Presence of lysinoalanine and histidinoalanine in bovine dentin phosphoprotein

Dehydroamino acid chemical biology: an example of functional group interconversion on proteins

In nature, dehydroalanine (Dha) and dehydrobutyrine (Dhb) residues are byproducts of protein aging, intermediates in the biosynthesis of lanthipeptides and products of bacterial phospholyases that inactivate host kinase immune responses. Recent chemical biology studies have demonstrated the possibility of mapping dehydroamino acids in complex proteomes in an unbiased manner that could further our understanding of the role of Dha and Dhb in biology and disease more broadly. From a synthetic perspective, chemical mutagenesis through site-selective formation of the unsaturated residue and subsequent addition chemistry has yielded homogeneous proteins bearing a variety of post-translational modifications (PTMs) which have assisted fundamental biological research. This Opinion discusses these recent advances and presents new opportunities for protein engineering and drug discovery.

The chemical biology of dehydroalanine and dehydrobutyrine in proteins is summarized and new concepts are presented.

Glutathionylation of lens proteins through the formation of thioether bond

Formation of lanthionine, a dehydroalanine crosslink, is associated with aging of the human lens and cataractogenesis. In this study we investigated whether modification of lens proteins by glutathione could proceed through an alternative pathway: that is, by the formation of a nonreducible thioether bond between protein and glutathione. Direct ELISA of the reduced water-soluble and water-insoluble lens proteins from human cataractous, aged and bovine lenses showed a concentration-dependent immunoreactivity toward human nonreducible glutathionyl-lens proteins only. The reduced water-insoluble cataractous lens proteins showed the highest immunoreactivity, while bovine lens protein exhibited no reaction. These data were confirmed by dot-blot analysis. The level of this modification ranged from 0.7 to 1.6 nmol/mg protein in water-insoluble proteins from aged and cataractous lenses. N-terminal amino acid determination in the reduced and alkylated lens proteins, performed by derivatization of these preparations with dansyl chloride followed by an exhaustive dialysis, acid hydrolysis and fluorescence detection of dansylated amino acids by RP-HPLC, showed that N-terminal glutamic acid was present in concentration of approximately 0.2 nmol/mg of lens protein. This evidence points out that at least some of the N-terminal amino groups of nonreducible glutathione in the reduced human lens proteins are not involved in a covalent bond formation. Since disulfides were not detected in the reduced and alkylated human lens proteins, GSH is most likely attached to lens proteins through thioether bonds. These results provide, for the first time, evidence that glutathiolation of human lens proteins can occur through the formation of nonreducible thioether bonds.

Intact growth factors are conserved in the extracellular matrix of ancient human bone and teeth: a storehouse for the study of human evolution in health and disease

For the first time we have extracted, solubilized and identified growth factors, such as insulin growth factor II (IGF-II), bone morphogenetic protein-2 (BMP-2), and transforming growth factor-beta (TGF-beta), from archaeological compact human bone and tooth dentin dating from the late pre-ceramic pottery Neolithic (late PPNB) and the early Middle Ages. These factors are typical of special physiological or pathological situations in the metabolism of bone. The extracellular matrix proteins from bone and teeth of individuals from the late PPNB and early Middle Ages were separated by 2-D electrophoresis and more than 300 different protein spots were detected by silver staining. The matrix protein patterns of compact bone and tooth from the same individual (early Middle Ages) are very different and only 16% of the protein spots were detected in both compact bone and tooth dentin.

Dehydroalanine crosslinks in human lens

This study was conducted to develop a methodology for the purification and detection of histidinoalanine, lanthionine and lysinoalanine in the lens tissue. Cataractous and aged human lens proteins were hydrolysed and fractionated by using anion-exchange chromatography. The fraction containing the bulk of dehydroalanine crosslinks was derivatized with dansyl chloride and then separated and quantified by means of RP-HPLC. The spectral and chromatographic properties of all three substances purified and quantified in this study were identical to those of their synthesized counterparts. Histidinoalanine and lanthionine were the most abundant dehydroalanine crosslinks in both water-soluble and water-insoluble lens proteins. Histidinoalanine levels in water-soluble proteins from the cataractous lenses of Indian origin were 6.2-fold higher than those in water-soluble proteins from normal lenses (1.68+/-0.75 vs 0.26+/-0.06 nmol/mg protein; p<0.001). In water-insoluble proteins, they were 2.2-fold higher in cataractous lenses compared with normal lenses (1.59+/-0.76 vs 0.73+/-0.17 nmol/mg protein; p<0.01). Lanthionine levels were significantly higher in water-insoluble proteins of cataractous lenses when compared to non-cataractous lenses (2.5+/-1.68 vs 0.95+/-0.08 nmol/mg protein; p<0.03). Unlike histidinoalanine, this crosslink appears to accumulate in relatively high concentrations in water-soluble lens proteins; its concentration was 9-fold higher than histidinoalanine from the same proteins (0.26+/-0.06 HAL vs 2.34+/-0.76 LAN nmol/mg protein; p<0.0004). The concentration of lysinoalanine was in the picomolar range and in cataractous lens proteins only.

Two new in vitro calcification systems showing the higher calcifiability of enamel proteins than dentin and bone matrices

One of the difficulties in simulating in vivo calcification by in vitro experiments is how to prepare and apply a suitable calcifying solution. We have previously developed an entirely new model system consisting of 40% acrylamide gel blocks that contains matrix proteins and is immersed in fetal calf serum at 37 degrees C. (40% gel system) for 18 hr (Connect. Tissue Res., 33, 185, 1995). The gels were analyzed for immobilized calcium. In this system bovine enamel proteins (0.1% in the gel) showed the highest calcifiability among the tested matrices, followed by insoluble bovine dentin, bone and skin collagens. The 40% gel system provides a barrier for high molecular weight inhibitor molecules in the body fluid. The new calcifying system developed in this study consists of the matrix protein sealed in dialysis tubing within a glass chromatography column that was eluted with a calcifying solution. In this system (dialysis tubing system), again the enamel protein showed higher calcifiability than dentin, bone and skin collagens. It was also shown that enamel proteins became not only a reversible opaque gel, but also a relatively-irreversible coagulant, if the solution contained calcium and phosphate ions at concentration below saturation (1 mM calcium and 1 mM phosphate). With both systems combined, deposition and crystal growth of minerals in enamel proteins will be better understood than with previous methods.

Chemistry, biochemistry, nutrition, and microbiology of lysinoalanine, lanthionine, and histidinoalanine in food and other proteins

Heat and alkali treatments of foods, widely used in food processing, result in the formation of dehydro and cross-linked amino acids such as dehydroalanine, methyldehydroalanine, beta-aminoalanine, lysinoalanine (LAL), ornithinoalanine, histidinoalanine (HAL), phenylethylaminoalanine, lanthionine (LAN), and methyl-lanthionine present in proteins and are frequently accompanied by concurrent racemization of L-amino acid isomers to D-analogues. The mechanism of LAL formation is a two-step process: first, hydroxide ion-catalyzed elimination of H(2)S from cystine and H(2)O, phosphate, and glycosidic moieties from serine residues to yield a dehydroalanine intermediate; second, reaction of the double bond of dehydroalanine with the epsilon-NH(2) group of lysine to form LAL. Analogous elimination-addition reactions are postulated to produce the other unusual amino acids. Processing conditions that favor these transformations include high pH, temperature, and exposure time. Factors that minimize LAL formation include the presence of SH-containing amino acids, sodium sulfite, ammonia, biogenic amines, ascorbic acid, citric acid, malic acid, and glucose; dephosphorylation of O-phosphoryl esters; and acylation of epsilon-NH(2) groups of lysine. The presence of LAL residues along a protein chain decreases digestibility and nutritional quality in rodents and primates but enhances nutritional quality in ruminants. LAL has a strong affinity for copper and other metal ions and is reported to induce enlargement of nuclei of rats and mice but not of primate kidney cells. LAL, LAN, and HAL also occur naturally in certain peptide and protein antibiotics (cinnamycin, duramycin, epidermin, nisin, and subtilin) and in body organs and tissues (aorta, bone, collagen, dentin, and eye cataracts), where their formation may be a function of the aging process. These findings are not only of theoretical interest but also have practical implications for nutrition, food safety, and health. Further research needs are suggested for each of these categories. These overlapping aspects are discussed in terms of general concepts for a better understanding of the impact of LAL and related compounds in the diet. Such an understanding can lead to improvement in food quality and safety, nutrition, microbiology, and human health.

Tissue reactions to various percutaneous materials with different surface properties and structures

Tissue responses to various percutaneous materials with different surface properties and structures were investigated. Dense hydroxyapatite (HA), tricalcium phosphate (TCP), glassy carbon (GC), and 2 types of porous HA were used. Cutaneous and subcutaneous tissues were tightly attached to the surface of the penetrating portion of an HA percutaneous device (PD). Neither bacterial infection nor serious epidermal downgrowth were observed in the area surrounding the shaft of the HA-PD. At the margin of the epidermis, fibroblasts and collagen fibrils of fibrous connective tissue were well oriented and formed perpendicular to the shaft. The tissue response to the TCP was mild and nearly the same as that to the HA. The GC induced serious epidermal downgrowth and inflammatory cell infiltration. In contrast to dense HA-PD, the insertion of both types of porous HA-PDs, 1 with a spongy structure and 1 with close pores was followed by acute infection within 1 month. Based upon these results, it was concluded that the dense HA was the best percutaneous material of those tested.

Extracellular processing of dentin matrix protein in the mineralizing odontoblast culture

Odontoblasts that we prepared from bovine incisors produced a dentin-specific protein, phosphophoryn, and accumulated it in mineralized nodules. The time course of mineralization was detected by measuring osteocalcin and mineral in the nodules. The sequence of developmental expression of proteins in this mineralizing dentin cell culture is very similar to that in bone cells, suggesting a common mechanism for matrix mineralization in bone and dentin. Casein kinase II, which phosphorylates bone phosphoproteins and dentin phosphorylates bone phosphoproteins and dentin phosphophoryn, also emerges coinciding with the initiation of mineralization. Furthermore, we have detected extracellular phosphorylation by casein kinase II of a dentin protein of M(r) 60,000, which we recovered from the phosphophoryn fraction in CaCl2 precipitate.

Conformational changes of bovine bone osteonectin induced by interaction with calcium

To clarify calcium-induced conformational changes in bovine bone osteonectin, the protein was labeled with fluorescein isothiocyanate (FITC) in the presence and absence of calcium. By calcium titration using fluorescence spectrometry, it was demonstrated that FITC-osteonectin labeled in the presence of 2 mM CaCl2 showed a much higher affinity for calcium ions than did that labeled in the absence of calcium ions. The midpoint for completion of the increase in the intrinsic fluorescence (K0.5) of the two were 1 x 10(-7) M and 5 x 10(-7) M, respectively. By tryptic digestion and isolation of the fluorescent peptide of both FITC-osteonectins, the site of FITC-labeling was determined to be Lys174. Furthermore, it was found that the efficacy of labeling in this specific binding site was three times higher in the FITC-osteonectin labeled in the presence of 2 mM CaCl2 than in that labeled in the absence of calcium. The results indicate that in the presence of 2 mM CaCl2 the microenvironment around Lys174 of osteonectin was more open to modification than in the absence of calcium.

Matrix mineralization and the differentiation of osteocyte-like cells in culture

Osteocyte-like cells were prepared by sequentially treating calvaria from newborn rats with collagenase and chelating agents. On a reconstituted gel of basement membrane components, cells from the third collagenase digest displayed a round shape and expressed the highest level of alkaline phosphatase with minimal osteocalcin deposition into the matrix. On the other hand, cells derived from the interior after EDTA treatment exhibited well-developed dendritic cell processes and expressed essentially no alkaline phosphatase. The latter population also showed quite distinct characteristics such as higher extracellular activities of casein kinase II and ecto-5'-nucleotidase and the extracellular accumulation of a large amount of osteocalcin associated with mineral. These diverse phenotypic and protein expressions as well as the sites from which each population of cells were recovered strongly suggest that we have isolated osteoblastic and osteocytic cells. Bone sialoprotein II was extracellularly phosphorylated by casein kinase II in osteocytic cells but not in osteoblastic cells. We discuss the possibility that differentiation of young osteocytes from osteoblasts may facilitate the biochemical sequence of mineral deposition in the bone matrix.

A new method for in vitro calcification using acrylamide gel and bovine serum

To investigate the mechanism of biological calcification in vitro, a model system consisting of an acrylamide gel block (1 x 3 x 3 mm) and fetal bovine serum was developed. Mineral deposition was induced in gel blocks which were immersed in 300 microliters of fetal bovine serum at 37 degrees C for 7 days in a CO2 incubator. X-ray diffraction indicated that the mineral was hydroxyapatite with low crystallinity. Effects of the concentration of acrylamide gel, the partial pressure of CO2 and matrix proteins within the gel on the mineral formation were investigated. In the gel concentration range of 10-60%, the largest amount of crystal grew in 40% acrylamide gel, where the serum protein did not penetrate. With an increase in the partial pressure of CO2 the Ca content in the gel block increased, reached the highest level at about 3.5% CO2 and then began to decrease. In 40% gel and at 5% CO2, the mineral formation was enhanced by phosvitin, phosphophoryn, demineralized dentin powder and alkaline phosphatase. Mineral deposition occurred around the collagen fibers immobilized in 40% acrylamide gel. These results indicate that 1) a putatively serum-derived inhibitor of calcification with high-molecular weight was prevented from penetrating into the 40% acrylamide gels, 2) immobilized polyanionic proteins and alkaline phosphatase were able to increase mineral deposition and 3) the partial pressure of CO2 greatly influenced the mineral deposition. It was concluded that this gel system is useful to investigate the mechanism of biological calcification in vitro.

In vitro and in vivo association of dentin phosphophoryn with alpha1CB6 peptide of type I collagen

A small number of molecules of phosphophoryns, dentin phosphoproteins, are associated to collagen tightly with the maturation of dentin. As an in vitro model of the process, we examined non-covalent association of phosphophoryns and collagen CNBr peptides. Cyanogen bromide peptides of type I collagen were separated with electrophoresis and transferred to a membrane, which was probed with labeled phosphophoryns. Phosphophoryns were bound preferentially to alpha1CB6 peptide. Another experiment using a cleavable crosslinking agent also demonstrated the affinity between phosphophoryns and the alpha1CB6 region of collagen. The matrix-bound fraction of phosphophoryns was solubilized by CNBr digestion of bovine dentin, and was partially purified. Compositional analysis revealed that the fraction was composed of association products of phosphophoryns and collagen at the ratio of 2:3. Considering the high molecular weight of the fraction, one phosphophoryn molecule should be associated with several collagen peptides. The fraction reacted with the antibody against alphalCB6 peptide. When the fraction was digested with lysyl endopeptidase, several peptides that coincided with peptides from alpha1CB6 were released. One of the peptides was sequenced and identified to be a peptide with Asp 975 of the alpha1(I) chain as an amino terminal residue. The alpha1CB6 peptide may be involved in the matrix-bound fraction, although involvement of other collagen peptides cannot be ruled out. Phosphophoryns may associate with collagen at the alpha1CB6 region at first, and then be immobilized on it, possibly by covalent crosslinking, with maturation of dentin. Binding of several alpha1CB6 peptides to a single phosphophoryn molecule is possible in current 3-dimensional models of collagen packing in mineralized tissue, which allows alignment of several hole zones in register.

Amino-terminal location of pyridinoline in dentin collagen

Cross-linking is believed to be one of the major factors that characterize the calcifiability of dentin and bone collagens. Dehydro-dihydroxylysinonorleucine and pyridinoline which constitute the principal cross-links of dentin collagen have so far been located only in the carboxy terminal telopeptide of the molecules [alpha 1(I)-chain 87 x alpha 1(I)-chain 16C]. This situation suggested that the amino terminal telopeptide portion might be "open" without intermolecular cross-linking in hard tissue collagen fibrils. However, the present study provided evidence that pyridinoline is also located in amino-terminal telopeptides (alpha 1-chain 9N or alpha 2-chain 5N) and alpha 1-chain 930. Bovine dentin collagen was digested with trypsin followed by heating at 60 degrees C before and after the digestion. This method gave complete trypsin peptides of dentin collagen. Fluorescent pyridinoline peptides with a smaller molecular size were isolated by Sephadex G-50 superfine, DEAE-cellulose and reverse-phase HPLC. Automatic Edman analysis of several isolated peptides revealed the five-residue sequence, Gly-Ile-X-Gly-His-Arg, the only assignment of which was alpha 1-chain 928-933. The above evidence together with the amino acid compositions of the peptides led to the conclusion that pyridinoline is located not only in the carboxy-terminal but also in the amino-terminal telopeptide in dentin collagen.

Separation of bone matrix proteins by calcium-induced precipitation

It was found that significant precipitation occurred immediately after calcium, at a concentration as low as 2 mM, was added to a desalted solution of EDTA extract of adult bovine femur. The maximal yield of the precipitates was observed at a calcium concentration of 30 mM. These precipitates were dissolved in 0.5 M EDTA, desalted, and characterized by Sepharose CL-6B gel filtration chromatography and high performance gel-exclusion chromatography. Results revealed that the precipitates were enriched in a 40 K protein and a higher molecular weight fraction as compared with the original extract of bone proteins. The 40 K fraction was isolated and identified as osteonectin, as judged from amino acid analysis, electrophoresis, and immunodetection. The supernatant after calcium-induced precipitation predominantly contained osteocalcin and a 50 K protein that was tentatively identified as alpha 2HS protein. Osteonectin was purified from the calcium-induced precipitates from the EDTA extract of bovine bone. By calcium titration using fluorescence spectrometry, the isolated osteonectin showed high affinity to calcium ions with an apparent dissociation constant (K0.5) of 8 x 10(-7) M. Thus, the use of calcium to separate bone proteins, especially osteonectin, was proved to be a useful technique. In addition, calcium-induced precipitation of osteonectin suggested a possible in vivo mechanism via which osteonectin might interact with calcium ions and participate in the initial immobilization of calcium to induce the nucleation of calcification in bone tissue.

Changes in levels of osteonectin in bovine dentine during tooth development

Bovine incisors were classified into three developmental stages and non-collagenous proteins extracted from them. Sodium dodecyl sulphate gel electrophoresis of the extracts showed a reduction in osteonectin with the various stages. The reduction was confirmed by enzyme immunoassay using antiserum against bone osteonectin. This change is in contrast to dentine phosphoprotein, indicating functional differences between these two proteins.

Abnormal amino acid analyses obtained from osteogenesis imperfecta dentin

Amino acid analyses were carried out on dentin proteins obtained from 33 normal teeth and 59 teeth from osteogenesis imperfecta patients. The analyses revealed that in the control teeth approximately 95% of the insoluble dentin fraction was collagen. The majority of the analyses for teeth obtained from the O.I. patients were biochemically abnormal (55 out of 59). Specifically, there was a significant increase in the acidic amino acids, with a corresponding decrease in the basic amino acids. A small group of patients showed double peaks in the histidine, hydroxylysine, and lysine areas of the chromatograms. These results emphasize that despite the fact that the teeth of O.I. patients may appear to be clinically normal, all but four of the teeth examined had abnormal dentin collagen.

Increase of dentin phosphophoryn with dentin formation

Dentin phosphophoryn was quantified on bovine and rabbit dentin at three developmental stages. Phosphophoryn was extracted from teeth with 0.6M HCl, and quantified as optical density on DEAE-cellulose chromatogram or as phosphoserine content. Bovine phosphophoryn showed progressive increase with formation of dentin. Matrix-associated phosphophoryn was also quantified as phosphoserine content in insoluble dentin residue which was extracted with 6 M urea after decalcification. This fraction increased with formation of dentin both in bovine and rabbit dentin. Phosphophoryn is thought to be related to the later stage of dentin formation.

High production of type VI collagen in multiple fibromatosis with multiple articular dysplasia

A patient with multiple fibromatosis occurring at the sites of multiple cartilagenous dysplasia was described. Collagen types solubilized with pepsin from the fibromatous tissue were fractionated by a different salt concentration and analyzed by SDS-polyacrylamide gel electrophoresis, which indicated that the tissue produces predominantly "short-chain" collagen. Western blotting of the subunits indicated a cross reaction with antisera of the type VI collagen. The results of rotatory shadowing electron microscopy confirmed the characteristic short-chain structure.

Histidinoalanine, a naturally occurring cross-link derived from phosphoserine and histidine residues in mineral-binding phosphoproteins

Native mineral-containing phosphoprotein particles were isolated from the Heterodont bivalve Macrocallista nimbosa. The native particles are discrete structures about 40 nm in diameter which migrate as a single band during electrophoresis in agarose gels. Removal of the mineral component with ethylenediaminetetraacetic acid dissociates the native protein into nonidentical subunits. The lower molecular weight subunits, representing 8% of the total protein, were obtained by differential centrifugation. The native protein is characterized by a high content of aspartic acid, phosphoserine, phosphothreonine, histidine, and the bifunctional cross-linking residue histidinoalanine. The low molecular weight subunits have the same amino acid composition except for a reduction in histidinoalanine and a corresponding increase in phosphoserine and histidine residues, demonstrating that the alanine portion of the cross-link is derived from phosphoserine residues. Ion-exchange chromatography and molecular sieve chromatography show that the low molecular weight subunits have a similar charge density but differ in molecular weight, and the relative mobilities of the subunits on agarose gels indicate that they are polymers of a single phosphoprotein molecule. The minimum molecular weight of the monomer is about 140 000 on the basis of the amino acid composition. The high molecular weight subunits are rich in histidinoalanine and too large to be resolved by either molecular sieve chromatography or gel electrophoresis. On the basis of the ultrastructural, electrophoretic, chromatographic, and compositional evidence, native phosphoprotein particles are composed of subunits ionically cross-linked via divalent cations. These subunits are variable molecular weight aggregates of a single phosphoprotein molecule covalently cross-linked via histidinoalanine residues. Evidence for a nonenzymatic cross-linking mechanism is discussed.

In vivo cleavage of dentin phosphophoryn following beta elimination of its phosphoserine residues

Phosphophoryn, a unique phosphoprotein in dentin, has a polydisperse character. Keto acid was found to be present in this protein by condensation reaction with dinitrophenyl (DNP)-hydrazine and was identified to be pyruvate from the result that [3H]lactate was formed by the reduction of phosphophoryn with [3H]NaBH4. In the case of phosvitin, incorporation of DNP-hydrazine was much less than in phosphophoryn, even after the same treatment as the extraction procedure of phosphophoryn. DNP-hydrazine incorporation was much higher in the phosphophoryn of mature dentin than in dentin of unerupted teeth. From these results, pyruvate was though to be formed time dependently in vivo by beta elimination of phosphoserine residues and subsequent cleavage of the peptide chain, which may contribute to the polydispersity of phosphophoryn.

In vivo decomposition of phosphoserine and serine in noncollagenous protein from human dentin

HCl-soluble proteins in human dentin ranging in age from 3 to 45 years exhibit amino acid compositional changes consistent with beta-elimination and hydrolysis of phosphoserine as well as dehydration and aldol cleavage of serine. This is the first evidence of nonenzymatic mechanisms for in vivo degradation of hydroxy and substituted hydroxy amino acids in dentin. Decomposition of phosphoseryl residues reduces the calcium-binding capacity of phosphoproteins. Elimination and dehydration reactions can produce variability in molecular weight. The rates of decomposition may be rapid enough to cause the heterogeneity or "maturational" degradation seen in dentin phosphoproteins during mineralization.