A plasmin-derived hexapeptide from the carboxyl end of osteocalcin counteracts oxytocin-mediated growth inhibition [corrected] of osteosarcoma cells.

We have previously described the presence of the functional plasminogen activator system on the surfaces of bone neoplastic cells and the fact that plasmin specifically cleaves bone matrix protein osteocalcin (OC). The cleavage of OC to NH2-midterminal (1-44) and COOH-terminal RFYGPV hexapeptide (44-49) proceeds with detachment of both products from bone mineral. Because the sequence of OC-derived hexapeptide (HP) is nearly identical to the E2 region of the oxytocin receptor (OTR), we set out to ascertain whether the HP interferes with the osteosarcoma (OS)-associated oxytocin (OT) system. We documented the presence and functional activity of OTRs in several OS cells by means of (a) OT-mediated inhibition of OS growth; (b) expression of OTR mRNA by means of reverse transcription-PCR; (c) immunofluorescence staining with IF3 monoclonal antibody specific for human OTR; and (d) saturation binding and Scatchard analysis of OT binding to the receptors of isolated membranes or intact OS cells. Although we could not demonstrate direct binding of HP to OT, the presence of HP in cultures of OS cells antagonizes the inhibitory effect of OT on these cells. Additionally, in competitive binding assays, the HP effectively competes with binding of OT to its cognate receptors. The results indicate the existence of an OTR/OT system in tumor cells of bone origin. Suggested plasminogen activator-OC-OTR/OT interactions may have an effect on the regulation of cell proliferation within the bone tissue as well as properties of the extracellular matrix surrounding the tumor foci in the bone.

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.

Rapid and sensitive method of quantitation of bone gla protein mRNA using competitive polymerase chain reaction.

A method for sensitive quantitation of bone gla protein (BGP, osteocalcin) mRNA has been developed using competitive polymerase chain reaction after reverse transcription (competitive RT-PCR). The complementary DNA (cDNA) were transcribed from sample RNA was co-amplified in a PCR with a known amount of mutant BGP cDNA (competitor) using the identical oligonucleotide primers. The mutant cDNA with its unique restriction site allowed quantitation of sample and mutant PCR products after densitometric analysis of ethidium bromide-stained agarose gels. A linear relationship between initial sample BGP amount and the ratio of BGP to mutant BGP band intensity was obtained and used to make a standard curve to determine the initial BGP mRNA of unknown samples. These standard curves were made with known amounts of recombinant BGP cDNA. The competitive RT-PCR for BGP allows measurement of twofold differences in 1 and 2 micrograms total RNA and requires at least 10 times less sample RNA than usual Northern blotting. Moreover, heteroduplexes with one BGP strand and one mutant BGP strand formed as a result of high PCR cycles were quantifiable. This provided the advantages of rapid quantitation from ethidium bromide-stained gels without blotting, hybridization, or autoradiography. Multiple samples could be assayed for greater confidence in the results. The sensitivity, accuracy, and ease of the assay will facilitate analysis of BGP mRNA from a small amount of sample. The assay has been used to confirm the BGP mRNA changes with hormonal treatment in cultured cells and the age-related changes in whole tibia in vivo.

Plasmin-mediated proteolysis of osteocalcin.

Plasmin cleaves osteocalcin at a site within its carboxyl end, thus creating an N-midterminal 1-43 and a short C-terminal 44-49 peptides. The products of the cleavage were identified by matrix assisted laser desorption ionization time of flight mass spectrophotometry and by reversed phase high performance liquid chromatography followed by N-terminal sequence determination. When separated by sodium dodecyl sulfide-polyacrylamide gel electrophoresis in the presence of reducing agents, large (LF; N-midterminal) and a small molecular weight (SF; C-terminal) fragments can be identified. The major cleavage site involves arg43-arg44 amino acid residues, and the resulting 44-49 C-terminal fragment appears as a slow migrating band on native gels (SFnat). Elevated levels of calcium ion inhibit the plasmin-mediated lysis of osteocalcin. Plasmin-mediated cleavage of osteocalcin occurs both in solution and when bound to hydroxyapatite. Both osteocalcin cleavage products detach from the hydroxyapatite substrate. Diisopropyl fluorophosphate-inhibited plasmin does not displace osteocalcin from the hydroxyapatite surface. Previously, the C-terminal pentapeptide has been shown to be chemotactic for bone cells while bone particles lacking osteocalcin were resistant to bone resorption. We therefore hypothesize that the plasmin-mediated digestion of free and hydroxyapatite-bound osteocalcin could play a role in the regulation of bone remodeling.

Presence of messenger ribonucleic acid encoding osteocalcin, a marker of bone turnover, in bone marrow megakaryocytes and peripheral blood platelets.

Osteocalcin (Oc), an abundant gamma-carboxylated protein (mol wt, 5800) of bone matrix, is used as a serum marker of bone turnover because it is considered to be uniquely synthesized by the osteoblast. Our finding of Oc messenger RNA (mRNA) in rat tibial diaphyseal marrow led us to investigate the cellular origins of Oc mRNA in peripheral blood and bone marrow. In anticoagulated blood, Oc mRNA was detected in total RNA prepared from buffy coat cells (BCC). Fractionation of rat and human blood showed that platelets contain Oc mRNA identical to that found in bone cells. In rat bone marrow, Oc mRNA is highly enriched in the platelet-producing megakaryocyte population. Depending upon the RIA used, immunoreactive Oc was either undetectable or present at very low levels in platelets and megakaryocytes, suggesting that synthesis of Oc by these cells may be under strong translational regulation. In addition, Oc levels were higher in serum vs. plasma obtained from the same blood, suggesting that Oc may be released by platelets during blood clotting. Interestingly, the magnitude of this difference was greater in female rats. Injection of 1,25-dihydroxyvitamin D3 dose-dependently increased plasma Oc, but did not cause correlative changes in steady state levels of Oc mRNA in BCC. During rat growth, plasma Oc was maximal, whereas Oc mRNA levels in BBC were low. This relationship was reversed during aging. A correlation between Oc mRNA levels in BCC and rat age suggests a developmental regulation of Oc mRNA levels in platelets. These data indicate that Oc mRNA is not restricted to cells on mineralizing surfaces, but is also found in megakaryocytes and peripheral blood platelets, which possibly contribute to the Oc levels in blood and the regulation of bone turnover.

Matrix Gla protein gene expression is elevated during postnatal development.

Matrix Gla protein (MGP) is a vitamin K-dependent extracellular matrix protein with a wide tissue distribution. Developmental expression of the MGP gene is characterized by competitive RT-PCR in kidney and calvaria. High levels of MGP mRNA were observed in kidneys and calvaria from 19-day-old embryos to 1-month-old rats. There was a peak in MGP mRNA at 7 days in both tissues. MGP mRNA expression was very low or undetectable in 3-, 5- and 7- month-old kidneys. Similar observations were seen in lung, heart and spleen. However, in connective tissues like calvaria, tibia and trachea, low levels of MGP mRNA are maintained throughout life. Kidney MGP protein was present from birth to 15 days, with the highest MGP protein level at 7 days. Calvarial MGP protein was present throughout development and maturation but peaked at 7 days. The highest MGP protein levels were coincident with peak levels of MGP mRNA. Thus, MGP protein level correlated with mRNA level during rat development. In situ hybridization revealed that MGP staining was most intense in the straight tubules of the developing kidney medulla at 7 days. Staining was absent in stromal cells and in mature nephrons. Taken together, our finding of high MGP mRNA and its intense in situ staining during the postnatal growth phase prove that increased MGP synthesis occurs at a specific time and place during development and maturation.

Quantitation of matrix Gla protein mRNA by competitive polymerase chain reaction using glyceraldehyde-3-phosphate dehydrogenase as an internal control.

Matrix Gla (gamma-carboxyglutamic acid) protein (MGP) is a vitamin-K-dependent extracellular matrix protein. A method was developed to quantitate MGP mRNA based on competitive polymerase chain reaction following reverse transcription (competitive RT-PCR). The MGP cDNA was coamplified with a mutant MGP cDNA (competitor). The ratio of MGP to competitor after the PCR reaction was compared to standards to determine the amount of MGP mRNA in RT samples. MGP mRNA in as little as 3.125 ng total RNA was accurately quantitated and was far more sensitive than RNA hybridization methods. To control for variations due to sample preparation, a second competitive RT-PCR was developed to measure the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA from the same sample as an internal control. Thus, the amount of MGP is normalized to the amount of the housekeeping gene GAPDH. The accuracy, sensitivity and ease of this new method enables rapid mRNA quantitation without blotting, hybridization or autoradiography. The method is particularly advantageous for MGP mRNA measurement from a small amount of sample. Using this assay, we established that MGP mRNA increases approx. fivefold with co-treatment of retinoic and ascorbic acids.

Matrix Gla protein binding to hydroxyapatite is dependent on the ionic environment: calcium enhances binding affinity but phosphate and magnesium decrease affinity.

Matrix Gla protein (MGP) is an inhibitor of mineralization found in bone, cartilage, developing tissues, smooth muscle, and atherosclerotic plaques. MGP interaction with hydroxyapatite (HA) has been inferred by its function, but has never been measured directly. In this study, the influence of MGP antibody (x-MGP) binding, plasmin digestion, and various ions, including calcium and phosphate, on (125)I-labeled MGP-HA binding was examined. Nonlinear regression analysis of MGP binding yielded K(a) (association constant; approximately 8.0 x 10(4) M(-1)) and B(max) (maximum specific bound fraction of MGP; approximately 0.53). Anti-MGP antiserum reduced K(a) to less than half of control (0.33% x-MGP). Plasmin-digested MGP decreased HA binding parameters by almost a third, showing that protein binding and limited proteolysis greatly affected HA binding. The presence of free calcium ions significantly increased binding in a dose-dependent manner, with approximately 1 mmol/L calcium increasing K(a) by a factor of 2. Phosphate ions decreased binding significantly in a dose-dependent fashion, with approximately 1 mmol/L PO(4) decreasing K(a) by a third. Magnesium at approximately 1 mmol/L decreased K(a) significantly by half, but the effect was not dose-dependent. Carbonate, sulfate, and sodium ions had no significant effect on binding. MGP binding to HA is sensitive to protein binding, limited proteolysis, and the surrounding ionic environment.

Cellular events associated with the induction of bone by demineralized bone.

Implantation of demineralized bone (DB) in the form of powder or intact segments in extra skeletal sites stimulates new bone formation. Urist and co-workers presented substantial evidence that there is a noncollagenous protein that has the ability to induce bone formation. One aim of this study was to trace the process of bone formation when DB, in the form of perforated rectangular plates, is implanted subcutaneously in 2-month-old rats. A second objective was to determine whether cartilage cells play a role in the formation of bone in this model. Various DB plates with 0.25 mm diameter holes were implanted subcutaneously for 1-4 weeks in rats. One week after implantation, DB plates were covered by vascularized connective tissue that invaded the perforations. Aggregates of chondrocytes were observed within the holes and on periosteal surfaces in only a few specimens. Further cartilage proliferation was not observed, and by the 2nd week there was no evidence of endochondral bone formation. Where these cartilage-like cells were present, a thin layer of mineral was deposited around them; resorption and fibrous tissue infiltration followed. This aborted form of endochondral calcification was not followed spatially by bone formation. Patent vascularized channels were invaded by alkaline phosphatase-positive mononuclear cells and fibroblasts, and became enlarged by the enzymatic action of macrophages. The next step involved the calcification of DB plates adjacent to the wide spaces. Osteoclasts now appeared leading to the resorption of this recalcified matrix. The eroded and now enlarged lacunar surfaces were lined by newly formed bone and osteoblasts. This process continued so that, at the end of 4 weeks following implantation, the original DB plates were replaced by trabecular bone. Biochemical data on calcium and alkaline phosphatase levels in the implants paralleled the morphological observations.

A colorimetric assay specific for gamma-carboxyglutamic acid-containing proteins: its utility in protein purification procedures.

A colorimetric method for the detection of gamma-carboxyglutamic acid (Gla)-containing proteins after reaction with 4-diazobenzenesulfonic acid is presented. Proteins can be visualized after electroblotting from polyacrylamide gels onto membrane supports, after dot-blotting onto membranes, or in solution as a red colored product with an absorbance maximum at 530 nm. The method is specific since other proteins without gamma-carboxyglutamic acid do not form a red color. The presence of other proteins does not inhibit or affect color production by gamma-carboxyglutamic acid-containing proteins. Application of the method for staining a Western blot of a crude extract of bone resulted in staining of only the gamma-carboxyglutamic acid-containing proteins. The usefulness of the method was verified when a second gamma-carboxyglutamic acid-containing protein, prothrombin, also resulted in red color production. A linear color response is seen up to 17 microM for the gamma-carboxyglutamic acid-containing protein bone Gla protein and up to 27 microM for the amino acid. The detection limit is down to 1 microgram of bone Gla protein or 0.17 nmol of the protein on electroblots or dot blots. The simplicity of the method allows rapid screening for gamma-carboxyglutamic acid-containing proteins or allows monitoring of purifications of these proteins in chromatographic or electrophoretic separations.

The vitamin K-dependent bone protein is accumulated within cultured osteosarcoma cells in the presence of the vitamin K antagonist warfarin.

Osteosarcoma cells grown in normal culture medium secrete bone gamma-carboxyglutamic acid protein (BGP, osteocalcin) which is identical with BGP purified from the bone matrix. Two tests indicate that the secreted medium protein contains the full complement of three gamma-carboxyglutamate residues present on BGP purified from the bone matrix. First, the secreted protein from ROS 17/2 and bone matrix BGP have identical isoelectric points (pI = 4.0). Second, they have identical hydroxyapatite binding behavior. If warfarin is added to the culture medium, the secreted protein has a higher isoelectric point (pI = 4.6) and a lower affinity for hydroxyapatite characteristic of thermally decarboxylated or non-gamma-carboxylated BGP. The observed shift in isoelectric point of secreted BGP after warfarin treatment from pH 4.0 to 4.6 is also reflected in the presence of pI = 4.1 and pI = 4.6 species intracellularly. These isoelectric species correspond to fully carboxylated BGP and noncarboxylated BGP, which are in the process of secretion. Addition of 10 micrograms/ml of warfarin causes a specific 47% reduction in secretion rate of BGP, while at the same time, the intracellular BGP concentration increases 3-fold. These phenomena appear related to the interruption by warfarin of the normal sequence of processing of precursor BGP proteins, as a new, immunoreactive species with a higher isoelectric pH not present in control cells appears to be responsible for the increased intracellular antigen within warfarin-treated cells. Our results show that vitamin K-dependent processing is important for normal secretion of BGP from the cell.

Radioimmunoassay for the vitamin K-dependent protein of bone and its discovery in plasma.

The vitamin K-dependent protein of bone has been detected in human plasma by radioimmunoassay at 4.5 ng per ml. The plasma protein has the same apparent molecular weight as the pure bone Gla protein (BGP) and other studies indicate the plasma protein is probably the intact bone protein. BGP also has been detected in bovine serum by radioimmunoassay. The bovine serum levels of BGP decrease with developmental age from 200 ng per ml in fetal calves to 26 ng per ml in adult cows. The implications of the discovery of BGP in plasma to the function of this unique protein are discussed. This assay employs rabbit antibody directed against calf BGP and has a sensitivity of 0.1 ng. The antibody crossreacts with purified human BGP but not with BGP from rat or rabbit bone. Studies with peptides of known structure derived from enzymatic digests of BGP indicate that the rabbit antibody recognizes the COOH-terminal region of the 49-residue calf bone protein.

Secretion of the vitamin K-dependent protein of bone by rat osteosarcoma cells. Evidence for an intracellular precursor.

Four clonal cell lines derived from a rat osteosarcoma were tested for the ability to secrete the gamma-carboxyglutamic acid-containing protein of bone (BGP) using a specific radioimmunoassay for this protein. Two cell lines secreted BGP into culture media while the other two did not. Other investigators have shown that these two cell lines are also the only ones with the high parathyroid hormone responsiveness and alkaline phosphatase activity expected for osteoblast cells in culture. Both cell lines also form a mineralized sarcoma when implanted in rats. The BGP in culture media is identical in molecular weight and in electrophoretic mobility with the 5800-dalton BGP purified from rat bone. Thus, BGP is probably secreted by osteosarcoma cells directly and not derived from an extracellular precursor by proteolytic cleavage. There are two immunoreactive components within osteosarcoma cells which secrete BGP. One component is identical in molecular weight and electrophoretic mobility with BGP from rat bone. The other component has a higher molecular mass (approximately 9000 daltons) and about half the electrophoretic mobility of BGP from bone. The presence of both components within these cells raises the possibility that the larger component may be an intracellular precursor which is processed to BGP prior to secretion.

Effect of aging and dietary restriction on matrix Gla protein and other components of rat tracheal cartilage.

Age-related changes in connective tissues can alter their functions of elasticity, compressibility and support. Matrix Gla protein (MGP) is a vitamin K-dependent connective tissue component of unknown function. We have purified bovine MGP, and developed a specific radioimmunoassay for it. Since it is found in highest concentration in cartilage, we have developed quantitative extraction methods for MGP, and examined the age-related changes of MGP relative to other components found in the cartilage matrix. The ratio of hydroxyproline to MGP increases with age, while the ratio of glycosaminoglycan to MGP is constant. No effect is seen for MGP in the dietary restricted rat with prolonged lifespan, while both hydroxyproline and glycosaminoglycan contents of tracheal cartilage are significantly increased by dietary restriction (p < or = .05). These data show that MGP and glycosaminoglycan concentration are relatively constant in rats from 6 to 30 months of age, while hydroxyproline concentration increases with age.

Proteoglycan core protein is accumulated in cultured chondrocytes in the presence of the ionophore monensin.

The nature of the proteoglycan antigen which accumulates in chick embryo chondrocytes that had been incubated in monensin was examined. Both the culture media and cell lysates were immunoprecipitated using antibody which primarily is directed against core protein. Gel filtration and electrophoresis of the immunoprecipitates showed that molecules corresponding to completed proteoglycan, core protein, and link protein were present in the immunoprecipitates. Monensin caused the cellular accumulation of core protein which was both underglycosylated and undersulfated. These results suggest that monensin affects early events of proteoglycan biosynthesis and that it may be useful for elucidating those events.

Isolation and characterization of the reaction product of 4-diazobenzenesulfonic acid and gamma-carboxyglutamic acid: modification of the assay for measurement of beta-carboxyaspartic acid.

gamma-Carboxyglutamic acid (Gla)-containing proteins are extracellular proteins with enhanced cation or mineral-binding properties. Discovery of new Gla-containing proteins is facilitated by methods that decrease the number of steps and time involved in assaying for Gla. Reaction of 4-diazobenzenesulfonic acid (DBS) and Gla or Gla-containing proteins produces an intensely red-colored product. This method has been used to identify Gla-containing proteins in crude extracts of proteins. The reaction product of Gla and DBS has been purified by reversed-phase HPLC and characterized by uv-visible spectroscopy and electrospray ionization (ESI) mass spectrometry (MS). The red-colored product exhibits an absorption maximum at 530 nm. The ESI-MS data of the colored derivative of Gla are consistent with replacement of the two gamma-carboxyl groups in Gla with two DBS groups. DBS also reacts with another malonic acid derivative, beta-carboxyaspartic acid (Asa). The optimum conditions for colorimetric assay of Asa were established, and the potential of this reaction as an assay for Asa and Asa-containing proteins was studied.

Calcification of osteoblastlike rat osteosarcoma cells in agarose suspension cultures.

Ros 17/2 clonal rat osteosarcoma cells calcify when cultured in the presence of 10 micrograms/ml beta-glycerol phosphate in an agarose gel. Culture in 1% agarose inhibited cell division while allowing cells to remain metabolically active and viable for over 21 days. Serial photography of the same microscopic field shows a progressive deposition of calcium phosphate during the course of the experiment. The deposition of calcium around cells was confirmed by calcium-specific stains, and by energy dispersive X-ray analysis (EDX) during scanning electron microscopy. Cells with high calcium content analyzed by EDX had Ca:P ratios similar to hydroxyapatite. Total calcium progressively increased in beta-glycerol phosphate-treated cultures whereas the control plates maintained a constant calcium content over 16 days. Alkaline phosphatase activity increased with time in culture whereas cells with beta-glycerol phosphate maintained the alkaline phosphatase values achieved at the time of initial calcification. Alkaline phosphatase staining revealed no correlation between the presence of the enzyme activity and calcification. Radioimmunoassay for the bone-specific vitamin K-dependent protein bone Gla protein showed that beta-glycerol phosphate-treated cells accumulate over sixfold greater amounts of this protein. Our studies show that ROS cells can calcify and accumulate bone-specific matrix components when cultured in a 3-dimensional agarose matrix.

The effect of food restriction and germ-free environment on age-related changes in bone matrix.

Studies were performed to determine the effects of food restriction or a germ-free environment on age-related changes in bone. Four groups of male Lobund-Wistar rats were examined at 6 months, 18 months, and 30 months of age. Conventional-free-fed rats were housed in routine laboratory cages and fed ad libitum. Conventional-restricted diet rats were fed 12 grams a day, which becomes restrictive at 8 weeks of age. Germ-free rats were maintained using gnotobiotic procedures and were free of pathogens. The germ-free rats were maintained on either the full-fed or restricted diet regimens. Serum bone Gla protein and matrix concentrations of calcium, magnesium, and hydroxyproline (reflecting collagen content) and bone Gla protein were not significantly different between the four treatment groups. All of these parameters except hydroxyproline, however, showed age-related declines in all four treatment groups. We conclude that prolongation of rat life span by dietary restriction and/or a germ-free environment did not alter the measured parameters in each age group, and did not alter or slow the age-related changes in the bone matrix.

Cells isolated from fetal rat calvaria by isopycnic separation express different collagen phenotypes.

Calvaria from 20-21 day old fetuses were obtained under sterile conditions and the endo- and exoperiosteum stripped off. Cells were dispersed by sequential collagenase-DNase treatment and suspended in 0.5% low Tm agarose in the presence of DMEM supplement with 10% FCS. After 4-5 days of incubation some 30% of these cells showed active synthesis of metachromatic extracellular matrix. Cells from skin, muscle and periosteum failed to show metachromatic matrix positive colonies to a comparable extent. The phenotypic expression of these cells was determined by analysis of collagen types. Eleven day old cultures were incubated in the presence of [3H]-proline plus beta-aminopropionitrile and ascorbic acid and the collagen extracted analyzed by polyacrylamide electrophoresis of their intact chains or CNBr-derived peptides. The results show that anchorage independence is a requirement for calvaria cells to express type II collagen. Type I collagen was preferentially expressed in monolayer culture or when pre-attached to a substrate before being cultured in agarose. Type II collagen was the predominant collagen when cells were cultured in agarose. Further characterization of cell populations was achieved by isopycnic centrifugation in a percoll gradient. Cell fractions were tested for their collagen phenotype when cultured in agarose. Cells recovered from densities 1.04 g/ml or higher synthesized type II collagen, while cells with densities lower than 1.04 g/ml synthesized mainly type I collagen. Isopycnic centrifugation appears to be a novel method for separation of phenotypically different cells from a heterogeneous population in fetal calvaria. The high density cell fractions may represent a mixture of pre-chondrocytes as well as pluripotential cells.

Discovery of bone gamma-carboxyglutamic acid protein in mineralized scales. The abundance and structure of Lepomis macrochirus bone gamma-carboxyglutamic acid protein.

The mineralized scale of the freshwater sunfish Lepomis macrochirus (bluegill) contains a Gla protein. The protein was identified in extracts of scale by a new colorimetric assay for Gla-containing proteins. The protein was purified by gel filtration chromatography followed by reversed phase high performance liquid chromatography (HPLC). Several tests establish the identity of scale Gla protein and bone Gla protein (BGP). First, the proteins exhibit identical mobilities on electrophoresis and by reversed phase HPLC. Second, they have identical amino-terminal amino acid sequences. Finally, identical peptides are generated by proteolytic digestion. The 45-residue amino acid sequence of the bone Gla protein from L. macrochirus has a high sequence homology with swordfish, as well as homology to mammalian bone Gla protein. The BGP of bluegill shares with swordfish BGP a truncated NH2 terminus and an extended COOH terminus. These features may be unique to fish, as they have not been observed in terrestrial vertebrates. The bluegill BGP is the first vitamin K-dependent protein to contain a non-gamma-carboxylated residue to the NH2-terminal side of all of its Gla residues. In all other vitamin K-dependent proteins, Gla always appears to the NH2-terminal side of the first Glu. The implications of this result are discussed. The bluegill rib bone is curiously enriched in BGP, as are other mineralized tissues of this species. One hypothesis is that this may be due to the acellular nature of the bone in this species. The abundance of BGP in the bones of this fish may provide clues to the unknown function of this bone protein.