Prolyl 3-hydroxylase: partial characterization of the enzyme from rat kidney cortex

Post-translational modification of type IV collagen with 3-hydroxyproline affects its interactions with glycoprotein VI and nidogens 1 and 2

Type IV collagen is a major component of the basement membrane and interacts with numerous other basement membrane proteins. Many of these interactions are poorly characterized. Type IV collagen is abundantly post-translationally modified with 3-hydroxyproline (3-Hyp), but 3-Hyp's biochemical role in type IV collagen's interactions with other proteins is not well established. In this work, we present binding data consistent with a major role of 3-Hyp in interactions of collagen IV with glycoprotein VI and nidogens 1 and 2. The increased binding interaction between type IV collagen without 3-Hyp and glycoprotein VI has been the subject of some controversy, which we sought to explore, whereas the lack of binding of nidogens to type IV collagen without 3-Hyp is novel. Using tandem MS, we show that the putative glycoprotein VI-binding site is 3-Hyp-modified in WT PFHR-9 type IV collagen, but not in PFHR-9 cells in which prolyl-3-hydroxylase 2 (P3H2) has been knocked out (KO). Moreover, we observed altered 3-Hyp occupancy across many other sites. Using amino acid analysis of type IV collagen from the WT and P3H2 KO cell lines, we confirm that P3H2 is the major, but not the only 3-Hyp-modifying enzyme of type IV collagen. These findings underscore the importance of post-translational modifications of type IV collagen for interactions with other proteins.

Molecular and cellular mechanisms of HIF prolyl hydroxylase inhibitors in clinical trials

Inhibition of the human 2-oxoglutarate (2OG) dependent hypoxia inducible factor (HIF) prolyl hydroxylases (human PHD1-3) causes upregulation of HIF, thus promoting erythropoiesis and is therefore of therapeutic interest. We describe cellular, biophysical, and biochemical studies comparing four PHD inhibitors currently in clinical trials for anaemia treatment, that describe their mechanisms of action, potency against isolated enzymes and in cells, and selectivities versus representatives of other human 2OG oxygenase subfamilies. The 'clinical' PHD inhibitors are potent inhibitors of PHD catalyzed hydroxylation of the HIF-α oxygen dependent degradation domains (ODDs), and selective against most, but not all, representatives of other human 2OG dependent dioxygenase subfamilies. Crystallographic and NMR studies provide insights into the different active site binding modes of the inhibitors. Cell-based results reveal the inhibitors have similar effects on the upregulation of HIF target genes, but differ in the kinetics of their effects and in extent of inhibition of hydroxylation of the N- and C-terminal ODDs; the latter differences correlate with the biophysical observations.

The supramolecular structure of bone: X-ray scattering analysis and lateral structure modeling

The evolution of vertebrates required a key development in supramolecular evolution: internally mineralized collagen fibrils. In bone, collagen molecules and mineral crystals form a nanocomposite material comparable to cast iron in tensile strength, but several times lighter and more flexible. Current understanding of the internal nanoscale structure of collagen fibrils, derived from studies of rat tail tendon (RTT), does not explain how nucleation and growth of mineral crystals can occur inside a collagen fibril. Experimental obstacles encountered in studying bone have prevented a solution to this problem for several decades. This report presents a lateral packing model for collagen molecules in bone fibrils, based on the unprecedented observation of multiple resolved equatorial reflections for bone tissue using synchrotron small-angle X-ray scattering (SAXS; ∼1 nm resolution). The deduced structure for pre-mineralized bone fibrils includes features that are not present in RTT: spatially discrete microfibrils. The data are consistent with bone microfibrils similar to pentagonal Smith microfibrils, but are not consistent with the (nondiscrete) quasi-hexagonal microfibrils reported for RTT. These results indicate that collagen fibrils in bone and tendon differ in their internal structure in a manner that allows bone fibrils, but not tendon fibrils, to internally mineralize. In addition, the unique pattern of collagen cross-link types and quantities in mineralized tissues can be can be accounted for, in structural/functional terms, based on a discrete microfibril model.

Protein Hydroxylation Catalyzed by 2-Oxoglutarate-dependent Oxygenases

The post-translational hydroxylation of prolyl and lysyl residues, as catalyzed by 2-oxoglutarate (2OG)-dependent oxygenases, was first identified in collagen biosynthesis. 2OG oxygenases also catalyze prolyl and asparaginyl hydroxylation of the hypoxia-inducible factors that play important roles in the adaptive response to hypoxia. Subsequently, they have been shown to catalyze N-demethylation (via hydroxylation) of N(ϵ)-methylated histone lysyl residues, as well as hydroxylation of multiple other residues. Recent work has identified roles for 2OG oxygenases in the modification of translation-associated proteins, which in some cases appears to be conserved from microorganisms through to humans. Here we give an overview of protein hydroxylation catalyzed by 2OG oxygenases, focusing on recent discoveries.

Posttranslational modifications in type I collagen from different tissues extracted from wild type and prolyl 3-hydroxylase 1 null mice

Type I collagen extracted from tendon, skin, and bone of wild type and prolyl 3-hydroxylase 1 (P3H1) null mice shows distinct patterns of 3-hydroxylation and glycosylation of hydroxylysine residues. The A1 site (Pro-986) in the α1-chain of type I collagen is almost completely 3-hydroxylated in every tissue of the wild type mice. In contrast, no 3-hydroxylation of this proline residue was found in P3H1 null mice. Partial 3-hydroxylation of the A3 site (Pro-707) was present in tendon and bone, but absent in skin in both α-chains of the wild type animals. Type I collagen extracted from bone of P3H1 null mice shows a large reduction in 3-hydroxylation of the A3 site in both α-chains, whereas type I collagen extracted from tendon of P3H1 null mice shows little difference as compared with wild type. These results demonstrate that the A1 site in type I collagen is exclusively 3-hydroxylated by P3H1, and presumably, this enzyme is required for the 3-hydroxylation of the A3 site of both α-chains in bone but not in tendon. The increase in glycosylation of hydroxylysine in P3H1 null mice in bone was found to be due to an increased occupancy of normally glycosylated sites. Despite the severe disorganization of collagen fibrils in adult tissues, the D-period of the fibrils is unchanged. Tendon fibrils of newborn P3H1 null mice are well organized with only a slight increase in diameter. The absence of 3-hydroxyproline and/or the increased glycosylation of hydroxylysine in type I collagen disturbs the lateral growth of the fibrils.

Comprehensive mass spectrometric mapping of the hydroxylated amino acid residues of the α1(V) collagen chain

BACKGROUND

α1(V) is an extensively modified collagen chain important in disease.

RESULTS

Comprehensive mapping of α1(V) post-translational modifications reveals unexpectedly large numbers of X-position hydroxyprolines in Gly-X-Y amino acid triplets.

CONCLUSION

The unexpected abundance of X-position hydroxyprolines suggests a mechanism for differential modification of collagen properties.

SIGNIFICANCE

Positions, numbers, and occupancy of modified sites can provide insights into α1(V) biological properties. Aberrant expression of the type V collagen α1(V) chain can underlie the connective tissue disorder classic Ehlers-Danlos syndrome, and autoimmune responses against the α1(V) chain are linked to lung transplant rejection and atherosclerosis. The α1(V) collagenous COL1 domain is thought to contain greater numbers of post-translational modifications (PTMs) than do similar domains of other fibrillar collagen chains, PTMs consisting of hydroxylated prolines and lysines, the latter of which can be glycosylated. These types of PTMs can contribute to epitopes that underlie immune responses against collagens, and the high level of PTMs may contribute to the unique biological properties of the α1(V) chain. Here we use high resolution mass spectrometry to map such PTMs in bovine placental α1(V) and human recombinant pro-α1(V) procollagen chains. Findings include the locations of those PTMs that vary and those PTMs that are invariant between these α1(V) chains from widely divergent sources. Notably, an unexpectedly large number of hydroxyproline residues were mapped to the X-positions of Gly-X-Y triplets, contrary to expectations based on previous amino acid analyses of hydrolyzed α1(V) chains from various tissues. We attribute this difference to the ability of tandem mass spectrometry coupled to nanoflow chromatographic separations to detect lower-level PTM combinations with superior sensitivity and specificity. The data are consistent with the presence of a relatively large number of 3-hydroxyproline sites with less than 100% occupancy, suggesting a previously unknown mechanism for the differential modification of α1(V) chain and type V collagen properties.

Distribution and prediction of catalytic domains in 2-oxoglutarate dependent dioxygenases

BACKGROUND

The 2-oxoglutarate dependent superfamily is a diverse group of non-haem dioxygenases, and is present in prokaryotes, eukaryotes, and archaea. The enzymes differ in substrate preference and reaction chemistry, a factor that precludes their classification by homology studies and electronic annotation schemes alone. In this work, I propose and explore the rationale of using substrates to classify structurally similar alpha-ketoglutarate dependent enzymes.

FINDINGS

Differential catalysis in phylogenetic clades of 2-OG dependent enzymes, is determined by the interactions of a subset of active-site amino acids. Identifying these with existing computational methods is challenging and not feasible for all proteins. A clustering protocol based on validated mechanisms of catalysis of known molecules, in tandem with group specific hidden markov model profiles is able to differentiate and sequester these enzymes. Access to this repository is by a web server that compares user defined unknown sequences to these pre-defined profiles and outputs a list of predicted catalytic domains. The server is free and is accessible at the following URL (http://comp-biol.theacms.in/H2OGpred.html).

CONCLUSIONS

The proposed stratification is a novel attempt at classifying and predicting 2-oxoglutarate dependent function. In addition, the server will provide researchers with a tool to compare their data to a comprehensive list of HMM profiles of catalytic domains. This work, will aid efforts by investigators to screen and characterize putative 2-OG dependent sequences. The profile database will be updated at regular intervals.

Null mutations in LEPRE1 and CRTAP cause severe recessive osteogenesis imperfecta

Classical osteogenesis imperfecta (OI) is a dominant genetic disorder of connective tissue caused by mutations in either of the two genes encoding type I collagen, COL1A1 and COL1A2. Recent investigations, however, have generated a new paradigm for OI incorporating many of the prototypical features that distinguish dominant and recessive conditions, within a type I collagen framework. We and others have shown that the long-sought cause of the recessive form of OI, first postulated in the Sillence classification, lies in defects in the genes encoding cartilage-associated protein (CRTAP) or prolyl 3-hydroxylase 1 (P3H1/LEPRE1). Together with cyclophilin B (PPIB), CRTAP and P3H1 comprise the collagen prolyl 3-hydroxylation complex, which catalyzes a specific posttranslational modification of types I, II, and V collagen, and may act as a general chaperone. Patients with mutations in CRTAP or LEPRE1 have a lethal to severe osteochondrodystrophy that overlaps with Sillence types II and III OI but has distinctive features. Infants with recessive OI have white sclerae, undertubulation of the long bones, gracile ribs without beading, and a small to normal head circumference. Those who survive to childhood or the teen years have severe growth deficiency and extreme bone fragility. Most causative mutations result in null alleles, with the absence or severe reduction of gene transcripts and proteins. As expected, 3-hydroxylation of the Pro986 residue is absent or severly reduced, but bone severity and survival length do not correlate with the extent of residual hydroxylation. Surprisingly, the collagen produced by cells with an absence of Pro986 hydroxylation has helical overmodification by lysyl hydroxylase and prolyl 4-hydroxylase, indicating that the folding of the collagen helix has been substantially delayed.

Biochemical characterization of the prolyl 3-hydroxylase 1.cartilage-associated protein.cyclophilin B complex

The rough endoplasmic reticulum-resident protein complex consisting of prolyl 3-hydroxylase 1 (P3H1), cartilage-associated protein (CRTAP), and cyclophilin B (CypB) can be isolated from chick embryos on a gelatin-Sepharose column, indicating some involvement in the biosynthesis of procollagens. Prolyl 3-hydroxylase 1 modifies a single proline residue in the alpha chains of type I, II, and III collagens to (3S)-hydroxyproline. The peptidyl-prolyl cis-trans isomerase activity of cyclophilin B was shown previously to catalyze the rate of triple helix formation. Here we show that cyclophilin B in the complex shows peptidyl-prolyl cis-trans isomerase activity and that the P3H1.CRTAP.CypB complex has another important function: it acts as a chaperone molecule when tested with two classical chaperone assays. The P3H1.CRTAP.CypB complex inhibited the thermal aggregation of citrate synthase and was active in the denatured rhodanese refolding and aggregation assay. The chaperone activity of the complex was higher than that of protein-disulfide isomerase, a well characterized chaperone. The P3H1.CRTAP.CypB complex also delayed the in vitro fibril formation of type I collagen, indicating that this complex is also able to interact with triple helical collagen and acts as a collagen chaperone.

The evolutionarily conserved leprecan gene: its regulation by Brachyury and its role in the developing Ciona notochord

In Ciona intestinalis, leprecan was identified as a target of the notochord-specific transcription factor Ciona Brachyury (Ci-Bra) (Takahashi, H., Hotta, K., Erives, A., Di Gregorio, A., Zeller, R.W., Levine, M., Satoh, N., 1999. Brachyury downstream notochord differentiation in the ascidian embryo. Genes Dev. 13, 1519-1523). By screening approximately 14 kb of the Ci-leprecan locus for cis-regulatory activity, we have identified a 581-bp minimal notochord-specific cis-regulatory module (CRM) whose activity depends upon T-box binding sites located at the 3'-end of its sequence. These sites are specifically bound in vitro by a GST-Ci-Bra fusion protein, and mutations that abolish binding in vitro result in loss or decrease of regulatory activity in vivo. Serial deletions of the 581-bp notochord CRM revealed that this sequence is also able to direct expression in muscle cells through the same T-box sites that are utilized by Ci-Bra in the notochord, which are also bound in vitro by the muscle-specific T-box activators Ci-Tbx6b and Ci-Tbx6c. Additionally, we created plasmids aimed to interfere with the function of Ci-leprecan and categorized the resulting phenotypes, which consist of variable dislocations of notochord cells along the anterior-posterior axis. Together, these observations provide mechanistic insights generally applicable to T-box transcription factors and their target sequences, as well as a first set of clues on the function of Leprecan in early chordate development.

Effect of the -Gly-3(S)-hydroxyprolyl-4(R)-hydroxyprolyl- tripeptide unit on the stability of collagen model peptides

In order to evaluate the role of 3(S)-hydroxyproline [3(S)-Hyp] in the triple-helical structure, we produced a series of model peptides with nine tripeptide units including 0-9 3(S)-hydroxyproline residues. The sequences are H-(Gly-Pro-4(R)Hyp)(l)-(Gly-3(S)Hyp-4(R)Hyp)(m)-(Gly-Pro-4(R)Hyp)(n)-OH, where (l, m, n) = (9, 0, 0), (4, 1, 4), (3, 2, 4), (3, 3, 3), (1, 7, 1) and (0, 9, 0). All peptides showed triple-helical CD spectra at room temperature and thermal transition curves. Sedimentation equilibrium analysis showed that peptide H-(Gly-3(S)Hyp-4(R)Hyp)(9)-OH is a trimer. Differential scanning calorimetry showed that replacement of Pro residues with 3(S)Hyp residues decreased the transition enthalpy, and the transition temperature increases by 4.5 degrees C from 52.0 degrees C for the peptide with no 3(S)Hyp residues to 56.5 degrees C for the peptide with nine 3(S)Hyp residues. The refolding kinetics of peptides H-(Gly-3(S)Hyp-4(R)Hyp)(9)-OH, H-(Gly-Pro-4(R)Hyp)(9)-OH and H-(Gly-4(R)Hyp-4(R)Hyp)(9)-OH were compared, and the apparent reaction orders of refolding at 10 degrees C were n = 1.5, 1.3 and 1.2, respectively. Replacement of Pro with 3(S)Hyp or 4(R)Hyp has little effect on the refolding kinetics. This result suggests that the refolding kinetics of collagen model peptides are influenced mainly by the residue in the Yaa position of the -Gly-Xaa-Yaa- repeated sequence. The experiments indicate that replacement of a Pro residue by a 3(S)Hyp residue in the Xaa position of the -Gly-Xaa-4(R)Hyp- repeat of collagen model peptides increases the stability, mainly due to entropic factors.

Characterization of recombinant human prolyl 3-hydroxylase isoenzyme 2, an enzyme modifying the basement membrane collagen IV

The single 3-hydroxyproline residue in the collagen I polypeptides is essential for proper fibril formation and bone development as its deficiency leads to recessive osteogenesis imperfecta. The vertebrate prolyl 3-hydroxylase (P3H) family consists of three members, P3H1 being responsible for the hydroxylation of collagen I. We expressed human P3H2 as an active recombinant protein in insect cells. Most of the recombinant polypeptide was insoluble, but small amounts were also present in the soluble fraction. P3H1 forms a complex with the cartilage-associated protein (CRTAP) that is required for prolyl 3-hydroxylation of fibrillar collagens. However, coexpression with CRTAP did not enhance the solubility or activity of the recombinant P3H2. A novel assay for P3H activity was developed based on that used for collagen prolyl 4-hydroxylases (C-P4H) and lysyl hydroxylases (LH). A large amount of P3H activity was found in the P3H2 samples with (Gly-Pro-4Hyp)5 as a substrate. The Km and Ki values of P3H2 for 2-oxoglutarate and its certain analogues resembled those of the LHs rather than the C-P4Hs. Unlike P3H1, P3H2 was strongly expressed in tissues rich in basement membranes, such as the kidney. P3H2 hydroxylated more effectively two synthetic peptides corresponding to sequences that are hydroxylated in collagen IV than a peptide corresponding to the 3-hydroxylation site in collagen I. These findings suggest that P3H2 is responsible for the hydroxylation of collagen IV, which has the highest 3-hydroxyproline content of all collagens. It is thus possible that P3H2 mutations may lead to a disease with changes in basement membranes.

Deficiency of cartilage-associated protein in recessive lethal osteogenesis imperfecta

Classic osteogenesis imperfecta, an autosomal dominant disorder associated with osteoporosis and bone fragility, is caused by mutations in the genes for type I collagen. A recessive form of the disorder has long been suspected. Since the loss of cartilage-associated protein (CRTAP), which is required for post-translational prolyl 3-hydroxylation of collagen, causes severe osteoporosis in mice, we investigated whether CRTAP deficiency is associated with recessive osteogenesis imperfecta. Three of 10 children with lethal or severe osteogenesis imperfecta, who did not have a primary collagen defect yet had excess post-translational modification of collagen, were found to have a recessive condition resulting in CRTAP deficiency, suggesting that prolyl 3-hydroxylation of type I collagen is important for bone formation.

CRTAP Is Required for Prolyl 3- Hydroxylation and Mutations Cause Recessive Osteogenesis Imperfecta

Prolyl hydroxylation is a critical posttranslational modification that affects structure, function, and turnover of target proteins. Prolyl 3-hydroxylation occurs at only one position in the triple-helical domain of fibrillar collagen chains, and its biological significance is unknown. CRTAP shares homology with a family of putative prolyl 3-hydroxylases (P3Hs), but it does not contain their common dioxygenase domain. Loss of Crtap in mice causes an osteochondrodysplasia characterized by severe osteoporosis and decreased osteoid production. CRTAP can form a complex with P3H1 and cyclophilin B (CYPB), and Crtap-/- bone and cartilage collagens show decreased prolyl 3-hydroxylation. Moreover, mutant collagen shows evidence of overmodification, and collagen fibrils in mutant skin have increased diameter consistent with altered fibrillogenesis. In humans, CRTAP mutations are associated with the clinical spectrum of recessive osteogenesis imperfecta, including the type II and VII forms. Hence, dysregulation of prolyl 3-hydroxylation is a mechanism for connective tissue disease.

Prolyl 3-Hydroxylase 1, Enzyme Characterization and Identification of a Novel Family of Enzymes

The collagen prolyl hydroxylases are enzymes that are required for proper collagen biosynthesis, folding, and assembly. They reside within the endoplasmic reticulum and belong to the group of 2-oxoglutarate and iron-dependent dioxygenases. Although prolyl 4-hydroxylase has been characterized as an alpha2beta2 tetramer in which protein disulfide isomerase is the beta subunit with two different alpha subunit isoforms, little is known about the enzyme prolyl 3-hydroxylase (P3H). It was initially characterized and shown to have an enzymatic activity distinct from that of prolyl 4-hydroxylase, but no amino acid sequences or genes were ever reported for the mammalian enzyme. Here we report the characterization of a novel prolyl 3-hydroxylase enzyme isolated from embryonic chicks. The primary structure of the enzyme, which we now call P3H1, demonstrates that P3H1 is a member of a family of prolyl 3-hydroxylases, which share the conserved residues present in the active site of prolyl 4-hydroxylase and lysyl hydroxylase. P3H1 is the chick homologue of mammalian leprecan or growth suppressor 1. Two other P3H family members are the genes previously called MLAT4 and GRCB. In this study we demonstrate prolyl 3-hydroxylase activity of the purified enzyme P3H1 on a full-length procollagen substrate. We also show it to specifically interact with denatured collagen and to exist in a tight complex with other endoplasmic reticulum-resident proteins. Immunohistochemistry with a monoclonal antibody specific for chick P3H1 localizes P3H1 specifically to tissues that express fibrillar collagens, suggesting that other P3H family members may be responsible for modifying basement membrane collagens.

The Peptides Acetyl-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 and Acetyl-(Gly-Pro-3(S)Hyp)10-NH2 Do Not Form a Collagen Triple Helix

Hydroxylation of proline residues in the Yaa position of the Gly-Xaa-Yaa repeated sequence to 4(R)-hydroxyproline is essential for the formation of the collagen triple helix. A small number of 3(S)-hydroxyproline residues are present in most collagens in the Xaa position. Neither the structural nor a biological role is known for 3(S)-hydroxyproline. To characterize the structural role of 3(S)-hydroxyproline, the peptide Ac-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 was synthesized and analyzed by circular dichroism spectroscopy, analytical ultracentrifugation, and 1H nuclear magnetic resonance spectroscopy. At 4 degrees C in water the circular dichroism spectrum indicates that this peptide was in a polyproline-II-like secondary structure with a positive peak at 225 nm similar to Ac-(Gly-Pro-4(R)Hyp)10-NH2. The positive peak at 225 nm almost linearly decreases with increasing temperature to 95 degrees C without an obvious transition. Although the peptide Ac-(Gly-Pro-4(R)Hyp)10-NH2 forms a trimer at 10 degrees C, sedimentation equilibrium experiments indicate that Ac-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 is a monomer in water at 7 degrees C. To study the role of 3(S)-hydroxyproline in the Yaa position, we synthesized Ac-(Gly-Pro-3(S)Hyp)10-NH2. This peptide also does not form a triple helix in water. 1H Nuclear magnetic resonance spectroscopy data (including line widths and nuclear Overhauser effects) are entirely consistent, with neither Ac-(Gly-3(S)Hyp-4(R)Hyp)10-NH2 nor Ac-(Gly-Pro-3(S)Hyp)10-NH2 forming a triple helix in water. Therefore 3(S)-hydroxyproline destabilizes the collagen triple helix in either position. In contrast, when 3(S)-hydroxyproline is inserted as a guest in the highly stable -Gly-Pro-4(R)Hyperepeated host sequence, Ac-(Gly-Pro-4(R)Hyp)3-Gly-3(S)Hyp-4(R)Hyp-(Gly-Pro-4(R)Hyp)4-Gly-Gly-NH2 forms as stable a trimer (Tm=49.6 degrees C) as Ac-(Gly-Pro-4(R)Hyp)8-Gly-Gly-NH2 (Tm=48.9 degrees C). Given that Ac-(Gly-Pro-4(R)Hyp)3-Gly-4(R)Hyp-Pro-(Gly-Pro-4(R)Hyp)4-Gly-Gly-NH2 forms a triple helix nearly as stable as the above two peptides (Tm=45.0 degrees C) and the knowledge that Ac-(Gly-4(R)Hyp-Pro)10-NH2 does not form a triple helix, we conclude that the host environment dominates the structure of host-guest peptides and that these peptides are not necessarily accurate predictors of triple helical stability.

Collagen hydroxylases and the protein disulfide isomerase subunit of prolyl 4-hydroxylases

Prolyl 4-hydroxylases catalyze the formation of 4-hydroxyproline in collagens and other proteins with an appropriate collagen-like stretch of amino acid residues. The enzyme requires Fe(II), 2-oxoglutarate, molecular oxygen, and ascorbate. This review concentrates on recent progress toward understanding the detailed mechanism of 4-hydroxylase action, including: (a) occurrence and function of the enzyme in animals; (b) general molecular properties; (c) intracellular sites of hydroxylation; (d) peptide substrates and mechanistic roles of the cosubstrates; (e) insights into the development of antifibrotic drugs; (f) studies of the enzyme's subunits and their catalytic function; and (g) mutations that lead to Ehlers-Danlos Syndrome. An account of the regulation of collagen hydroxylase activities is also provided.

Purification and cloning of a proline 3-hydroxylase, a novel enzyme which hydroxylates free L-proline to cis-3-hydroxy-L-proline

Proline 3-hydroxylase was purified from Streptomyces sp. strain TH1, and its structural gene was cloned. The purified enzyme hydroxylated free L-proline to cis-3-hydroxy-L-proline and showed properties of a 2-oxoglutarate-dependent dioxygenase (H. Mori, T. Shibasaki, Y. Uosaki, K. Ochiai, and A. Ozaki, Appl. Environ. Microbiol, 62:1903-1907, 1996). The molecular mass of the purified enzyme was 35 kDa as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The isoelectric point of the enzyme was 4.3. The optimal pH and temperature were 7.0 and 35 degrees C, respectively. The K(m) values were 0.56 and 0.11 mM for L-proline and 2-oxoglutarate, respectively. The Kcat value of hydroxylation was 3.2 s-1. Determined N-terminal and internal amino acid sequences of the purified protein were not found in the SwissProt protein database. A DNA fragment of 74 bp was amplified by PCR with degenerate primers based on the determined N-terminal amino acid sequence. With this fragment as a template, a digoxigenin-labeled N-terminal probe was synthesized by PCR. A 6.5-kbp chromosome fragment was cloned by colony hybridization with the labeled probe. The determined DNA sequence of the cloned fragment revealed a 870-bp open reading frame (ORF 3), encoding a protein of 290 amino acids with a calculated molecular weight of 33,158. No sequence homolog was found in EMBL, GenBank, and DDBJ databases. ORF 3 was expressed in Escherichia coli DH1. Recombinants showed hydroxylating activity five times higher than that of the original bacterium, Streptomyces sp. strain TH1. It was concluded that the ORF 3 encodes functional proline 3-hydroxylase.

2-oxoglutarate-dependent dioxygenase and related enzymes: biochemical characterization

Hydroxylation reactions are catalysed by a few major subclasses of enzymes which are ubiquitously distributed in nature. Dioxygenases generally occur as soluble enzymes where they catalyse a diversity of oxygenation reactions in a large number of metabolic pathways in animals, plants and micro-organisms. This review discusses recent advances in the biochemistry and molecular biology of dioxygenases occurring in different biological systems.

Ascorbic acid accumulation and transport in human fibroblasts

As the initial step in the use of fibroblasts as a model system for 'in situ kinetics', ascorbic acid (vitamin C) accumulation in normal human fibroblasts was investigated for the first time. Ascorbic acid was transported into fibroblasts and accumulated against a concentration gradient up to 20-fold, as measured by h.p.l.c. with coulometric electrochemical detection. Ascorbic acid accumulation was mediated by two concentration-dependent transport activities. The first was a high-affinity activity with an apparent Km of 6 microM and an apparent Vmax. of 203 microM/h, and the second was a low-affinity activity with an apparent Km of 5 mM and an apparent Vmax. of 1.8 mM/h. Both activities were inhibited by metabolic inhibitors and inhibitors of ascorbic acid transport in human neutrophils. The low-affinity transporter could not be accounted for by diffusion. Although the high-affinity transport activity was comparable with that described for human neutrophils, the low-affinity transporter was different. These data provide the first evidence that two-component ascorbic acid transport may be a generalized mechanism for accumulation of this vitamin in humans.

Amino-acid sequence and cell-adhesion activity of a fibril-forming collagen from the tube worm Riftia pachyptila living at deep sea hydrothermal vents

We have determined the amino acid sequence of the alpha chain of a fibril-forming collagen from the body wall of the marine invertebrate Riftia pachyptila (vestimentifera) by Edman degradation. The pepsin-solubilized collagen chain consists of a 1011-residue triple-helical domain and short remnants of N- and C-telopeptides. The triple-helical sequence showed one imperfection of the collagen Gly-Xaa-Yaa triplet repeat structure due to a Gly-->Ala substitution. This imperfection is correlated to a prominent kink in the molecule observed by electron microscopy. No strong sequence similarity was found with the fibril-forming vertebrate collagen types I-III, V and XI except for the invariant Gly residues. However, one of the two consensus cross-linking sequences was well conserved. The Riftia collagen shared with the vertebrate collagens many post-translational modifications. About 50% of the Pro and Lys residues are found in the Yaa position and were extensively hydroxylated to 4-hydroxyproline (4Hyp) and hydroxylysine (Hyl). A few proline residues in Xaa position were partially hydroxylated to either 4Hyp or 3Hyp. Despite the low sequence similarity, Riftia collagen was a potent adhesion substrate for two human cell lines. Cell adhesion could be inhibited by antibodies against the integrin beta 1 subunit but not by RGD peptides. This biological activity is apparently conserved in fibril-forming collagens of distantly related species but does not require the two RGD sequences present in Riftia collagen.

Type IV collagen synthesis by cultured mouse keratinocytes (Pam cells)

The collagens synthesized by Pam cells were biochemically characterized. The collagen in the cell layers was identified as type IV collagen by immunoprecipitation with type IV collagen specific antibody. The apparent molecular weights were 180 Kd and 175 Kd, which converted to major (160 Kd) and several minor smaller fragments with pepsin treatment. The ratios of 3-hydroxyproline/4-hydroxyproline and 4-hydroxyproline/proline in the 160 Kd polypeptide were 0.11 and 1.18, respectively. The result shows that Pam cells are able to produce type IV collagen whose biochemical characteristics are similar to those from other epithelial cells. In the kinetic study, the synthesis of type IV collagen reached its maximum level rapidly on the 1st day after seeding, suggesting that type IV collagen may play a role in cell attachment and proliferation.

Hydroxylation of aspartic acid in domains homologous to the epidermal growth factor precursor is catalyzed by a 2-oxoglutarate-dependent dioxygenase

3-Hydroxyaspartic acid and 3-hydroxyasparagine are two rare amino acids that are present in domains homologous to the epidermal growth factor precursor in vitamin K-dependent plasma proteins as well as in proteins that do not require vitamin K for normal biosynthesis. They are formed by posttranslational hydroxylation of aspartic acid and asparagine, respectively. The first epidermal growth factor-like domain in factor IX (residues 45-87) was synthesized with aspartic acid in position 64, replacing 3-hydroxyaspartic acid. It was used as substrate in a hydroxylase assay with rat liver microsomes as the source of enzyme and reaction conditions that satisfy the requirements of 2-oxoglutarate-dependent dioxygenases. The synthetic peptide stimulated the 2-oxoglutarate decarboxylation in contrast to synthetic, modified epidermal growth factor (Met-21 and His-22 deleted and Glu-24 replaced by Asp) and synthetic peptides corresponding to residues 60-71 in human factor IX. This indicates that the hydroxylase is a 2-oxoglutarate-dependent dioxygenase with a selective substrate requirement.

Abnormal collagen metabolism in cultured skin fibroblasts from patients with Duchenne muscular dystrophy

Total collagen synthesis is decreased by about 29% (P less than 0.01) in skin fibroblasts established in vitro from male patients with Duchenne muscular dystrophy (DMD) as compared with that in normal male skin fibroblasts in vitro. The reduction in collagen synthesis is associated with an approximately 2-fold increase in collagen degradation in DMD fibroblasts. Correlated to these alterations in the metabolism of collagen, DMD fibroblasts express a significantly higher hydroxyproline/proline ratio (DMD: 1.36-1.45; P less than 0.01) than do normal fibroblasts (controls: 0.86-0.89). The increased hydroxylation of proline residues of collagen (composed of type I and type III) could be the cause for the enhanced degradation of collagen in DMD fibroblasts.

Determination of radioactive proline and hydroxyproline by a simple paper-chromatographic procedure

A simple method to separate and determine radioactive proline and hydroxyproline by paper chromatography is described. The localization of the imino acids after separation is achieved by direct nondestructive staining with 1-fluoro-2,4-dinitrobenzene dye. The imino acids are quantitated directly by liquid scintillation counting in the presence of paper strips. The method was applied to bone cultures with good reproducibility, sensitivity and linearity over a wide range of radioactivity. The procedure was also tested in fibroblast cultures. The results for hydroxyproline were in good correlation with the widely used method of Juva and Prockop (Juva, K. and Prockop, D.J. (1966) Anal. Biochem. 15, 77-83), in which hydroxyproline is oxidized to pyrrole, and then extracted and purified by column chromatography before counting radioactivity.

Posttranslational modifications in the biosynthesis of type IV collagen by a human tumor cell line

Factors responsible for the high extent of intracellular posttranslational modifications in type IV collagens were studied in a cultured human tumor cell line, HT-1080. These cells do not synthesize any detectable amounts of interstitial collagens but produce type IV collagen at a high rate, corresponding to about one-third of the production of interstitial collagens by cultured human skin fibroblasts. Prolyl 4-hydroxylase activity was lower in the HT-1080 cells than in human skin fibroblasts, there being a rough correlation between this enzyme activity and the rate of 4-hydroxyproline formation in these two cell types. The differing extents of the respective modifications could largely be explained by differences in the activities of lysyl hydroxylase and the hydroxylysyl glycosyltransferases between the two cell types. No difference ws found in prolyl 3-hydroxylase activity, however, even though the extent of 3-hydroxylation of proline residues was about 6-fold in the type IV collagens. In experiments where the HT-1080 cells were studied in suspension, a lag of about 100 min was found before the secretion of type IV collagen from the cells became linear. Pulse-chase experiments in suspension indicated that all the intracellular enzyme reactions proceeded for about 40 min, presumably due to the slow triple-helix formation in type IV collagens. This slow helix formation apparently contributed to the high extent of all the intracellular modifications but was not a major factor.

Effect of prevention of procollagen triple-helix formation on proline 3-hydroxylation in freshly isolated chick-embryo tendon cells

Inhibition of procollagen triple-helix formation by the addition of cis-hydroxyproline or azetidine-2-carboxylic acid increased the synthesis of 3-hydroxy[14C]proline 1.7-1.8-fold in pulse-chase experiments with freshly isolated chick-embryo tendon cells. The amount of 3-hydroxy[14C]proline, expressed as a percentage of the total 14C radioactivity in hydroxyproline, reached 8.4%. Control experiments indicated that the two analogues had no effect on the prolyl 3-hydroxylase activity of these cells. The data suggest that the time available before triple-helix formation in part regulates the extent of the 3-hydroxylation of proline in the biosynthesis of collagen in intact cells.

Glomerular basement membrane collagen and activities of the intracellular enzymes of collagen biosynthesis in congenital nephrotic syndrome of the Finnish type

The composition of pepsin-solubilized glomerular basement membrane (GBM) collagen was studied in kidneys from patients with congenital nephrotic syndrome of the Finnish type (CNF). The 3-hydroxyproline content in the GBM collagen in CNF was only about one half of that noted in the controls. The alanine content was slightly higher in CNF, but no differences were found in the contents of the other amino acids or carbohydrates. Analysis of the GBM collagen by sodium dodecyl sulphate polyacrylamide gel electrophoresis after reduction indicated a lower proportion of one of the major polypeptide chains in CNF than in the controls, while a higher proportion of one low molecular weight minor component was noted. The activities of the five intracellular enzymes of collagen biosynthesis, including prolyl 3-hydroxylase, were not significancy altered in the CNF kidney cortex samples when compared with the controls.