-phytyl-2,3-dihydronaphthoquinone

An enzyme from bovine liver microsomes that catalyzes the reduction of vitamin K 2,3-epoxide to 2- and 3-hydroxy-2-methyl-3-phytyl-2,3-dihydronaphthoquinone was purified 1152-fold to apparent homogeneity. Microsomes were solubilized with 3-[3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), and the enzyme was purified by chromatography on PBE-94 ion exchanger, hydroxylapatite, and DEAE-cellulose, and then gel filtration on Sephacryl S-200. The homogeneity of the final preparation was established by polyacrylamide slab gel electrophoresis in the presence of sodium dodecyl sulfate. The molecular weight of the native enzyme is 25,000 and that of denatured enzyme is 12,400, which suggests that the enzyme is a dimer with identical subunits. No chromophoric cofactors are associated with the enzyme. Dithiothreitol and CHAPS are essential for activity, but high concentrations of glycerol reduces the activity. The enzyme is not inhibited by warfarin, a potent inhibitor of the vitamin K epoxide reductase, which catalyzes the conversion of vitamin K 2,3-epoxide to vitamin K. Evidence is presented indicating that the purified enzyme is not simply a fragment of the warfarin-sensitive vitamin K epoxide reductase.

Responses of renal and hepatic vitamin K dependent gamma-glutamyl carboxylase substrates to warfarin and vitamin K treatments

A single intraperitoneal injection of warfarin (5 mg/kg) in the rat causes maximal accumulation of hepatic vitamin K dependent carboxylase substrate by 8 hr. In the kidney accumulation is slower with maximal amounts of the substrate appearing at about 16 hr. Vitamin K administered intravenously to warfarin-treated rats causes the complete disappearance of the hepatic substrate in 2 hr. In contrast, neither a single nor multiple injections of the vitamin decrease the renal substrate level by more than 30%. However, this decrease can be augmented by inhibition of protein synthesis with cycloheximide.

Cloning and sequencing of liver cDNA coding for bovine protein C

cDNA coding for protein C has been cloned from a bovine liver library in plasmid vector pBR322 and its sequence has been determined. Two overlapping clones code for the entire light and heavy chains of the mature protein, as well as a previously unreported connecting dipeptide (Lys-Arg) and a 39-amino acid leader peptide region. Identification and characterization of the clones establishes the liver as a site of protein C biosynthesis. A contiguous coding region reveals that a one-chain precursor protein is made that upon limited proteolysis yields both the mature light and heavy chains. The codon for aspartic acid is found at light chain amino acid position 71, showing that the beta-hydroxyaspartic acid that exists in this position of the mature protein is the result of post-translational modification of an aspartic acid residue. Amino acid sequence homology in the amino-terminal region of the light chain with other vitamin K-dependent coagulation factors is continued into the leader peptide region.

Rapid assay for gamma-carboxyglutamic acid in urine and bone by precolumn derivatization and reversed-phase liquid chromatography

A reversed-phase high-performance liquid chromatographic assay for the analysis of gamma-carboxyglutamic acid (Gla) in urine and bone protein hydrolyzates is described. The method employs precolumn derivatization with o-phthalaldehyde and mercaptoethanol. Gla was quantified by reference to an internal standard (beta-carboxyaspartic acid). The "within-run" coefficient of variation of the assay for Gla in urine was between 2.1 and 3.4%, and that for bone protein hydrolyzates was 3.2%. The "between-run" coefficient of variation ranged from 4.1 to 5.5%. There was good agreement between the measurement of urinary Gla by high-performance liquid chromatography and amino acid analyzer. Free Gla could not be detected in serum.

Solubilization and characterization of vitamin K epoxide reductase from normal and warfarin-resistant rat liver microsomes

Two procedures have been developed for the solubilization of vitamin K epoxide reductase from rat liver microsomal membranes using the detergent Deriphat 160 at pH 10.8. The methods are applicable to both normal and Warfarin-resistant-strain rat liver microsomes and yield material suitable for further purification. The preparations retain dithiothreitol-dependent vitamin K quinone reductase activity as well as vitamin K epoxide reductase and are free of vitamin K-dependent carboxylase and epoxidase activities. Optimal epoxide reductase activity is obtained at 0.1 M KCl and pH 9 in the presence of sodium cholate. Artifactual formation of vitamin K metabolites was eliminated through the use of mercuric chloride to remove excess dithiothreitol prior to extraction and metabolite assay. Using the solubilized enzyme, valid initial velocities were measured, and reproducible kinetic data was obtained. The substrate initial velocity patterns were determined and are consistent with a ping-pong kinetic mechanism. The kinetic parameters obtained are a function of the cholate concentration, but do not vary drastically from those obtained using intact microsomal membranes. At 0.8% cholate, the enzymes solubilized from normal Warfarin-sensitive- and Warfarin-resistant-strain rat livers exhibit respective values of Vmax = 3 and 0.75 nmol/min/g liver; Km for vitamin K epoxide = 9 and 4 microM; and Km for dithiothreitol of 0.6 and 0.16 mM.

Vitamin K-dependent carboxylase: effect of detergent concentrations, vitamin K status, and added protein precursors on activity

Activity of the rat liver microsomal vitamin K-dependent carboxylase has been studied at various concentrations of detergent. The activity which could be solubilized by 0.25% Triton X-100 was low but could be greatly increased if vitamin K-deficient rats were given vitamin K a few minutes before they were killed. At higher concentrations of Triton, more activity was solubilized and this effect was not seen. In vitro carboxylation of endogenous microsomal proteins was decreased by 80-90% if vitamin K was administered 1 min before rats were killed, but the amount of assayable prothrombin precursor was decreased by only 20%. Decarboxylated vitamin K-dependent rat plasma proteins were not substrates for the carboxylase and did not influence peptide carboxylase activity significantly. Purified microsomal prothrombin precursors did, however, stimulate carboxylation of peptide substrate and were used as a substrate for the carboxylase in a preparation from precursor depleted vitamin K-deficient rats.

Studies on a family with combined functional deficiencies of vitamin K-dependent coagulation factors

Two siblings with m ild hemorrhagic symptoms had combined functional deficiencies of vitamin K-dependent clotting factors. Prothrombin (0.18-0.20 U/ml) and Stuart factor (Factor X, 0.18-0.20 U/ml) and Stuart factor (Factor X, 0.18-0.20 U/ml) were most severely affected. Antigenic amounts of affected coagulation factors were normal and normal generation of thrombin activity occurred in the patients' plasmas after treatment with nonophysiologic activators that do not require calcium for prothrombin activation. Hepatobilary disease, malabsorptive disorders, and plasma warfarin were not present. Both parents had normal levels of all coagulation factors. The patients' plasmas contained prothrombin that reacted both with antibody directed against des-gamma-carboxyprothrombin and native prothrombin. Crossed immunoelectrophoresis of patients' plasmas and studies of partially purified patient prothrombin suggested the presence of a relatively homogeneous species of dysfunctional prothrombin, distinct from the heterologous species found in the plasma of warfarin-treated persons. These studies are most consistent with a posttranslational defect in hepatic carboxylation of vitamin K-dependent factors. This kindred uniquely possesses an autosomal recessive disorder of vitamin K-dependent factor formation that causes production of an apparently homogeneous species of dysfunctional prothrombin; the functional deficiencies in clotting factors are totally corrected by oral or parenteral administration of vitamin K1.

Characterization of vitamin K-dependent carboxylase from the livers from the adult ox and dicoumarol-treated calf

The properties of the microsomal vitamin K-dependent carboxylase from the livers of the adult ox and dicoumarol-treated calf were investigated. The enzymes from both sources utilized glutamic residues of synthetic peptides as substrates and could be solubilized with Triton X-100 similarly to the enzyme from vitamin K-deficient rat liver. Under the optimal assay conditions, the microsomes from calf liver had peptide carboxylase activity comparable with that of the rat liver microsomes and 6.5-fold that of adult ox liver microsomes. The apparent Km for reduced vitamin K and the ionic strength optima of the calf and adult ox enzyme clearly differ from those of the rat enzyme. Pyridoxal phosphate activated the adult ox carboxylase only slightly, whereas the calf enzyme was activated by pyridoxal phosphate as effectively as was the enzyme from the vitamin K-deficient rat. Mn2+ activated the adult ox enzyme 9-fold and calf enzyme 22-fold under optimal conditions (no KCl). Three other divalent metal cations (Ca2+, Ba2+, and Mg2+) activated the adult ox and calf enzymes to about half the extent caused by Mn2+, KCl inhibited this activation. The vitamin K-dependent carboxylase from the dicoumarol-treated calf is apparently more tightly bound to the microsomal membrane than is the adult ox enzyme. In many other respects (pH optimum), temperature optimum, Km values for peptide substrate, substrate specificity, inhibitor effects), the properties of the adult ox and calf enzymes resemble closely those of the rat enzyme.

Elevation of quinone reductase activity by anticarcinogenic antioxidants

NAD(P)H:quinone reductase exhibits broad specificity in the reduction of endogenous and exogenous quinones and quinone imines, such as those derived from polycyclic aromatic carcinogens, phenolic steroids, vitamin K, and numerous therapeutic drugs. This enzyme is found in several cell compartments and is widely distributed among tissues. In contrast to several other flavoprotein dehydrogenases, quinone reductase catalyzes obligatorily two electron reductions. Extensive studies by Huggins and by others have shown that the quinone reductase in liver and some other tissues of rats is inducible by various polycyclic hydrocarbons and aromatic amines, as well as by certain azo dyes. Huggins perceived that the relative effectiveness of such compounds in inducing quinone reductase correlated with their abilities to protect against toxicity and carcinogenesis. Certain antioxidants are also known to protect against the tumorigenic and toxic effects of carcinogens. Studies on the mechanisms underlying the protective effects of BHA, BHT, ethoxyquin, and disulfiram have revealed that these compounds alter the activity profiles of several enzymes which metabolize carcinogenic and toxic compounds. We have observed that quinone reductase specific activity is increased markedly in mouse liver and several extrahepatic tissues in response to dietary BHA, ethoxyquin, and disulfiram, whereas BHT has been shown by others to enhance this enzymatic activity in rat liver. These findings confirm and extend the correlation between the ability to elevate quinone reductase activity and to confer protection against carcinogenesis and toxicity. The broad specificity of quinone reductase, its apparent inability to catalyze one electron reductions of quinones, its widespread distribution, and its inducibility by a variety of structurally dissimilar protective compounds, suggest that quinone reductase may play a significant local protective role in various regions of the cell.

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.

Blood ionized calcium cycles in the chicken

Changes in blood ionized calcium concentration (Ca++) appear to be dependent on the physiological state of the chicken. In the laying hen, blood Ca++ levels were affected by the ovulatory cycle; Ca++ was elevated when the shell gland was empty, declined following entry of an egg into that organ, and reached a minimum level approximately 16 hr prior to the subsequent oviposition. Pullets not in laying condition had blood Ca++ that varied with the photoperiod and/or rate of feed intake. In those pullets, blood Ca++ levels were depressed during the 9.5 hr of light, then rose in plateaued during the dark period. These studies indicate that blood Ca++ in laying hens is affected by the reproductive state first and the photoperiod and/or pattern of feed intake second.

Vitamin K-dependent carboxylation and vitamin K epoxidation. Evidence that the warfarin-sensitive microsomal NAD(P)H dehydrogenase reduces vitamin K1 in these reactions

Passage of a Triton X-100-solubilized microsomal systems in vitro that are used to study these reactions is the warfarin-sensitive NAD(P)H dehydrogenase.