Human prolidase and prolidase deficiency: an overview on the characterization of the enzyme involved in proline recycling and on the effects of its mutations.

Here we summarized what is known at the present about function, structure and effect of mutations in the human prolidase. Among the peptidases, prolidase is the only metalloenzyme that cleaves the iminodipeptides containing a proline or hydroxyproline residue at the C-terminal end. It is relevant in the latest stage of protein catabolism, particularly of those molecules rich in imino acids such as collagens, thus being involved in matrix remodelling. Beside its intracellular functions, prolidase has an antitoxic effect against some organophosphorus molecules, can be used in dietary industry as bitterness reducing agent and recently has been used as target enzyme for specific melanoma prodrug activation. Recombinant human prolidase was produced in prokaryotic and eukaryotic hosts with biochemical properties similar to the endogenous enzyme and represents a valid tool both to better understand the structure and biological function of the enzyme and to develop an enzyme replacement therapy for the prolidase deficiency (PD). Prolidase deficiency is a rare recessive disorder caused by mutations in the prolidase gene and characterized by severe skin lesions. Single amino acid substitutions, exon splicing, deletions and a duplication were described as causative for the disease and are mainly located at highly conserved amino acids in the sequence of prolidase from different species. The pathophysiology of PD is still poorly understood; we offer here a review of the molecular mechanisms so far hypothesized.

Zebrafish Collagen Type I: Molecular and Biochemical Characterization of the Major Structural Protein in Bone and Skin.

Over the last years the zebrafish imposed itself as a powerful model to study skeletal diseases, but a limit to its use is the poor characterization of collagen type I, the most abundant protein in bone and skin. In tetrapods collagen type I is a trimer mainly composed of two α1 chains and one α2 chain, encoded by COL1A1 and COL1A2 genes, respectively. In contrast, in zebrafish three type I collagen genes exist, col1a1a, col1a1b and col1a2 coding for α1(I), α3(I) and α2(I) chains. During embryonic and larval development the three collagen type I genes showed a similar spatio-temporal expression pattern, indicating their co-regulation and interdependence at these stages. In both embryonic and adult tissues, the presence of the three α(I) chains was demonstrated, although in embryos α1(I) was present in two distinct glycosylated states, suggesting a developmental-specific collagen composition. Even though in adult bone, skin and scales equal amounts of α1(I), α3(I) and α2(I) chains are present, the presented data suggest a tissue-specific stoichiometry and/or post-translational modification status for collagen type I. In conclusion, this data will be useful to properly interpret results and insights gained from zebrafish models of skeletal diseases.

Stability of type I collagen CNBr peptide trimers.

As indices of triple helix stability of type I collagen CNBr peptide homotrimers, deltaG degrees for monomer-trimer transitions and melting temperatures were obtained from circular dichroism measurements at increasing temperatures. The data were compared with the stability of the parent native molecule. Peptides were found to have a lower stability than the whole collagen molecule. The general implication is that the coordinated water molecules play a key role in determining collagen triple helical stability and high cooperativity at melting. Other factors (monomer stability, ionic and hydrophobic factors, variations of composition, specific sequences) could also contribute towards peptide stability; these factors could explain the data obtained in the case of peptide alpha1(I) CB3.

Stability and networks of hydrogen bonds of the collagen triple helical structure: influence of pH and chaotropic nature of three anions.

The thermal stability of the trimeric species formed by seven type I collagen CNBr peptides was determined at neutral and acidic pH. Melting temperature of peptide trimers and free energy change for monomer to trimer transition were used as indices of trimer stability. A greater stability at neutral pH than at acidic pH was found for all peptides analysed because in most conditions an entropic gain overwhelms an enthalpic cost. Enthalpic reasons are prevailing only in some conditions of the more acidic peptides. The overlap zone of type I collagen fibrils is more basic than the gap zone and is therefore more sensitive to variations of pH from neutral to acidic, e.g. in bone degradation when osteoclasts acidify the lacuna lying between cell and bone. Peptide trimer stability in neutral conditions is influenced also by the chaotropic nature and the concentration of three anions: chloride, sulfate and phosphate. This was more evident for sulfate at the highest concentration used (0.5 M) when a greater stability is caused by entropic reasons. The contribution of hydroxyproline to the stability of peptide trimers is greater at neutral than at acidic pH, particularly at the highest concentration of sulfate. All our data indicate that pH, chaotropic nature and concentration of three anions influence the networks of hydrogen bonds present in the collagen triple helical structure.

Conformational study of a collagen peptide by 1H NMR spectroscopy: observation of the 14N-1H spin-spin coupling of the Arg guanidinium moiety in the triple-helix structure.

CB2, a CNBr peptide of 36 residues from type I collagen alpha1(I) chain has been studied by NMR spectroscopy as a function of temperature. At low temperature, the guanidinium protons of Arg9 showed sharp 1:1:1 NMR triplets around 6.95 ppm, characteristic of 14N coupled protons (1J(NH)=52 Hz) when the quadrupolar relaxation rate is drastically reduced. These spectral characteristics and the low temperature coefficient of the 1:1:1 triplets (delta delta/delta T of -3.6 ppb/degrees C) suggest that the H atoms of the protonated guanidinium moiety of Arg9 in the triple helix are slowly exchanging with bulk water, most likely involved in hydrogen bonds. On the basis of conformational energy computations on a model segment of type I collagen (Vitagliano, L., Némethy, G., Zagari, A. and Scheraga, H.A. (1993) Biochemistry 32, 7354-7359), similar to CB2, our data could indicate that the guanidinium group of Arg9 form hydrogen bonds with a backbone carbonyl of an adjacent chain probably by using the N(epsilon) hydrogen, leaving the four N(eta) hydrogens bound to water molecules that must be in slow exchange with bulk water and that could therefore be considered structural elements of the trimeric alpha1(I) CB2 triple helix. The behaviour of Arg9 has been investigated also in terms of equilibrium between random monomer and helical trimer conformations controlled by temperature. The thermal unfolding process was found to be reversible and the melting point resulted to be 17 degrees C.

Comparison between urinary pyridinium cross-links and hydroxylysine glycosides in monitoring the effects of ovariectomy and 17 beta-estradiol replacement in aged rats.

This study was undertaken to assess the sensitivity of hydroxylysylpyridinoline (HP), lysylpyridinoline (LP), galactosylhydroxylysine (GHyl) and glucosylgalactosylhydroxylysine (GGHyl) to monitor bone response to estrogen deficiency and replacement by comparing their excretory patterns in ovariectomized aged (11-14 months old) rats. The ovariectomized (OVX) rats were randomized into two groups: (1) OVX plus vehicle; (2) OVX plus 17 beta-estradiol (17-beta E, 10 micrograms/kg, s.c., 4 days/week). Treatment with 17-beta E started immediately after OVX and continued for 60 days. The collagen catabolites were measured in urine for 1 month before OVX and thereafter for 60 days. In temporal coincidence with urine collection, bone area and bone mineral density (BMD) of lumbar vertebrae, femoral diaphysis and distal metaphysis were measured by dual-energy X-ray absorptiometry. In the untreated rats, BMD of the femoral metaphysis and lumbar vertebrae decreased significantly and the urinary excretion of LP, HP, GHyl and GGHyl increased with different patterns. In the treated rats, 17-beta E replacement prevented the increment in LP excretion, partially prevented the increase in HP excretion, but had no effect on the excretion of GHyl and GGHyl. In conclusion pyridinolines and glycosides have different sensitivities to the bone response to OVX. Glycoside excretion after OVX also reflects metabolic processes not strictly related to bone loss and, in contrast with LP, is not sensitive to estrogen replacement.

Possible role of overglycosylation in the type I collagen triple helical domain in the molecular pathogenesis of osteogenesis imperfecta.

The underlying defect in patients affected by a form of osteogenesis imperfecta (OI) clarified at the molecular level regards the amount or the structure of type I collagen synthesized. This leads to a decreased and/or abnormal mineral deposition in bone and affects bone mass and/or strength. Abnormal interactions between collagen molecules in the presence of mutant trimers could give rise to abnormal fibrils, which, in turn, can determine incorrect interactions with noncollagenous matrix macromolecules. The interactions can be disturbed or modulated by an abnormal distribution on the collagen fibril surface of electrically charged or hydrophobic groups, or by an increased presence of sugar moieties linked to hydroxylysyl residues due to chain post-translational overmodifications (lysyl overhydroxylation and hydroxylysyl overglycosylation) of the portion of the triple helical domain of abnormal type I collagen molecules N-terminal with respect to the defect localization.

Prolidase deficiency in two siblings with chronic leg ulcerations. Clinical, biochemical, and morphologic aspects.

Prolidase deficiency occurred in two sisters suffering from recurrent leg ulcers that appeared in early childhood. The patients presented the typical clinical symptoms of the disease, including characteristic facies, dermatologic manifestations of the lower extremities, splenomegaly, and hematologic anomalies. Large amounts of iminodipeptides were excreted into the urine, and prolidase activity in their erythrocytes was virtually absent. Changes associated with a connective-tissue disorder were demonstrated by light and electron microscopic studies of the patients' apparently normal skin. Collagen fibers were smaller than in controls and were irregularly packed; the fibrils had normal aspect but were significantly smaller than in one age-matched control. Elastin fibers appeared altered both in size and structure.

Preparation from keratin sulfate of substrates for the measurement of 2-acetamido-2-deoxy-D-glucose 6-sulfate sulfatase and (1 goes to 3)-N-acetyl-beta-D-glucosaminidase.

An extract of bacterial cells Pseudomonas sp. IFO-13309 grown on medium containing 0.1% bovine cornea keratan sulfate of low sulfate content degraded exhaustively bovine cornea keratan sulfate to give 2-acetamido-2-deoxy-beta-D-gluco-pyranosyl 6-sulfate-(1 goes to 3)-D-galactose, isolated by gel filtration on Sephadex G-25 and purified by preparative paper chromatography. This was reduced with sodium borotritide to give 2-acetamido-2-deoxy-beta-D-glucopyranosyl 6-sulfate-(1 goes to 3)-D-[1-3H]galactitol, purified by gel filtration on Sephadex G-15, which was an excellent substrate for the measurement of 2-acetamido-2-deoxy-D-glucose 6-sulfate sulfatase. The reduced, radioactive monosulfated disaccharide was desulfated with methanolic 70mM hydrogen chloride and purified by gel filtration on Sephadex G-15 to give O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-(1 goes to 3)-D-[1-3H]galactitol, which allowed the measurement of (1 goes to 3)-N-acetyl-beta-D-glucosaminidase. This enzyme may participate in the normal degradation of keratan sulfate.

A simple method for quantitative estimation of collagen type III to type I ratio in soft tissues.

In the present paper a relatively rapid and simple method for estimating the ratio of collagen type III to type I in soft tissues is proposed. The ratio Gly/Ala is determined in pure collagen samples obtained from pepsin digests of the tissues. Since this ratio varies linearly depending on the composition of the mixtures of the two collagen types, it is shown that the percentage content of the two collagen types is easily calculated.

Influence of flavonoid-copper complexes on cross linking in elastin.

A protective and curative effect of some flavonoids on collagen maturation in lathyrism has been previously demonstrated. This action appeared to involve cross linking of collagen. This paper deals with the effect in vitro of flavonoids and flavonoid-copper complexes on the oxidative deamination of lysine epsilon-amino groups in [4,5-3H]-lysine-labelled elastin. Flavonoids alone do not affect the reaction but it is evident that some flavonoid copper complexes are strongly effective: the aldehyde groups that are quickly formed then enable cross linking to occur. The lysine epsilon-amino groups of elastin are specifically concerned: in fact no effect was observed on free [4,5-3H]-lysine or on [4,5-3H]-lysine-labelled proteins obtained from mouse liver. The lysyl oxidase seems to interfere with the flavonoid-copper complees to regulate the oxidative deamination of lysine epsilon-amino groups.

Hydroxylysine glycosides: preparation and analysis by reverse phase high performance liquid chromatography.

Hydroxylysine diglycoside was prepared from hydrolyzed sponge by a combination of ion-exchange chromatography and gel filtration. The product obtained was chemically pure on amino acid analyzer and was active as substrate of the enzyme alpha-(1----2)glucosidase. Hyl monoglycoside was prepared by mild acid hydrolysis from diglycoside. A reverse phase HPLC system was devised for Hyl glycosides, hydroxylysine and other basic amino acids. The separation was achieved using an octadecyl bonded silica column on the compounds derivatized with dabsyl chloride to produce di-dabsyl derivatives. Elution was followed in the visible region at 436 nm. In the conditions used no or very low amount of mono-dabsyl derivatives was observed. Hyl di- and monoglycoside resulted a mixture of two diastereoisomers, which form during the alkaline hydrolysis. The separation of the diastereoisomers of each compound depended on pH and ionic strength of the eluent in the HPLC column, whereas they were not separated by our short column on amino acid analyzer. The HPLC system was also used for the analysis of Hyl glycosides on two collagen preparations.

Effect of the triterpenoid fraction of Centella asiatica on macromolecules of the connective matrix in human skin fibroblast cultures.

The mechanism of action of the total triterpenoid fraction extracted from Centella Asiatica (TTFCA) was evaluated using human skin fibroblasts cultures as the experimental system. In particular its influence on the biosynthesis of collagen, fibronectin and proteoglycans was considered. The presence of TTFCA (25 micrograms/ml) does not seem to affect cell proliferation, total protein synthesis or the biosynthesis of proteoglycans in a significant way. A statistically important increase was observed in the percentage of collagen and, as revealed by immunofluorescence measurements, in cell layer fibronectin. This effect on collagen and fibronectin may help to explain the action of TTFCA in promoting wound healing, and suggests an interesting working hypothesis for its action on basal endothelia.

Localization of a structural defect in type I procollagen in a patient affected with the severe non-lethal form of Osteogenesis imperfecta.

A case of severe non-lethal Osteogenesis imperfecta was studied. The patient's cultured skin fibroblasts synthesised a mixed population of type I collagen chains some of which showed abnormal behaviour on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Further analysis revealed that two types of alpha 1(I) chains were synthesised, both an abnormal, slower migrating and a normal species. A small defect in one allele of one of the type I procollagen chains could lead to the larger size of the abnormal chains, probably caused by overmodifications of the triple helical region. CNBr peptide mapping allowed us to localise the defect midway along the triple helix: the defect site could be assigned to the region between the alpha 1(I)CB-3 and CB-7 peptides. The abnormal alpha 1(I) chains synthesised by the patient's cells had a melting temperature which was about 2 degrees C lower than normal chains. The results appear to be in agreement with the defect localisation and the phenotype.

Hypothesis on the role of hydroxylysyl glycosides in collagen fibre organization.

Hydroxylysine (Hyl) glycosides were determined both on the collagen produced by in-vitro cultures of human skin fibroblasts and pig articular chondrocytes and on the insoluble collagen extracted from bovine cornea and sclera. The disorganized collagen present in the culture medium showed a Hyl di- to mono-glycoside ratio markedly higher than the molecules extracted from the cell layer, where electron microscopy investigations demonstrated the real presence of collagen fibres. Moreover in bovine cornea the insoluble collagen showed a ratio Hyl di- to mono-glycoside significantly higher than in sclera, which, on the contrary, possesses fibres with larger mean diameter. The possible conversion of Hyl diglycoside to monoglycoside was suggested by the demonstration of an alpha (1----2) glucosidase activity on Hyl diglycoside , in an enzyme extract from cultured human skin fibroblasts. A role for the processes of collagen glycosylation and deglycosylation was thus considered.

Glycosaminoglycan alterations in osteogenesis imperfecta.

Urinary GAGs from patients affected with Osteogenesis Imperfecta (O.I.) type I, type II and type III - according to Sillence et al. (1979) - have been investigated. Galactosamine to glucosamine ratio resulted significantly decreased in O.I. type II and III, whereas smaller differences in the mildest type of the disease were observed. Cellulose polyacetate electrophoresis of urinary GAGs purified from some patients showed the presence of a slowly moving polysaccharide substance, which did not appear in normal subjects. Moreover some pathological fractions, mainly constituted of ChS on the basis of chemical analysis and electrophoretic behaviour, were not digested by testicular hyaluronidase and presented anomalous structures as compared with the corresponding normal ones. These results seem to indicate that in some forms of O.I. the metabolic defect(s) not only affects the synthesis or the catabolism of a particular type of collagen, but also involves the proteoglycan component of the connective tissue.

Moderately severe osteogenesis imperfecta: biochemical studies showing variable defect localization in the triple-helical domain of type I collagen.

This report describes the biochemical investigations on six patients affected by a moderate form of Osteogenesis Imperfecta (type IV according to the Sillence classification). Biochemical characterization of type I collagen produced by skin fibroblasts showed considerable heterogeneity: in three patients out of six, collagen appeared normal; while in the three others a structural defect in the protein was present. In these probands the mutations were localized in different regions of the triple helix domain (corresponding to peptides alpha 1(I)CB6 and alpha 1(I)CB7). In two probands showing the defect in alpha 1(I)CB7, a decrease of the thermal stability of the protein was present.

Osteogenesis imperfecta mutations may probe vital functional domains (e.g. proteoglycan binding sites) of type 1 collagen fibrils.

Osteogenesis imperfecta (OI) is a disease characterized by bone malformations caused by mutations in type 1 collagen. Since many of the 338 possible glycine mutations have not been observed in clinical practice, is this due to chance alone? Because only 83 mutations have been reported in 126 patients, we conclude that many mutations are absent from clinical data for non-random causes. Mutations affecting vital intermolecular interactions in the extracellular matrix (e.g. potential collagen binding sites for proteoglycans) may result in non-viable fetuses that do not progress to clinical status. Some mutations may be silent because they do not significantly affect normal function. The total number of clinically active mutations that will be observed may be far fewer than the potential 338 maximum.

The structural characterization and bilirubin-binding properties of albumin Herborn, a [Lys240-->Glu] albumin mutant.

We report the molecular defect of albumin Herborn, a new genetic variant of human serum albumin which has been found in Germany. Isoelectric focusing analysis of CNBr fragments from the purified variant allowed us to localize the mutation in fragment CNBr 3 (residues 124-298). This fragment was isolated on a preparative scale and subjected to tryptic and V8 protease digestion. Sequence determination of the abnormal tryptic and V8 peptides revealed that the variant arises from the substitution Lys240-->Glu. The -2 charge change of albumin Herborn, which is probably due to a A-->G transition in the first position of the corresponding codon in the structural gene, has no significant effect on its electrophoretic mobility under non-denaturating conditions. Therefore we have assumed that residue 240, which has been implicated in the bilirubin primary binding site (Jacobsen, C. (1978) Biochem. J. 171, 453-459), is buried. The binding of bilirubin and biliverdin by albumin Herborn was quantified using the fluorescence quenching method. The apparent equilibrium association constants (Ka +/- SD) and the number of high-affinity binding sites (n) of the defatted variant for bilirubin and biliverdin were Ka = 1.03 +/- 0.18 x 10(8) M-1, n = 1.07; and Ka = 7.48 +/- 1.10 x 10(6) M-1, n = 1.01, respectively. The Ka values are about 93.3% and 99.1% of the values found for the normal protein under the same conditions. These results strongly suggest that Lys240 of human serum albumin is not the basic residue involved in ion pairing with one of the carboxylate groups of bilirubin at its high-affinity site.