Purification and properties of collagen proline hydroxylase from newborn rat skin

Human prolyl hydroxylase expression in uterine leiomyoma during the menstrual cycle

BACKGROUND

To investigate the role of prolyl hydroxylase (PH), a key enzyme of collagen synthesis, in human uterine leiomyoma, PH expression was determined in the normal uterine myometrium and the leiomyoma tissues during the menstrual cycle.

METHODS

The tissues were obtained from 40 regularly cycling women (aged 29 to 53 yr) who were undergoing abdominal hysterectomy for symptomatic uterine leiomyoma. Immunohistochemistry for human PH with specific monoclonal antibody was used for analysis.

RESULTS

Immunohistochemical staining for PH revealed intense staining of leiomyoma cells in the uterine leiomyoma throughout the menstrual cycle, as compared with the adjacent normal myometrium. In the secretory phase, weak or no immunostaining for PH was detected in the normal myometrial tissues.

CONCLUSIONS

These results suggest that increased expression of PH might play an role in the physiology of uterine leiomyoma during the menstrual cycle.

Prolyl hydroxylase in human corpora lutea during menstrual cycle and early pregnancy

Immunohistochemical staining for human prolyl hydroxylase revealed intense staining of the human corpora lutea (CL) parencyma during early pregnancy compared with those in the menstrual cycle. These results suggest that human prolyl hydroxylase might play an important role in determining the physiology and structure of the CL during the menstrual cycle and early pregnancy.

Ascorbic acid: much more than just an antioxidant

Vitamin C (ascorbic acid (AA)) is very popular for its antioxidant properties. Consequently, many other important aspects of this multifaceted molecule are often underestimated or even ignored. In the present paper, we have tried to bring to the foreground some of these aspects, including the peculiarities of the AA biosynthetic pathway in different organisms, the remarkable function of AA as a co-substrate of many important dioxygenases, the role of AA-regenerating enzymes and the known pathways of AA catabolism, as well as the intriguing function of AA in gene expression.

Phytic acid, an iron chelator, attenuates pulmonary inflammation and fibrosis in rats after intratracheal instillation of asbestos

Reactive oxygen species, especially iron-catalyzed hydroxyl radicals (.OH) are implicated in the pathogenesis of asbestos-induced pulmonary toxicity. We previously demonstrated that phytic acid, an iron chelator, reduces amosite asbestos-induced .OH generation, DNA strand break formation, and injury to cultured pulmonary epithelial cells (268[1995, Am. J. Physiol.(Lung Cell. Mol. Physiol.) 12:L471-480]). To determine whether phytic acid diminishes pulmonary inflammation and fibrosis in rats after a single intratracheal (it) instillation of amosite asbestos, Sprague-Dawley rats were given either saline (1 ml), amosite asbestos (5 mg; 1 ml saline), or amosite treated with phytic acid (500 microM) for 24 hr and then instilled. At various times after asbestos exposure, the rats were euthanized and the lungs were lavaged and examined histologically. A fibrosis score was determined from trichrome-stained specimens. As compared to controls, asbestos elicited a significant pulmonary inflammatory response, as evidence by an increase (approximately 2-fold) in bronchoalveolar lavage (BAL) cell counts at 1 wk and the percentage of BAL neutrophils (PMNs) and giant cells at 2 wk (0.1 vs 6.5% and 1.3 vs 6.1%, respectively; p < 0.05). Asbestos significantly increased the fibrosis score at 2 wk (0 +/- 0 vs 5 +/- 1; p < 0.05). The inflammatory and fibrotic changes were, as expected, observed in the respiratory bronchioles and terminal alveolar duct bifurcations. The increased percentage of BAl PMNs and giant cells persisted at 4 wk, as did the fibrotic changes. Compared to asbestos alone, phytic acid-treated asbestos elicited significantly less BAL PMNs (6.5 vs 1.0%; p < 0.05) and giant cells (6.1 vs 0.2%; p < 0.05) and caused significantly less fibrosis (5 vs 0.8; p < 0.05) 2 wk after exposure. We conclude that asbestos causes pulmonary inflammation and fibrosis in rats after it instillation and that phytic acid reduces these effects. These data support the role of iron-catalyzed free radicals in causing pulmonary toxicity from asbestos in vivo.

Ascorbate system in plant development

By using lycorine, a specific inhibitor of ascorbate biosynthesis, it was possible to demonstrate that plant cells consume a high quantity of ascorbate (AA). The in vivo metabolic reactions utilizing ascorbate are the elimination of H2O2 by ascorbate peroxidase and the hydroxylation of proline residues present in the polypeptide chains by means of peptidyl-proline hydroxylase. Ascorbate acts in the cell metabolism as an electron donor, and consequently ascorbate free radical (AFR) is continuously produced. AFR can be reconverted to AA by means of AFR reductase or can undergo spontaneous disproportion, thus generating dehydroascorbic acid (DHA). During cell division and cell expansion ascorbate consumption is more or less the same; however, the AA/DHA ratio is 6-10 during cell division and 1-3 during cell expansion. This ratio depends essentially on the different AFR reductase activity in these cells. In meristematic cells AFR reductase is very high, and consequently a large amount of AFR is reduced to AA and a small amount of AFR undergoes disproportionation; in expanding cells the AFR reductase activity is lower, and therefore AFR is massively disproportionated, thus generating a large quantity of DHA. Since the transition from cell division to cell expansion is marked by a large drop of AFR reductase activity in the ER, it is suggested here that AFR formed in this compartment may be involved in the enlargement of the ER membranes and provacuole acidification. DHA is a toxic compound for the cell metabolism and as such the cell has various strategies to counteract its effects: (i) meristematic cells, having an elevated AFR reductase, prevent large DHA production, limiting the quantity of AFR undergoing disproportionation (ii) Expanding cells, which contain a lower AFR reductase, are, however, provided with a developed vacuolar system and segregate the toxic DHA in the vacuole. (iii) Chloroplast strategy against DHA toxicity is efficient DHA reduction to AA using GSH as electron donor. This strategy is usually poorly utilized by the surrounding cytoplasm. DHA reduction does play an important role at one point in the life of the plant, that is, during the early stage of seed germination. The dry seed does not store ascorbate, but contains DHA, and several DHA-reducing proteins are detectable. In this condition, DHA reduction is necessary to form a limited AA pool in the seed for the metabolic requirements of the beginning of germination. After 30-40 h ascorbate ex novo synthesis starts, DHA reduction declines until a single isoform remains, as is typical in the roots, stem, and leaves of seedlings.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence that an iron chelator regulates collagen synthesis by decreasing the stability of procollagen mRNA

Iron chelation has been shown previously to decrease collagen synthesis at a posttranslational level by inhibiting prolyl 4-hydroxylase, one of the key enzymes in collagen metabolism. On the other hand, recent in vivo studies of iron overload in rats suggest that iron could specifically activate collagen gene expression in liver tissues. These findings led us to investigate whether iron chelation might also affect collagen gene expression and posttranslational modification. Our data indicate that alpha,alpha'-dipyridyl, an iron chelator, at a concentration of 1 mmol/L, decreased steady-state levels of type I procollagen messenger RNA by 42% (p less than 0.001) without affecting beta-actin messenger RNA levels. Nuclear runoff studies demonstrated that transcription of the type I procollagen gene was unchanged by alpha,alpha'-dipyridyl. However, the turnover rate of type I procollagen messenger RNA was increased by 30%. This pretranslational inhibition of collagen synthesis was not due to decreased lipid peroxidation, because thiobarbituric acid-reactive substances were unchanged by alpha,alpha'-dipyridyl. However, cycloheximide totally abolished the effect, indicating that de novo protein synthesis was required.

Comparison between avian and human prolyl 4-hydroxylases: studies on the holomeric enzymes and their constituent subunits

Prolyl 4-hydroxylase, a key enzyme in collagen biosynthesis, catalyzes the conversion of selected prolyl residues to trans-hydroxyproline in nascent or completed pro-alpha chains of procollagen. The enzyme is a tetramer composed of two nonidentical subunits, designated alpha and beta. To compare the enzyme and its subunits from different sources, the chick embryo and human placental prolyl 4-hydroxylases were purified to homogeneity and their physicochemical and immunological properties were determined. Both enzymes were glycoproteins with estimated apparent molecular weights ranging between 400 and 600 kDa. Amino acid and carbohydrate analyses showed slight differences between the two holomeric enzymes, consistent with their deduced amino acid sequences from their respective cDNAs. Human placental prolyl 4-hydroxylase contained more tightly bound iron than the chick embryo enzyme. Immunodiffusion of the human placental enzyme with antibodies raised against the purified chick embryo prolyl 4-hydroxylase demonstrated partial identity, indicating different antigenic determinants in their tertiary structures. The enzymes could be separated by high-resolution capillary electrophoresis, indicating differential charge densities for the native chick embryo and human placental proteins. Electrophoretic studies revealed that the human prolyl 4-hydroxylase is a tetrameric enzyme containing two nonidentical subunits of about 64 and 62 kDa, in a ratio of approximately 1 to 2, designated alpha and beta, respectively. In contrast, the chick embryo alpha and beta subunit ratio was 1 to 1. Notably, the human alpha subunit was partially degraded when subjected to electrophoresis under denaturing conditions. Analogously, when the chick embryo enzyme was subjected to limited proteolysis, selective degradation of the alpha subunit was observed. Finally, only the alpha subunit was bound to Concanavalin A demonstrating that the alpha subunits of prolyl 4-hydroxylase in both species were glycosylated. Using biochemical techniques, these results demonstrated that the 4-trans-hydroxy-L-proline residues in human placental collagens are synthesized by an enzyme whose primary structure and immunological properties differ from those of the previously well-characterized chick embryo enzyme, consistent with their recently deduced primary structures from cDNA sequences.

Biochemical markers of hepatic fibrosis

Most liver diseases lead to a pathobiochemical reaction termed liver fibrosis. This is a dynamic process implying different rates of progression or regression. Thus, histological examination of a liver biopsy is essential for a diagnosis but biochemical tests are necessary for assessing the activity of the process and monitoring its evolution. We review the most important constituents of liver connective tissue and the biochemical tests developed for evaluating liver fibrosis. The aminopeptide of type III procollagen is the most widely used parameter: two different radioimmunoassays have been developed with different affinities for the two circulating forms of the molecule. The determination of serum P3P reveals an elevation of blood levels both in acute and chronic liver diseases. In the first, serum P3P is an index of hepatic necrosis and inflammation which correlates with other biochemical parameters. In the second it is an index of active fibrogenesis. Moreover, in primary biliary cirrhosis this parameter is an independent prognostic variable and an important predictor of survival. Other immunoassays exist for different collagen cleavage products, but their clinical value is not established. Laminin and fibronectin are the principal structural glycoproteins in liver. Fibronectin determination does not seem to be of clinical value in liver disease. In contrast, serum laminin correlates with the severity of portal venous pressure in advanced liver disease. Its concentration parallels the severity of varices and may indicate the risk of bleeding. Hyaluronate is a high molecular weight polysaccharide, raised serum concentrations reflect both its increased synthesis by activated fibroblasts and its impaired catabolism by the liver. Thus, it may be useful for evaluating and monitoring the progression of chronic liver disease. The measurement of the activity of prolyl 4-hydroxylase as well as that of lysine oxidase and other enzymes has been proposed, but their clinical value is not sufficiently demonstrated. A panel of tests (e.g., laminin, hyaluronate and the aminopeptide of type III procollagen) seems to be recommended for a biochemical assessment of liver fibrosis in clinical practice.

Effect of the reducing environment on the accumulation of elastin and collagen in cultured smooth-muscle cells

We show here that cultured neonatal-rabbit aortic smooth-muscle cells produce and accumulate significant amounts of insoluble elastin. When grown in the presence of ascorbic acid, the amount of insoluble elastin in these cultures decreases, whereas the accumulation of collagen increases. These changes have been attributed to increased hydroxylation of proline in elastin. The function of ascorbic acid in proline hydroxylation is thought to be that of a reductive cofactor that maintains the proper oxidation state of molecular iron in the enzyme complex. This study shows that both ascorbic and isoascorbic acids act similarly to modify the accumulation of elastin and collagen in culture. On the other hand, cultures grown in the presence of dithiothreitol, a reducing agent previously shown to act as a cofactor for prolyl hydroxylase, do not demonstrate altered elastin accumulation. These studies are consistent with the suggestion that there is a specific role for ascorbic acid in this cellular system that cannot be replaced by other reducing cofactors.

Review: pharmacologic control of wound healing in glaucoma filtration surgery

An integrated understanding of the wound healing process, pharmacological agents, and surgical techniques are required for comprehensive treatment of wound healing in glaucoma filtration surgery. More well controlled basic and clinical studies are required to clarify the existing ambiguities in the selection of proper pharmacological agents, dosage, timing, and method of delivery. The most promising treatment modalities may be combination drug therapy and the use of bioerodible compounds for a sustained and localized delivery system.

Isolation of cDNA clones and genomic DNA clones of beta-subunit of chicken prolyl 4-hydroxylase

Prolyl 4-hydroxylase (EC 1.14.11.2) is a key enzyme in collagen biosynthesis. The active enzyme is a tetramer composed of two pairs of non-identical subunits, alpha and beta. Sheep antiserum directed against chicken proly 4-hydroxylase was initially used to screen two cDNA expression libraries. The cDNA was prepared from chicken smooth muscle mRNA and cloned into the plasmids pUC8- and pUC9. Antibodies identified twenty-five clones among the approximately 2 x 10(5) clones in the libraries. Ten clones were isolated pure and used in the subsequent analysis. Monospecific antibodies directed against beta subunit of the enzyme were used in Western-blot analyses of extracts of bacteria carrying the cDNA clones. The results indicated that the clone CPH 9-10B encodes a portion of beta-subunit. The cDNA from CPH 9-10B was used to screen another cDNA library prepared from mRNA from chicken skeletal muscle. Several overlapping clones were isolated. Together the cDNAs correspond to 2.4 kb which is the same as the corresponding mRNA. Three regions of the amino acid sequence deduced from the cDNA sequence matched with that of the NH2-terminus of beta-subunit and two CNBr peptides derived from beta-subunit. The cDNA of CPH 9-10B was also used to screen a genomic DNA library constructed with lambda EMBL-3. Two overlapping genomic clones lambda gCPH beta-22 and beta-50 were isolated and characterized by restriction enzyme analysis. The results indicate that lambda gCPH beta-22 contains the portion of the beta-subunit gene that is transcribed into the 5' portion of beta-subunit mRNA, whereas lambda gCPH beta-50 contains the 3' portion.

A sandwich immunoassay for human prolyl 4-hydroxylase using monoclonal antibody

Monoclonal antibody was used in a sandwich enzyme immunoassay and in a radioimmunoassay for human serum immunoreactive prolyl 4-hydroxylase. The enzyme immunoassay utilized a monoclonal antibody as a solid phase and horseradish peroxidase-labeled rabbit antibody (Fab') to human prolyl 4-hydroxylase as a conjugate. Sensitivity was 0.1 ng (0.4 fmol) of enzyme per tube. With a conjugate purified by an enzyme-bound affinity column, sensitivity was increased to 0.01 ng (0.04 fmol) per tube, and linearity was obtained between 0.01 to 30 ng (0.04-125 fmol) per tube. The radioimmunoassay used a 125I-labeled rabbit antibody (IgG) as the conjugate. Sensitivity of this technique was 0.4 ng of enzyme per tube. The enzyme immunoassay gave reproducible quantitation and evidenced a higher enzyme concentration in the serum of patients with liver disorders. Protein immunoblotting showed that the serum immunoreactive prolyl 4-hydroxylase trapped in the sandwich immunoassay was mainly the beta-subunit.

Human prolyl hydroxylase. Purification, radioimmunoassay and clinical studies in liver diseases

Prolyl hydroxylase was purified from human placentae, specific antiserum against it was prepared, and a new radioimmunoassay system employing 125I-labelled enzyme preparation was established. The molecular weight of the placental enzyme was shown to be 320,000 by gel filtration. SDS-polyacrylamide gel electrophoresis showed two bands of unequal intensity having molecular weights of 60,000 and 130,000. Their amino acid compositions were identical to each other, suggesting the polypeptide with a molecular weight of 130,000 might be a dimer of the polypeptide with a molecular weight of 60,000. The new radioimmunoassay established had a sensitivity of the order of 10 ng/ml, indicating it was more sensitive than previous radioimmunoassay employing 3H-labelling method. Clinical studies on patients with liver diseases disclosed that the concentrations of serum immunoreactive prolyl hydroxylase were elevated both in cases of hepatocellular damage and in cases of cholestasis. In cases of hepatocellular damage the enzyme behaved like cytoplasmic enzymes such as glutamic oxaloacetic transaminase, glutamic pyruvic transaminase and lactic dehydrogenase, but in cases of cholestasis it resembled biliary enzymes such as alkaline phosphatase and gamma-glutamyl transpeptidase. This result might be associated with the peculiar location of the enzyme within the cell, in the membrane of rough endoplasmic reticulum.

Characterization of nematocyst proteins from the sea anemones Aiptasia pallida and Pachycerianthus torreyi (Cnidaria: Anthozoa)

Nematocysts from the anemones Aiptasia pallida and Pachycerianthus torreyi were investigated. SDS-polyacrylamide electrophoresis of solubilized Aiptasia nematocysts revealed one major protein band (mol. wt 31,800) and several minor components. Coelectrophoresed whole venom contained numerous protein components, of which a major one appeared to be identical to the major nematocyst protein. Nematocyst capsules and everted threads from both species contained levels of glycine and proline-hydroxyproline characteristic of vertebrate collagens. Cysteine was present in significant amounts. Aiptasia whole venom contained high levels of glutamic acid and/or glutamine (71%) with no detectable cysteine or proline-hydroxyproline. The 31,800-dalton venom protein possessed only glycine (80%) and glutamyl and/or glutaminyl (20%) residues.

Hepatic peptidyl prolyl hydroxylase activity and liver fibrosis--a prospective study of 94 infants and children with hepatobiliary disorders

To assess whether hepatic peptidyl prolyl hydroxylase (PPH) activity could serve as a practical quantitative indicator of hepatic fibrosis or aid in the categorization, diagnosis or prognosis of hepatobiliary disorders in infancy and childhood, the activity of this enzyme has been determined prospectively by a tritium release method in 97 biopsies from 94 infants and children with the following conditions: acute hepatitis of infancy, 10 patients; extrahepatic biliary atresia, 13; previous hepatitis of infancy, 8; alpha-1-antitrypsin deficiency, 6; chronic active hepatitis, 17; chronic persistent hepatitis, 5; glycogen storage disease, 5; and 25 patients with a miscellanea of other liver disorders. PPH activity was considered in relation to diagnosis, biochemical and histological abnormality and subsequent prognosis over a 4-year period. Five liver biopsies which showed no histological abnormality were considered as "controls" having PPH values of 0.72 +/- 0.47 (mean +/- S.D.). PPH activity was significantly elevated in acute hepatitis of infancy, 9 of the 10 infants having PPH greater than 1.66 units (i.e., mean +/- 2 S.D. of the "control" value). Nine infants (70%) with extrahepatic biliary atresia also had PPH activity above this value, as did two with alpha-1-antitrypsin deficiency and 12 patients all in different diagnostic categories. PPH activity did not correlate with hepatic fibrosis as indicated by hepatic hydroxyproline concentration or by histological assessment, or with biochemical tests of liver function within any diagnostic group or in the series as a whole. PPH activity was similar in biopsies with and without histological features of cirrhosis.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between ascorbic acid and cell division

Proliferating cells require large amounts of ascorbic acid to reach cell division. The decrease in ascorbic acid caused by adding lycorine, an inhibitor of ascorbic acid biosynthesis, induces profound inhibition of cell division: the cell cycle is arrested in G1 and G2 phase, more than 90% of the cells being accumulated in G1 after some time. The effect of lycorine on mitotic index (MI) has been reversed by increasing experimentally the concentration of ascorbic acid in tissues. Ascorbic acid control on cell division is found to be specific, since isoascorbic acid is wholly ineffective. It is suggested that the principal role of ascorbic acid in the cell cycle may be related to its action in controlling the synthesis of hydroxyproline-containing proteins, which can be essential requirements for development of G1 and G2.

Conformational implications of enzymatic proline hydroxylation in collagen

In 1979 it was proposed that prolyl hydroxylase (prolyl-glycyl-peptide,2-oxoglutarate:oxygen oxidoreductase, EC 1.14.11.2) recognizes the beta-turn conformation in nascent procollagen chains and that the hydroxylation process involves a conformational change resulting in "straightening" of the beta-turn segments into the linear triple-helical conformation of native collagen. We present experimental data that verify both these postulates. The following peptides were synthesized and studied for their conformation and interaction with prolyl hydroxylase: tBoc-Pro-Gly-Ala-OH, tBoc-Pro-Gly-Val-OH, tBoc-Gly-Val-Pro-Gly-Val-OH, and tBoc-Pro-DAla-Ala-OH. Spectral data showed that these peptides preferred a beta-turn conformation. All of them acted as inhibitors of the enzyme; the pentapeptide also acted as a substrate. To mimic the biosynthetic event, a collagen model polypeptide, (Pro-Pro-Gly)10, was incubated at 37 degrees C with purified prolyl hydroxylase and the necessary cosubstrates and cofactors at pH 7.8. A progressive change from the initially nonhelical to the triple-helical conformation, as monitored by CD spectra and gel filtration, occurred during the course of proline hydroxylation. In addition to leading to increased thermal stability of the triple-helical conformation in (Pro-Pro-Gly)10 and (Pro-Pro-Gly)5, the enzymatic incorporation of the hydroxyproline residues was found to enable these polypeptides to fold into this conformation faster than the unhydroxylated counterparts. These conformational implications of proline hydroxylation in collagen may also be of use in the study of the complement subcomponent Clq and of acetylcholine esterase which contain collagen-like regions in them.

An improved method for the purification of vertebrate prolyl hydroxylase by affinity chromatography

Prolyl hydroxylase was purified from extracts of 13-day old chick embryos by an improved affinity column technique. Prolyl hydroxylase was released from the affinity column by poly-L-proline and was separated from the other proteins and from the poly-L-proline by ion exchange chromatography. This improvement allowed eluates from up to six affinity columns to be pooled and chromatographed in a single step. Furthermore, a major contaminating protein which was difficult to remove from the eluted enzyme using the previous procedure was easily separated by the ion exchange column. The overall advantage of the new technique allows much larger amounts of prolyl hydroxylase to be prepared at a single time than was previously possible.

The function of ascorbate with respect to prolyl 4-hydroxylase activity

1. Incubation in the presence of 2-oxoglutarate and oxygen inactivates prolyl 4-hydroxylase (prolyl-glycyl-peptide, 2-oxoglutarate:oxygen oxidoreductase, EC 1.14.11.2), with a t 1/2 of 80 s at 37 degrees C. This inactivation is not affected by the presence or absence of the prolyl peptide substrate or added Fe(II). 2. This inactivation can be prevented by either ascorbate or dithiothreitol. It can be reversed by dithiothreitol but not by ascorbate. 3. Although the iron-containing form of prolyl 4-hydroxylase requires ascorbate for activity, ascorbate is not stoicheiometrically consumed in the reaction catalysed by the enzyme. Ascorbate cannot be replaced by alloxan, lactate, NADH plus phenazine methosulphate, dithiothreitol or L-cysteine. 4. Ascorbate has a double function with respect to prolyl 4-hydroxylase activity. On the one hand, it is required to initiate the reaction when the enzyme has become oxidized during isolation. On the other hand it is required for the protection against inactivation induced by 2-oxoglutarate and oxygen, presumably by preventing S-S bridge formation. The latter function may be of physiological importance.

The assembly of tetrameric prolyl hydroxylase in tendon fibroblasts from newly synthesized alpha-subunits and from preformed cross-reacting protein

Embryonic-chick tendon cells were incubated in suspension for 4h with (14)C-labelled amino acids, cell extracts were subjected to gel filtration, and the effluent was examined by rocket immunoelectrophoresis by using antibodies specific for the beta-subunit of chick prolyl hydroxylase. Two peaks of immunoreactive protein were found. The first peak contained 40% of the immunoreactive protein eluted from the column and 100% of the enzyme activity. Polyacrylamide-slab-gel electrophoresis in sodium dodecyl sulphate of an immunoprecipitate of this peak demonstrated that it consisted of the tetrameric form of prolyl hydroxylase, subunit composition alpha(2)beta(2) where alpha and beta are non-identical subunits. Only the alpha-subunits were labelled, indicating that they were synthesized during the 4h labelling period. The beta-subunits were unlabelled, indicating that they had been synthesized before the labelling period. The second peak eluted from the gel-filtration column contained 60% of the immunoreactive protein eluted from the column and was enzymically inactive. Polyacrylamide-slab-gel electrophoresis of an immunoprecipitate of this peak indicated that it consisted of a single labelled polypeptide chain, identified as cross-reacting protein, which was related to, but not identical with, the beta-subunit of prolyl hydroxylase. Pulse-chase experiments were performed on cultured chick tendon cells to demonstrate that alpha-subunits and cross-reacting protein had half-lives of about 60h. The half-life of beta-subunits was considerably longer, and the kinetic pattern was consistent with their being derived from a labelled precursor such as cross-reacting protein. The data presented here indicate that the active tetrameric form of prolyl hydroxylase in cells is assembled from alpha-subunits which are newly synthesized, and from beta-subunits which are derived from cross-reacting protein.

Properties of prolyl 4-hydroxylase containing firmly-bound iron

1. Prolyl 4-hydroxylase (prolyl-glycyl-peptide, 2-oxoglutarate:oxygen oxidoreductase, EC 1.14.11.2) was isolated in a form containing iron (0.85-1.1 mol Fe/mol enzyme). 2. The enzyme was pure according to gel electrophoresis and had a high specific activity (1.8-2.6 mumol . mi-1 . mg-1). 3. Experiments with metal chelators showed this iron to be firmly bound and to be required for catalytic activity. 4. According to EPR spectrometry the bound iron is not part of a [2Fe-2S] or a [4Fe-4S] cluster. 5. The enzyme activity is to a large extent independent of added Fe2+. 6. The enzyme activity is completely dependent on ascorbate. 7. In the absence of ascorbate but the presence of substrates the enzyme is irreversibly inactivated. 8. Continuous measurement of enzyme activity was possible by following oxygen uptake.

Hepatic prolyl hydroxylase and collagen synthesis in patients with alcoholic liver disease

Hepatic prolyl hydroxylase activity and collagen synthesis were measured in patients with alcoholic liver disease to determine the feasibility of using the enzyme prolyl hydroxylase as a marker of hepatic fibrogenesis. Alcoholic patients with liver histopathology consistent with normal, steatosis, alcoholic hepatitis, early cirrhosis, or advanced cirrhosis were analysed for liver prolyl hydroxylase activity and in vitro collagen synthesis. Prolyl hydroxylase activity and the rate of in vitro collagen synthesis were correlated when these parameters were measured in samples of the same liver biopsy. Mean prolyl hydroxylase activity was significantly raised in all groups of alcoholic patients with alcoholic liver disease, except those with steatosis, when compared with alcoholic patients with normal morphology. Alcoholic patients with early cirrhosis had enzyme activity (mean +/- SE: 1.367 +/-0.162 mU/mg protein) significantly raised over all other groups. Mean enzyme activity was less raised (0.985 +/- 0.097 mU/mg protein) in patients with advanced cirrhosis. The percentage of collagen synthesis in patients with early or advanced cirrhosis was also raised compared with alcoholic patients with normal morphology. Prolyl hydroxylase activity and the rate of collagen synthesis are significantly correlated (r=0.62). These findings suggest that hepatic prolyl hydroxylase activity is a useful indicator of hepatic fibrogenesis and its measurement on available liver biopsy tissue should be a potent diagnostic tool reflecting active fibrogenesis and predicting progression of alcoholic liverdisease.

Iron: a possible heterotropic effector of prolyl hydroxylase

The mechanism of ferrous ion binding to prolyl hydroxylase (prolyl-glycyl-peptide,2-oxoglutarate:oxygen oxidoreductase, EC 1.14.11.2) was studied according to Koshland's curve-fitting procedure (Cornish-Bowden, A. and Koshland, Jr., D.E. (1970) Biochemistry 9, 3325--3336). The calculated data obtained by means of the unrestricted Adair equation were found to provide an adequate fit with experimentally obtained values, whereas those obtained on the basis of Michaelis-Menten kinetics did not. This suggests that prolyl hydroxylase could be an allosteric enzyme under positive heterotropic control.