Inhibition of hydroxyapatite crystal growth by bone-specific and other calcium-binding proteins

Mineralization of bone matrix may be influenced by the presence of specific, noncollagenous bone proteins. The quantitative influence of two bone-specific proteins--bone gamma-carboxyglutamic acid (Gla) protein and osteonectin--and other proteins that decreased the rate of crystal growth was measured by adding seed crystals of hydroxyapatite to a solution of CaCl2 and KH2PO4, pH 7.4 at 37 degrees C. The molar concentrations of proteins needed to inhibit the rate of crystal growth by 50% were as follows: osteonectin, 0.15 microM; bone Gla protein, 0.8 microM; prothrombin, 0.9 microM; prothrombin fragment 1, 1.0 microM; soybean trypsin inhibitor, 3 microM; prethrombin 1, 9 microM; cytochrome c, 30 microM. Calmodulin and parvalbumin were found to be less active than prothrombin fragment 1 and had no activity in the micromolar range. The combination of two inhibitors resulted in a mixture with an inhibitory activity that was the sum of the two inhibitors. Decarboxylation of bone Gla protein significantly reduced its inhibitory activity. These results indicate that the inhibitory activity of a protein does not correlate with Ca2+-binding affinity under these conditions, that the mixture of inhibitors has an additive effect, and that gamma-carboxyglutamic acid residues enhance the ability of a protein to inhibit hydroxyapatite-seeded crystal growth.     

Direct identification of gamma-carboxyglutamic acid in the sequencing of vitamin K-dependent proteins.

We report the first direct method for the identification of the vitamin K-dependent Ca2+ binding amino acid, gamma-carboxyglutamic acid (Gla), in the sequencing of proteins. The carboxyl groups on the protein are first converted to methyl esters with methanolic HCl, a procedure that reduces the polarity of the resulting ATZ derivative of dimethyl-Gla and so greatly improves its extraction from the polybrene-treated glass fiber filter. After conversion to the PTH derivative in methanolic HCl, the resulting dimethyl ester of PTH Gla can be identified directly by a simple modification of the standard HPLC program for the separation of PTH derivatives. This methylation procedure can be used to identify Gla residues in proteins bound to PVDF membranes, as we demonstrate for matrix Gla protein and prothrombin, and to evaluate directly the degree of partial gamma-carboxylation at given glutamic acid residues, as we demonstrate for the 50% gamma-carboxylation of residue 17 in human bone Gla protein.     

Matrix Gla protein, a new gamma-carboxyglutamic acid-containing protein which is associated with the organic matrix of bone

A new protein has been isolated from CaCl2/urea extracts of demineralized bovine bone matrix. This protein has five to six residues of the vitamin K-dependent amino acid, gamma-carboxyglutamic acid (Gla), and we have accordingly designated it matrix Gla protein. Matrix Gla protein is a 15,000 dalton protein whose amino acid composition includes a single disulfide bond. The absence of 4-hydroxyproline in matrix Gla protein demonstrates that it is not a precursor to bone Gla protein, 5,800 dalton protein which has a residue of 4-hydroxyproline at position 9 in its sequence. Matrix Gla protein also does not cross-react with antibodies raised against bone Gla protein.     

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.     

Calcium binding of bovine protein S. Effect of thrombin cleavage and removal of the gamma-carboxyglutamic acid-containing region

Thrombin cleaves protein S at arginine residues 52 and 70 resulting in loss of cofactor activity and reduced Ca2+ ion binding. After thrombin cleavage the NH2-terminal region containing gamma-carboxyglutamic acid (Gla) is linked to the large COOH-terminal fragment by a disulfide bond. Measurements of the rate of disulfide bond reduction by thioredoxin in intact protein S showed that the disulfide bonds are largely inaccessible to thioredoxin in the presence of Ca2+ ions, whereas in the presence of EDTA apparently all of the disulfide bonds are rapidly reduced. Probing the reactivity of the disulfide bonds in thrombin-modified proteins indicated that the thrombin cleavage induces a conformational change in the protein. After thrombin cleavage of protein S, the domain containing gamma-carboxyglutamic acid could be removed by selective reduction with thioredoxin followed by alkylation of the sulfhydryl groups. Ca2+ ion binding was compared in intact protein S, thrombin-modified protein S, and Gla domainless protein S. The intact protein S bound several Ca2+ ions, and the binding was not saturable. Thrombin-modified protein S, whether intact or with the Gla domain removed by selective reduction, bound two to three Ca2+ ions with a KD of 15-20 microM. The Gla domain in thrombin-modified protein S thus does not contribute significantly to the high affinity Ca2+ ion binding. Thrombin cleavage of protein S may be of physiological importance in the regulation of blood coagulation.     

Periostin, a member of a novel family of vitamin K-dependent proteins, is expressed by mesenchymal stromal cells

The modification of glutamic acid residues to gamma-carboxyglutamic acid (Gla) is a post-translational modification catalyzed by the vitamin K-dependent enzyme gamma-glutamylcarboxylase. Despite ubiquitous expression of the gamma-carboxylation machinery in mammalian tissues, only 12 Gla-containing proteins have so far been identified in humans. Because bone tissue is the second most abundant source of Gla-containing proteins after the liver, we sought to identify Gla proteins secreted by bone marrow-derived mesenchymal stromal cells (MSCs). We used a proteomics approach to screen the secretome of MSCs with a combination of two-dimensional gel electrophoresis and tandem mass spectrometry. The most abundant Gla-containing protein secreted by MSCs was identified as periostin, a previously unrecognized gamma-carboxylated protein. In silico amino acid sequence analysis of periostin demonstrated the presence of four consensus gamma-carboxylase recognition sites embedded within fasciclin-like protein domains. The carboxylation of periostin was confirmed by immunoprecipitation and purification of the recombinant protein. Carboxylation of periostin could be inhibited by warfarin in MSCs, demonstrating its dependence on the presence of vitamin K. We were able to demonstrate localization of carboxylated periostin to bone nodules formed by MSCs in vitro, suggesting a role in extracellular matrix mineralization. Our data also show that another fasciclin I-like protein, betaig-h3, contains Gla. In conclusion, periostin is a member of a novel vitamin K-dependent gamma-carboxylated protein family characterized by the presence of fasciclin domains. Furthermore, carboxylated periostin is produced by bone-derived cells of mesenchymal lineage and is abundantly found in mineralized bone nodules in vitro.     

An improved method for the determination of γ-carboxyglutamic acid in proteins,bone, and urine

A method of high-performance liquid chromatographic separation of the fluorescence derivative of γ-carboxyglutamic acid (Gla) is presented. Alkaline hydrolysates of protein samples were reacted with o-phthalaldehyde in the presence of ethanethiol for 2 min, and the fluorescence derivative of γ-carboxyglutamic acid was resolved from the other amino acids by a short column packed with silica-based anion exchanger under isocratic conditions. By this method, as low as 200 fmol of γ-carboxyglutamic acid can be quantitatively analyzed within 10 min. The method presented here shortened the analysis time for Gla and was at least 10 times more sensitive than the method we described previously (Anal. Biochem.117, 259–265, 1981). The application of this method to the formic acid-soluble or insoluble γ-carboxyglutamic acid-containing proteins in chicken bone and the concomitant increase of γ-carboxyglutamic acid content in chicken bone with age are reported.     

The biosynthesis of dentine gamma-carboxyglutamic acid-containing proteins by rat incisor odontoblasts in organ culture.

Rat odontoblasts were shown to synthesize and secrete gamma-carboxyglutamic acid(Gla)-containing proteins into dentine after organ culture in the presence of radiolabelled amino acid precursors. Purified dentine Gla-containing protein from rat incisors was used as antigen to prepare rabbit antisera as a probe of dentine Gla-containing-protein biosynthesis in organ cultures of dentine (rat incisor) and bone (rat calvaria). Use of the antiserum also pointed out the cross-reactivity of a high-M, glycoprotein present within the dentine matrix. The present results are significant in identifying dentine gla-containing protein as endogenous to mineralizing dentine and may relate to the commonality between calcifying connective tissues in general.     

The propeptides of the vitamin K-dependent proteins possess different affinities for the vitamin K-dependent carboxylase.

The vitamin K-dependent gamma-glutamyl carboxylase catalyzes the modification of specific glutamates in a number of proteins required for blood coagulation and associated with bone and calcium homeostasis. All known vitamin K-dependent proteins possess a conserved eighteen-amino acid propeptide sequence that is the primary binding site for the carboxylase. We compared the relative affinities of synthetic propeptides of nine human vitamin K-dependent proteins by determining the inhibition constants (Ki) toward a factor IX propeptide/gamma-carboxyglutamic acid domain substrate. The Ki values for six of the propeptides (factor X, matrix Gla protein, factor VII, factor IX, PRGP1, and protein S) were between 2-35 nM, with the factor X propeptide having the tightest affinity. In contrast, the inhibition constants for the propeptides of prothrombin and protein C are approximately 100-fold weaker than the factor X propeptide. The propeptide of bone Gla protein demonstrates severely impaired carboxylase binding with an inhibition constant of at least 200,000-fold weaker than the factor X propeptide. This study demonstrates that the affinities of the propeptides of the vitamin K-dependent proteins vary over a considerable range; this may have important physiological consequences in the levels of vitamin K-dependent proteins and the biochemical mechanism by which these substrates are modified by the carboxylase.     

Calcium binding to a human factor IXa derivative lacking gamma-carboxyglutamic acid: evidence for two high-affinity sites that do not involve beta-hydroxyaspartic acid

A derivative of human blood clotting factor IXa beta lacking gamma-carboxyglutamic acid (Gla) residues was prepared by limited proteolysis with chymotrypsin, and subsequently examined for its ability to bind calcium ions. By amino acid analysis, Gla-domainless human factor IXa beta contained 0.3-0.4 moles of beta-hydroxyaspartic acid per mole of protein. Equilibrium dialysis experiments demonstrated that Gla-domainless human factor IXa beta retained two high-affinity calcium binding sites (Kd=52 microM), a finding essentially identical to that observed for Gla-domainless bovine factor IX that contains 0.8-0.9 moles of beta-hydroxyaspartic acid per mole of protein. These data strongly suggest that the beta-hydroxyaspartic acid residue in these proteins does not participate in their high affinity calcium sites.     

Central role of epidermal growth factor (EGF) receptor density in anchorage-independent growth of normal rat kidney cells

The gamma-carboxyglutamic acid (Gla) content of several variants of human prothrombin has been measured by using capillary electrophoresis and laser-induced fluorescence (CE-LIF). Both plasma-derived prothrombin and recombinant prothrombin contain ten residues of Gla per molecule of protein. In contrast, a variant of human prothrombin (containing the second kringle domain of bovine prothrombin) was separated into two populations that differed in their Gla content. Direct measurement of the Gla content showed an association with the presence or absence of the calcium-dependent conformational change that is required for prothombinase function. Thus, the CE-LIF assay is useful in determining the carboxylation status of recombinant proteins.     

Alterations of the gamma-carboxyglutamic acid and osteocalcin concentrations in vitamin D-deficient chick bone

The content of osteocalcin and protein bound gamma-carboxyglutamic acid (Gla) was studied as a function of bone maturation and mineralization in normal and vitamin D-deficient, rachitic chickens. The Gla/Ca2+ ratio was elevated in rachitic bone, particularly in the most undermineralized regions. For example, there is a 10- to 20-fold elevation in Gla/Ca2+ in the newly synthesized, least mineralized rachitic bone fraction, which progressively decreases to a 1.5-fold elevation in the most highly mineralized areas of rachitic tissue. Osteocalcin, which is the principal Gla-containing protein of mature bone, was quantitated by radioimmunoassay using specific antiserum to the 5670-dalton chicken protein. Surprisingly, the osteocalcin concentration is decreased 50% in vitamin D-deficient bone. From this we infer that accumulated Gla-containing protein in vitamin D-deficient and poorly mineralized bone may possibly represent a precursor of osteocalcin.     

Cell adhesion to matrix Gla protein and its inhibition by an Arg-Gly-Asp-containing peptide.

Matrix Gla protein (MGP) is a 14-kDa protein found in bone and cartilage which contains the unusual amino acid gamma-carboxyglutamic acid (Gla). The biological function of this protein has not been elucidated. Here we have demonstrated the adherence of chondrocytes, fibroblasts, osteosarcoma cells, and kidney mesangial cells to MGP purified from bovine bone. Maximum adherence occurred at MGP concentrations of 0.5-1.0 micrograms/ml. Removal of the calcium-binding Gla residues by thermal decarboxylation of MGP destroyed the proteins' cell adherence properties. Cell adherence to MGP was not affected by the presence of antibodies directed against the C-terminal (non-Gla) portion of the protein or the presence of cycloheximide during the adherence assay. However, the Arg-Gly-Asp-containing synthetic peptide Gly-Arg-Gly-Asp-Ser-Pro significantly inhibited cell attachment to MGP, whereas the control peptide Gly-Arg-Gly-Glu-Ser-Pro had minimal effect. These data indicate that MGP may function in mediating cell attachment to the extracellular matrix via a receptor that requires intact Gla residues and that can be inhibited by Arg-Gly-Asp-containing peptides.     

Properties of chemically modified protein S: effect of the conversion of gamma-carboxyglutamic acid to gamma-methyleneglutamic acid on functional properties

Protein S, the protein cofactor for activated protein C in the proteolytic inactivation of factor Va, was chemically modified with a mixture of morpholine and formaldehyde. This treatment resulted in the conversion of the gamma-carboxyglutamic acid (Gla) residues of this vitamin K dependent protein to gamma-methyleneglutamic acid. With a 10,000-fold molar excess of morpholine and formaldehyde over protein S it was found that between 10 and 11 Gla residues could be modified. The degree of modification was proportional to the concentration of the modifying reagents used. The modification of as few as two residues resulted in the 70% loss of activity. Calcium inhibited the modification of several residues. In the presence of 3.2 mM calcium ion, a derivative with 2.5 residues modified was prepared that appeared to have full activity. Modification of protein S resulted in the alteration of a number of its properties. The quenching of intrinsic fluorescence by calcium decreased. The quenching effect of terbium ions was also decreased. However, the modified protein and the native protein were equivalent when protein-dependent terbium fluorescence was measured. When modified, protein S would no longer bind to phospholipid vesicles. Finally, the ability of protein S to self-associate was decreased by modification. These findings suggest that the gamma-carboxyglutamic acid residues of protein S may play several roles in the maintenance of structure.     

The presence of γ-carboxyglutamic acid in the proteins associated with ectopic calcification

γ-Carboxyglutamic acid (Gla) is identified in the proteins associated with several types of ectopic calcifications in which hydroxyapatite is the major mineral component. These included the calcified skin and subcutaneous plaques from a patient with dermatomyositis, the calcium containing material extruded from the skin of a patient with scleroderma, and the calcified, atheromatous plaques from aorta. Alkaline hydrolysis of biopsy material from these and from normal tissue revealed the presence of Gla only in the plaque specimens. Since a γ-carboxyglutamic acid-containing protein is normally present in bone and absent in unmineralized tissues, the presence of Gla in soft tissue calcifications is a potentially significant finding, especially in view of its known calcium and phospholipid binding properties.     

Derivatives of blood coagulation factor IX contain a high affinity Ca2+-binding site that lacks gamma-carboxyglutamic acid

We have examined the calcium-binding properties and metal ion-dependent conformational changes of proteolytically modified derivatives of factor IX that lack gamma-carboxyglutamic acid (Gla) residues. Equilibrium dialysis experiments demonstrated that a Gla-domainless factor IX species retained a single high affinity calcium ion-binding site (Kd = 85 +/- 5 microM). Ca2+ binding to this site was accompanied by a decrease in intrinsic fluorescence emission intensity (Kd = 63 +/- 15 microM). These spectral changes were reversed upon the addition of EDTA. Titration with Sr2+ resulted in little change in fluorescence intensity below 1 mM, while titration with Tb3+ caused fluorescence changes similar to those observed with Ca2+. Tb3+ and Ca2+ appear to bind to the same site because tryptophan-dependent terbium emission was reduced by the addition of Ca2+. Similar results were obtained with a Gla-domainless factor IX species lacking the activation peptide. Gla domain-containing factor IX species exhibited fluorescence changes similar to those of the Gla-domainless proteins at low Ca2+, but an additional structural transition was found at higher Ca2+ concentrations (apparent Kd greater than 0.8 mM). Thus, the conformations of factor IX proteins are nucleated and/or stabilized by calcium binding to a high affinity site which does not contain Gla residues. The binding of Ca2+ to lower affinity Gla domain-dependent metal ion-binding sites elicits an additional conformational change. The strong similarities between these results and those obtained with protein C (Johnson, A. E., Esmon, N. L., Laue, T. M. & Esmon, C. T. (1983) J. Biol. Chem. 258, 5554-5560), coupled with the remarkable sequence homologies of the vitamin K-dependent proteins, suggest that the high affinity Gla-independent Ca2+-binding site may be a common feature of vitamin K-dependent proteins.     

Metal ion blockage of tritium incorporation into gamma-carboxyglutamic acid of prothrombin. Stoichiometry of gamma-carboxyglutamic acid to Gd3+ for the high affinity sites

Prothrombin possesses two high affinity and four low affinity gamma-carboxyglutamic acid (Gla)-dependent gadolinium binding sites. Earlier work (Price, P. A., Williamson, M. K., and Epstein, D. J. (1981) J. Biol. Chem. 256, 1172-1176) has shown that tritium can be specifically incorporated at the gamma-carbon of Gla in proteins at pH 5. In the present work we show that inclusion of saturating concentrations of Ca2+ in nondenaturing buffer systems ranging from pH 5.5 to 8.5 prevents the exchange of tritium into all 10 Gla residues of prothrombin. Similarly, saturating concentrations of Gd3+ prevent tritium incorporation into Gla at pH 5.5. Positive cooperativity was observed for the binding of Gd3+ to human prothrombin (at pH 5.5) for the two high affinity sites (Kd congruent to 35 nM). The four low affinity sites bind Gd3+ with a Kd congruent to 5 microM. Incubation of prothrombin ranging in concentrations from 10 to 40 microM with 2 eq of Gd3+ at pH 5.5 prevents 5.7 (average of seven determinations) Gla residues from tritium incorporation. Sedimentation velocity experiments conducted at pH 5.5 indicate that prothrombin in the presence of saturating concentrations of Gd3+ polymerizes, most likely, to a trimer. Further, in the presence of 2 eq of Gd3+, calculated percent weight average concentration of monomer prothrombin is congruent to 100% at 10 microM, approximately equal to 95% at 20 microM, and congruento to 80% at 40 microM protein concentration. Thus, it appears that under conditions in which prothrombin primarily exists as a monomer, occupancy of the initial two metal binding sites by Gd3+ involves six Gla residues.     

Serum bone Gla protein (BGP) during treatment of hyperthyroidism and hypothyroidism. A longitudinal study

Serum bone gamma-carboxyglutamic acid-containing (Gla) protein (BGP) was measured before and with initially 2 weeks, later 4-8 weeks intervals for 20-58 weeks during treatment of patients with hyperthyroidism (n = 10) and hypothyroidism (n = 4). Biochemical euthyroidism was obtained in the hyperthyroid patients after a median of 3 weeks (range 1-8 weeks), and in the hypothyroid patients after a median of 17 weeks (range 10-27 weeks). Serum BGP levels closely followed the thyroid state, being high respectively low in the hyperthyroid and hypothyroid state and reaching a stable plateau just at the time biochemical euthyroidism was obtained. These data suggest that osteoblastic activity is enhanced in hyperthyroidism and reduced in hypothyroidism, and that normalization occurs in close conjunction with the normalization of the thyroid state, without any delay, indicating a direct effect on the function of the excisting osteoblasts by the thyroid hormones.     

Distribution of gamma-carboxyglutamic acid residues in partially carboxylated human prothrombins

The role of gamma-carboxyglutamic acid in prothrombin has been examined using partially carboxylated variant prothrombins isolated from a person with a hereditary defect in vitamin K-dependent carboxylation. These species differ in gamma-carboxyglutamic acid content, distribution, and function, as monitored by metal binding properties, conformational transitions, phospholipid binding, and calcium-dependent coagulant activity (Borowski, M., Furie, B. C., Goldsmith, G. H., and Furie, B. (1985) J. Biol. Chem. 260, 9258-9264). The distribution of gamma-carboxyglutamic acids in the variant prothrombin species was determined by specific tritium incorporation into gamma-carboxyglutamic acid residues, thermal decarboxylation, and automated Edman degradation. gamma-Carboxyglutamic acid residues in the partially carboxylated prothrombins were identified by the assay of tritium in the resultant glutamic acid residues in the acarboxyprothrombins. The results indicate that variant prothrombins 1-3 are nearly homogeneous populations of partially carboxylated prothrombins. The ability of prothrombin to undergo a metal-induced conformational change and to bind to phospholipid vesicles correlated closely to the presence of a gamma-carboxyglutamic acid at residue 16. This residue is likely involved in the formation of a critical high affinity metal-binding site, possibly formed by Gla 16 and Gla 25 and/or Gla 26. A second high affinity metal-binding site, present in all of the variant prothrombin species, is defined, as an upper limit, by Gla 6, Gla 14, Gla 19, and Gla 20. This region is likely responsible for the interaction of certain of the conformation-specific antibodies to the metal-stabilized conformer of prothrombin.     

The identification of matrix Gla protein in cartilage

The vitamin K-dependent bone protein matrix gamma-carboxyglutamic acid (Gla) protein (MGP) has been identified by radioimmunoassay in the guanidine extract of rat cartilage. MGP was present in all cartilages tested at levels comparable to the MGP level in bone. Western blot analysis indicated that the molecular weight of cartilage MGP is the same as bone MGP, and Northern blot analysis revealed that MGP mRNA from cartilage is the same size as the MGP mRNA from bone. The structurally related vitamin K-dependent protein bone Gla protein could not be detected in cartilage by radioimmunoassay or by Northern blot analysis. The discovery that MGP is synthesized by growth plate cartilage could provide an explanation for the excessive growth plate mineralization disorder seen in rats treated with the vitamin K antagonist warfarin and the punctate mineralization of the growth plate seen in infants whose mothers received warfarin in the first trimester of pregnancy (the fetal warfarin syndrome). Both disorders appear to be caused by the inactivation of a vitamin K-dependent mineralization inhibitor in cartilage, an inhibitor which we suggest is MGP.