Sequence of the precursor to rat bone gamma-carboxyglutamic acid protein that accumulates in warfarin-treated osteosarcoma cells

Proprotein convertase furin regulates osteocalcin and bone endocrine function

Osteocalcin (OCN) is an osteoblast-derived hormone that increases energy expenditure, insulin sensitivity, insulin secretion, and glucose tolerance. The cDNA sequence of OCN predicts that, like many other peptide hormones, OCN is first synthesized as a prohormone (pro-OCN). The importance of pro-OCN maturation in regulating OCN and the identity of the endopeptidase responsible for pro-OCN cleavage in osteoblasts are still unknown. Here, we show that the proprotein convertase furin is responsible for pro-OCN maturation in vitro and in vivo. Using pharmacological and genetic experiments, we also determined that furin-mediated pro-OCN cleavage occurred independently of its γ-carboxylation, a posttranslational modification that is known to hamper OCN endocrine action. However, because pro-OCN is not efficiently decarboxylated and activated during bone resorption, inactivation of furin in osteoblasts in mice resulted in decreased circulating levels of undercarboxylated OCN, impaired glucose tolerance, and reduced energy expenditure. Furthermore, we show that Furin deletion in osteoblasts reduced appetite, a function not modulated by OCN, thus suggesting that osteoblasts may secrete additional hormones that regulate different aspects of energy metabolism. Accordingly, the metabolic defects of the mice lacking furin in osteoblasts became more apparent under pair-feeding conditions. These findings identify furin as an important regulator of bone endocrine function.

Matrix Gla protein and osteocalcin: from gene duplication to neofunctionalization

Osteocalcin (OC or bone Gla protein, BGP) and matrix Gla protein (MGP) are two members of the growing family of vitamin K-dependent (VKD) proteins. They were the first VKD proteins found not to be involved in coagulation and synthesized outside the liver. Both proteins were isolated from bone although it is now known that only OC is synthesized by bone cells under normal physiological conditions, but since both proteins can bind calcium and hydroxyapatite, they can also accumulate in bone. Both OC and MGP share similar structural features, both in terms of protein domains and gene organization. OC gene is likely to have appeared from MGP through a tandem gene duplication that occurred concomitantly with the appearance of the bony vertebrates. Despite their relatively close relationship and the fact that both can bind calcium and affect mineralization, their functions are not redundant and they also have other unrelated functions. Interestingly, these two proteins appear to have followed quite different evolutionary strategies in order to acquire novel functionalities, with OC following a gene duplication strategy while MGP variability was obtained mostly by the use of multiple promoters and alternative splicing, leading to proteins with additional functional characteristics and alternative gene regulatory pathways.

Oral anticoagulant therapy: Antithrombotic Therapy and Prevention of Thrombosis, 9th ed: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines

BACKGROUND

The objective of this article is to summarize the published literature concerning the pharmacokinetics and pharmacodynamics of oral anticoagulant drugs that are currently available for clinical use and other aspects related to their management.

METHODS

We carried out a standard review of published articles focusing on the laboratory and clinical characteristics of the vitamin K antagonists; the direct thrombin inhibitor, dabigatran etexilate; and the direct factor Xa inhibitor, rivaroxaban

RESULTS

The antithrombotic effect of each oral anticoagulant drug, the interactions, and the monitoring of anticoagulation intensity are described in detail and discussed without providing specific recommendations. Moreover, we describe and discuss the clinical applications and optimal dosages of oral anticoagulant therapies, practical issues related to their initiation and monitoring, adverse events such as bleeding and other potential side effects, and available strategies for reversal.

CONCLUSIONS

There is a large amount of evidence on laboratory and clinical characteristics of vitamin K antagonists. A growing body of evidence is becoming available on the first new oral anticoagulant drugs available for clinical use, dabigatran and rivaroxaban.

Pharmacology and management of the vitamin K antagonists: American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition)

This article concerning the pharmacokinetics and pharmacodynamics of vitamin K antagonists (VKAs) is part of the American College of Chest Physicians Evidence-Based Clinical Practice Guidelines (8th Edition). It describes the antithrombotic effect of the VKAs, the monitoring of anticoagulation intensity, and the clinical applications of VKA therapy and provides specific management recommendations. Grade 1 recommendations are strong and indicate that the benefits do or do not outweigh the risks, burdens, and costs. Grade 2 recommendations suggest that the individual patient's values may lead to different choices. (For a full understanding of the grading, see the "Grades of Recommendation" chapter by Guyatt et al, CHEST 2008; 133:123S-131S.) Among the key recommendations in this article are the following: for dosing of VKAs, we recommend the initiation of oral anticoagulation therapy, with doses between 5 mg and 10 mg for the first 1 or 2 days for most individuals, with subsequent dosing based on the international normalized ratio (INR) response (Grade 1B); we suggest against pharmacogenetic-based dosing until randomized data indicate that it is beneficial (Grade 2C); and in elderly and other patient subgroups who are debilitated or malnourished, we recommend a starting dose of < or = 5 mg (Grade 1C). The article also includes several specific recommendations for the management of patients with nontherapeutic INRs, with INRs above the therapeutic range, and with bleeding whether the INR is therapeutic or elevated. For the use of vitamin K to reverse a mildly elevated INR, we recommend oral rather than subcutaneous administration (Grade 1A). For patients with life-threatening bleeding or intracranial hemorrhage, we recommend the use of prothrombin complex concentrates or recombinant factor VIIa to immediately reverse the INR (Grade 1C). For most patients who have a lupus inhibitor, we recommend a therapeutic target INR of 2.5 (range, 2.0 to 3.0) [Grade 1A]. We recommend that physicians who manage oral anticoagulation therapy do so in a systematic and coordinated fashion, incorporating patient education, systematic INR testing, tracking, follow-up, and good patient communication of results and dose adjustments [Grade 1B]. In patients who are suitably selected and trained, patient self-testing or patient self-management of dosing are effective alternative treatment models that result in improved quality of anticoagulation management, with greater time in the therapeutic range and fewer adverse events. Patient self-monitoring or self-management, however, is a choice made by patients and physicians that depends on many factors. We suggest that such therapeutic management be implemented where suitable (Grade 2B).

The elastic lamellae of devitalized arteries calcify when incubated in serum: evidence for a serum calcification factor

OBJECTIVE

To determine whether serum contains an activity that induces artery calcification.

METHODS AND RESULTS

The elastic lamellae of devitalized rat aortas calcify rapidly in rat or bovine serum, or in human serum provided [Pi] > or =2 mmol/L. This calcification is attributable to a potent serum calcification factor (SCF), one that causes devitalized aortas to calcify when incubated in DMEM containing as little as 1.5% serum but not in DMEM alone. The SCF that initiates medial elastin calcification has the same 50- to 150-kDa size and protease sensitivity as the SCF shown previously to initiate calcification of type I collagen. Our working hypothesis is that the same SCF initiates calcification of collagen and elastin, and that this SCF arises from sites of normal bone mineralization and, like alkaline phosphatase, is released into general circulation. The SCF does not initiate medial elastin calcification in living arteries, which suggests that vascular cells may prevent this calcification. This hypothesis is supported by the observations that living arteries secrete the calcification inhibitor matrix Gla protein (MGP); that inactivation of MGP with warfarin causes living arteries to calcify; and that addition of MGP to medium containing warfarin prevents this calcification.

CONCLUSIONS

The elastic lamellae of devitalized aortas calcify rapidly in serum.

The pharmacology and management of the vitamin K antagonists: the Seventh ACCP Conference on Antithrombotic and Thrombolytic Therapy

This article concerning the pharmacokinetics and pharmacodynamics of vitamin K antagonists (VKAs) is part of the Seventh American College of Chest Physicians Conference on Antithrombotic and Thrombolytic Therapy: Evidence-Based Guidelines. The article describes the antithrombotic effect of VKAs, the monitoring of anticoagulation intensity, the clinical applications of VKA therapy, and the optimal therapeutic range of VKAs, and provides specific management recommendations. Grade 1 recommendations are strong, and indicate that the benefits do, or do not, outweigh the risks, burdens, and costs. Grade 2 suggests that individual patient's values may lead to different choices (for a full understanding of the grading see Guyatt et al, CHEST 2004; 126:179S-187S). Among the key recommendations in this article are the following: for dosing of VKAs, we suggest the initiation of oral anticoagulation therapy with doses between 5 and 10 mg for the first 1 or 2 days for most individuals, with subsequent dosing based on the international normalized ratio (INR) response (Grade 2B). In the elderly and in other patient subgroups with an elevated bleeding risk, we suggest a starting dose at < or = 5 mg (Grade 2C). We recommend basing subsequent doses after the initial two or three doses on the results of INR monitoring (Grade 1C). The article also includes several specific recommendations for the management of patients with INRs above the therapeutic range and for patients requiring invasive procedures. For example, in patients with mild to moderately elevated INRs without major bleeding, we suggest that when vitamin K is to be given it be administered orally rather than subcutaneously (Grade 1A). For the management of patients with a low risk of thromboembolism, we suggest stopping warfarin therapy approximately 4 days before they undergo surgery (Grade 2C). For patients with a high risk of thromboembolism, we suggest stopping warfarin therapy approximately 4 days before surgery, to allow the INR to return to normal, and beginning therapy with full-dose unfractionated heparin or full-dose low-molecular-weight heparin as the INR falls (Grade 2C). In patients undergoing dental procedures, we suggest the use of tranexamic acid mouthwash (Grade 2B) or epsilon amino caproic acid mouthwash without interrupting anticoagulant therapy (Grade 2B) if there is a concern for local bleeding. For most patients who have a lupus inhibitor, we suggest a therapeutic target INR of 2.5 (range, 2.0 to 3.0) [Grade 2B]. In patients with recurrent thromboembolic events with a therapeutic INR or other additional risk factors, we suggest a target INR of 3.0 (range, 2.5 to 3.5) [Grade 2C]. As models of anticoagulation monitoring and management, we recommend that clinicians incorporate patient education, systematic INR testing, tracking, and follow-up, and good communication with patients concerning results and dosing decisions (Grade 1C+).

Isolation and complete amino acid sequence of osteocalcin from canine bone

Osteocalcin was purified in high yield and to homogeneity from the diaphysis of dog femora by the following steps: (1) acid demineralization of bone powder, (2) solid-phase extraction of acid-soluble proteins on Sep-Pak C18 cartridges, (3) gel filtration on Sephadex G-50, and (4) fast protein liquid chromatography on an Accell-QMA anion-exchange column. Starting from 30 g washed bone powder, approximately 7-10 mg pure protein was obtained in 2 days. The key step is the initial solid-phase extraction of osteocalcin from a large volume of a demineralized bone solution. The primary structure was established by automated sequence analyses of two tryptic peptides, of two endoproteinase Glu-C carboxy-terminal peptides, and of the first 30 amino acid residues of the intact protein. Dog osteocalcin contains 49 amino acids, has a molecular mass of 5654 daltons, contains no Thr, Met, Hyp, or Trp, has a disulfide bond between Cys 23 and 29, and is fully gamma-carboxylated at residues 17, 21, and 24. Dog osteocalcin does not contain a pair of basic amino acids found at positions 43-44 in most other osteocalcins from mammals and birds. A computer search for homology indicated 88, 90, 84, 88, 66, and 57% sequence identity of dog osteocalcin with human, bovine, cat, monkey, chicken, and swordfish osteocalcin, respectively, and weaker homologies with the gamma-carboxylated domains of blood-clotting proteins and the Pro-rich N-terminal extensions of myosin light-chain A1 and beta-crystalline B1. The possible relevance of these homologies to the structure and potential functions of osteocalcin is discussed.

Secretion of osteocalcin and its propeptide from human osteoblastic cells: dissociation of the secretory patterns of osteocalcin and its propeptide

Specific immunoassay systems for intact human osteocalcin (I-OC) and its 26-residue propeptide have been newly developed to assess their usefulness as biochemical markers of bone metabolism. Using human cultured osteoblastic periosteal cells, we monitored 24 h secretion of these molecules from the osteoblastic cells and also examined the deposition of Ca, P, and I-OC on the extracellular matrix. At day 5, both I-OC and its propeptide were secreted by osteoblastic cells in a concentration-dependent manner by treatment with 1,25-(OH)2D3. This propeptide was not detected in the serum of adult subjects but was detected in the serum of normal children, which confirmed this in vitro result of propeptide secretion. The secretion of I-OC into medium transiently decreased at day 11, when the rapid accumulation of I-OC, Ca, and P, namely mineralization, was observed on the extracellular matrix of osteoblastic cells, although secretion of the propeptide constantly increased throughout the culture period. Therefore, the ratio of the amount of propeptide to I-OC in the supernatant markedly increased when mineralization started. These data demonstrate the superior specificity of propeptide as a marker of osteoblastic function in vitro compared with I-OC and that monitoring the changes in propeptide to I-OC ratios in the culture supernatant may be useful for predicting the timing of mineralization on the extracellular matrix of osteoblastic cells.

Dose-dependent effects of aluminum on osteocalcin synthesis in osteoblast-like ROS 17/2 cells in culture

This in vitro study evaluates the effect of aluminum (Al3+) on osteocalcin, a small protein that is produced by the osteoblast. After stimulation with various doses of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3; 10(-11) to 10(-9) M], osteocalcin was consistently lower in the culture medium of ROS 17/2 osteoblastic cells conditioned with 5 microM Al(3+)-saturated transferrin (AlTR) than in apotransferrin (ApoTR)-treated controls. In a second experiment, cultures were conditioned with various doses of AlTR or ApoTR (1.6-8.0 microM) and stimulated with 10(-9) M 1,25(OH)2D3. High doses of AlTR (4.8-8.0 microM) resulted in lower medium and unchanged intracellular content of osteocalcin than treatment with equal amounts of ApoTR. However, in the same experiment, lower doses of AlTR or ApoTR (1.6 and 3.2 microM) yielded different results, i.e., increased medium and intracellular contents of osteocalcin in the Al(3+)-treated cells. Expression of osteocalcin mRNA was not altered in cultures conditioned with low (1.6 microM) or high (8.0 microM) concentrations of AlTR or ApoTR. Similarly, no effect of Al3+ was observed on total protein content, the rate of total protein synthesis, and the degradation of secreted osteocalcin in cultures conditioned with various doses of AlTR or ApoTR. These findings suggest that AlTR affects osteocalcin synthesis in a specific manner, without concomitant effects on the rate of total protein synthesis or on the rate of degradation of osteocalcin. This effect is dose dependent, i.e., low doses of AlTR stimulate and high doses suppress osteocalcin synthesis and/or secretion, and it appears to be posttranscriptional, since the expression of osteocalcin mRNA is not affected.

Evidence for the formation of a complex between osteopontin and osteocalcin

We hypothesize that the mechanisms governing bone formation and remodeling involve the assembly of some of the components of the extracellular matrix into supramolecular complexes. We have examined the associations of osteopontin (OPN) with other proteins isolated from demineralized rat long bones. Three ligand binding techniques were used to demonstrate the formation of complexes between osteopontin and osteocalcin (OCN). Using gel overlay assays, the binding between soluble 125I-OPN and OCN immobilized in acrylamide gels was visualized. Competition for 125I-OPN-OCN complexes was demonstrated when unlabeled OCN-enriched bone extract was included in gel overlay solutions. Also, gel overlay assays showed 125I-OCN binding to OPN. Saturable binding was shown in solid-phase filter binding assays, which yielded an equilibrium binding constant of moderately high affinity (approximately 10(-8) M). Specificity of OPN-OCN complex formation was confirmed by measuring binding in the presence of unlabeled OPN and OCN versus a bone-localized serum protein, alpha 2HS-glycoprotein. Finally, the formation of soluble complexes were demonstrated in a modified Hummel-Dreyer gel filtration assay. These results indicate that OPN and OCN form complexes in vitro. The possible functions of OPN-OCN complexes in osteoclast recruitment and attachment are discussed.

The osteocalcin propeptide is not secreted in vivo or in vitro

Osteocalcin is initially synthesized as an 11 kD molecule consisting of a 23-residue translocation signal peptide that is cleaved during translation, a 26-residue propeptide that targets the protein for gamma-carboxylation, and the 49-residue mature protein. Although the majority of newly synthesized osteocalcin is deposited into bone matrix, a small amount can be detected in blood, and it is this characteristic that has led to its current clinical use as a specific index of osteoblastic activity. Nothing is known, however, about the fate of the propeptide. If osteocalcin and the propeptide are cosecreted, then the concentration of the propeptide could also be useful as a marker of osteoblastic function and, further, may be superior to osteocalcin because it would be unaffected by binding to bone. To test this hypothesis, we synthesized a peptide corresponding to 21 residues of the osteocalcin propeptide from humans and produced a polyclonal antibody to this peptide. Human sera were screened for the presence of the propeptide, and the human osteosarcoma cell line MG-63 was tested for secretion of the propeptide. We could not detect any osteocalcin propeptide in sera from normal adults or individuals with renal failure or primary hyperparathyroidism or those on long-term coumadin therapy. Likewise there was no propeptide present in media from cells grown in the presence of vitamin K, 1,25-(OH)2D3, warfarin, or warfarin plus 1,25-(OH)2D3. In contrast, the cell extract, characterized by high-performance liquid chromatography, contained mature osteocalcin, free propeptide, and the proosteocalcin precursor when cells were grown in the presence of 1,25-(OH)2D3 alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of warfarin on early rat tooth development

Rat pups were treated from birth to 5 days of age with the vitamin K antagonist warfarin in order to investigate possible functions of the vitamin K-dependent dentin Gla protein (DGP) in tooth development. Warfarin completely eliminated the immunocytochemically detectable DGP which is a prominent feature of dentin in control rat pups, and also caused an increased concentration of DGP in odontoblasts. Warfarin treatment did not affect the ultrastructure of cells or the extracellular matrix in the tooth germs. The width of the predentin layer, which is considered to be correlated with the rate of mineralization, was unchanged. These results are the first to demonstrate that warfarin treatment prevents the accumulation of DGP in dentin, and that the deposition of DGP has no influence on the overall rate of dentin matrix mineralization in tooth germs.

Osteocalcin: diagnostic methods and clinical applications

Osteocalcin is a small (Mr 5800), very interesting bone specific protein, synthesized by osteoblasts and measured in plasma as a biochemical indicator of bone formation. Many immunoassays for osteocalcin have been developed, including radio- and enzymoimmunoassays, with the use of monoclonal and polyclonal antibodies. These are used in many different clinical settings, including bone, kidney, and liver diseases. However, there is a wide range of published values for plasma osteocalcin concentrations in control and patient samples and this has hindered a more widespread adoption of osteocalcin measurement by clinicians. This review discusses how various immunoassays for osteocalcin may contribute to the wide variation of published values and suggests approaches for the development of standardized assays. For example, epitope specificity and immunoreactivity with multiple forms of osteocalcin and osteocalcin peptides in plasma are discussed. It also includes a recent update on interesting clinical applications of osteocalcin.

Effect of sodium warfarin on vitamin K-dependent proteins and skeletal development in the rat fetus

Sodium warfarin was administered daily to Sprague-Dawley rats from gestational day 8 to day 22 to examine the effects of this compound on the developing fetal skeleton and on the vitamin K-dependent bone and cartilage proteins. At a dose of 175 micrograms/kg of sodium warfarin there was a 43% mortality rate among the dams. Maternal prothrombin times and serum osteocalcin levels were slightly elevated but not significantly. In the surviving litters, fetal bone osteocalcin and gamma-carboxyglutamic acid were significantly reduced (50 and 57%, respectively, on gestational day 22) when compared to age- and/or weight-matched control pups. The high correlation of osteocalcin content in long bone (R = 0.64) and calvariae (R = 0.77) to fetal body weight observed in control fetuses was not seen in the warfarin-exposed pups. Examination of alizarin-stained warfarin-exposed fetal skeletons for ossification centers showed no difference from controls. However, analysis of the tibial growth showed several changes compared to control that included (1) widened hypertrophic zones, (2) increased calcification of the hypertrophic zones, and (3) disorganization of the hypertrophic cells. These results suggest that the growth plate abnormalities seen with prenatal warfarin exposure relate to the inhibition of the vitamin K-dependent proteins of the skeletal system.

Dentin matrix proteins: composition and possible functions in calcification

Dentin may be regarded as a mineralized connective tissue. In its composition as well as its mode of formation, dentin exhibits several similarities with bone, but also definite differences. The dentin organic phase, the matrix, determines its morphology and is believed to be instrumental in the formation of the mineral phase. A fibrous web of collagen type I dominates the organic matrix. Also, minor amounts of other collagen types may be present. The noncollagenous proteins (NCPs), which constitute about 10% of the matrix, fall into several categories: phosphoproteins, Gla-proteins of the osteocalcin type as well as matrix Gla-protein, proteoglycans, different acidic glycoproteins, and serum proteins. Some of these NCPs have unique chemical compositions that give them specific properties. Dentinogenesis occurs by two simultaneous processes: the formation of a collagenous web in predentin, which is followed by the formation of the inorganic phase at the mineralization front. The composition of the predentin organic matrix differs from that of dentin, as some NCP components are secreted extracellularly just in advance of the mineralization front. In addition, some constituents of predentin seem to be metabolized. The NCPs may be important to several processes during dentinogenesis. Much evidence indicates that noncollagenous components in the matrix are instrumental in mineral formation. New data show that polyanionic NCPs, such as phosphoprotein and proteoglycans, when immobilized on a solid support, induce apatite formation under physiological conditions. These data indicate that polyanionic NCPs may function as mineral nucleators in vivo. They may also act as size and rate regulators for crystallization and promote calcium ion diffusion in the tissue. In addition, NCPs may regulate collagen fibrillogenesis.

Structure and evolution of the bovine prothrombin gene

The cloned bovine prothrombin gene has been characterized by partial DNA sequence analysis, including the 5' and 3' flanking sequences and all the intron-exon junctions. The gene is approximately 15.4 x 10(3) base-pairs in length and comprises 14 exons interrupted by 13 introns. The exons coding for the prepro-leader peptide and the gamma-carboxyglutamic acid-containing region are similar in organization to the corresponding exons in the factor IX and protein C genes. This region has probably evolved as a result of recent gene duplication and exon shuffling events. The exons coding for the kringles and the serine protease region of the prothrombin gene are different in organization from the homologous regions in other genes, suggesting that introns have been inserted into these regions after the initial gene duplication events.

Retinoic acid and glucocorticoids enhance the effect of 1,25-dihydroxyvitamin D3 on bone gamma-carboxyglutamic acid protein synthesis by rat osteosarcoma cells

Two 1,25-dihydroxyvitamin D3-controlled parameters in the osteoblastlike osteosarcoma cell line ROS 17/2, bone gamma-carboxyglutamic acid-containing protein (BGP) and collagen synthesis, were measured after pretreatments with either retinoic acid (RA), or triamcinolone acetate (TRM). RA and TRM both caused double the expected increase in BGP secretion at 16 hr after treatment with 1,25-dihydroxyvitamin D3. Triamcinolone acetate concentrations of 10(-8) and 10(-9) M or 10(-6) M retinoic acid were effective in enhancing the 1,25-dihydroxyvitamin D3 stimulation of BGP secretion. Treatment with RA or TRM alone did not stimulate BGP secretion. RA alone had no effect on BGP secretion, while TRM inhibited BGP secretion. Collagen synthesis is inhibited by 1,25-dihydroxyvitamin D3. Neither retinoic acid nor triamcinolone acetate enhanced the 1,25-dihydroxyvitamin D3-mediated inhibition of collagen synthesis. Retinoic acid by itself inhibited collagen synthesis but did not change the 1,25 dihydroxyvitamin D3-mediated inhibition of collagen synthesis. Triamcinolone acetate by itself or together with 1,25-dihydroxyvitamin D3 increased collagen synthesis. We conclude that, although both triamcinolone acetate and retinoic acid increase the 1,25-dihydroxyvitamin D3 stimulation of BGP secretion by ROS 17/2 cells, they have different effects on the regulation of collagen production. Thus, although both hormones increase the 1,25-dihydroxyvitamin D3 receptor concentration in these cells, their actions are not mediated solely by this mechanism.

Two proteins with gamma-carboxyglutamic acid in frog bone: isolation and comparative characterization

Two gamma-carboxyglutamic acid-containing proteins were purified from neutral (pH 7.5) EDTA-extract of frog, Rana catesbiana, cortical bone by Sephadex G-75 gel filtration, DEAE-Sephadex A-25 chromatography and successive hydroxyapatite column chromatography. These two bone gamma-carboxyglutamic acid-containing proteins, termed osteocalcin, P-1 and P-2, had molecular weights of about 5100 and 4900, respectively, based on their amino-acid composition. Both species of osteocalcin have two gamma-carboxyglutamic acid residues, one disulfide bond, but there was no 4-hydroxyproline in either molecule. Each N-terminus of both proteins was acetylated and each C-terminal amino acid was lysine. The isoelectric points of P-1 and P-2 are 4.02 and 3.91, respectively, and their pI values shifted to more neutral pH in the presence of calcium ions. Equilibrium dialysis has indicated that each of these two proteins binds specifically 2 mol Ca2+, and nonspecifically more, 4-5 mol, Ca2+ in 0.02 M Tris-HCl/0.15 M NaCl (pH 7.4), at 4 degrees C. By the best-fitted calculation, P-1 had one high affinity Ca2+-binding site (Kd1 = 0.17 mM) and one lower affinity site (Kd2 = 0.29 mM), and P-2 contained one high affinity site (Kd1 = 0.154 mM) and one lower affinity site (Kd2 = 0.67 mM).