Lipoprotein(a) levels in relation to albumin concentration in childhood nephrotic syndrome

BACKGROUND

Lipoprotein(a) [Lp(a)] is a lipoprotein consisting of a low-density lipoprotein (LDL) particle linked to a polymorphic glycoprotein, apoprotein(a) [apo(a)]. Prior studies have reported high Lp(a) levels in the nephrotic syndrome, but it is still controversial whether this is due to the degree of hypoalbuminemia or proteinuria.

METHODS

To investigate a model of nephrotic syndrome in the absence of renal failure, we studied a group of 84 children in different clinical stages of the disease for a period of five years. We evaluated the direct relationships between lipoproteins, including Lp(a), and/or plasma albumin and proteinuria.

RESULTS

Lp(a) levels were significantly higher in the subjects with the active disease compared with patients in remission, and were also significantly different when subjects were ranked by albumin quartiles. Multiple regression analysis revealed that Lp(a) levels were inversely correlated with apo(a) isoform size and plasma albumin levels but not with the proteinuria/creatinine clearance ratio. Among subjects in complete remission, Lp(a) levels were different in patients with albumin levels below or above the fifth percentile. After the improvement of the clinical stage of the disease, the Delta% variation of albumin levels was related to the Delta% of apoB and LDL cholesterol (LDL-C), but not with the Delta% variation of Lp(a), whereas the Delta% variation of LDL-C was, in turn, related to the Delta% of Lp(a) levels.

CONCLUSIONS

These results suggest that in the childhood nephrotic syndrome, the increased Lp(a) levels are mainly related to hypoalbuminemia, probably through a mechanism involving apoB overproduction, which leads to an increased number of LDL particles to be converted into Lp(a).

High-dose cyclophosphamide followed by autografting can improve the outcome of relapsed or resistant non-Hodgkin's lymphomas with involved or hypoplastic bone marrow

We report our experience of high-dose cyclophosphamide (HDCY) followed by high-dose therapy (HDT) and peripheral blood progenitor cell (PBPC) autografting in patients with diffuse, intermediate and high-grade non-Hodgkin's lymphomas who have failed conventional treatment. From 1991 to 1996, 54 consecutive patients pre-treated with a median of two chemotherapy lines entered the study. Eighteen patients (33%) were still responders to conventional chemotherapy (sensitive relapse), and 20 patients (37%) were in partial response (PR) after chemotherapy (CT). Sixteen patients (30%) were resistant to conventional CT either at presentation (non responder) or in relapse (resistant relapse). Thirty-nine patients had bone marrow involved by disease and fifteen had an hypoplastic marrow following conventional treatment. Patients received HDCY (7gr/m2) and G-CSF or GM-CSF in order to collect PBPC. Median collected CD34+ cells was 12.3 x 10(6)/Kg (range 0.7-197). After HDT (BEAM or Melphalan + TBI) 50 patients underwent PBPC autografting. According to intention to treat, 44 (81%) of 54 patients achieved complete remission (CR) (50% after HDCY and 31% after HDT). Procedure related death occurred in 6 patients (11%), one after HDCY and 5 after autografting. Twenty-nine (66%) of 44 patients are still in CR, 7 to 63 months (median 27 months) after the procedure. Three-year probability of survival, disease-free survival and progression-free survival are 63%, 64% and 52% respectively. In conclusion, HDCY is an effective procedure not only in mobilizing PBPC, but also in reducing tumour burden. HDT with PBPC support may further improve the outcome in this category of high-risk non-Hodgkin's lymphomas.

Structural investigations on human erythrocyte acylpeptide hydrolase by mass spectrometric procedures

The complete primary structure of human erythrocyte acylpeptide hydrolase has been determined by using a combination of different mass spectrometric procedures and sequencing techniques. These data allowed us to correct the incomplete nucleotide sequence of the DNF15S2 locus on the short arm of human chromosome 3 at region 21, coding for the enzyme. The protein consists of 732 amino acid residues and is acetylated at the N-terminus. Alkylation experiments on the native enzyme demonstrated that all 17 cysteine residues present in the polypeptide chain are in reduced form. Multiple sequence alignment did not reveal striking similarity with proteases of known tertiary structure with the exception of members of the serine oligopeptidase family. Limited proteolysis experiments generated a C-terminal portion, containing all the catalytic triad elements responsible for proteolytic activity, and an N-terminal domain of unknown function, both still strongly associated in a completely active nicked form. The site of tryptic hydrolysis was identified as Arg193. The secondary structural organization of the protease domain of the enzyme is consistent with the alpha/beta hydrolase fold.

[Doppler echography in the intraoperative assessment of the liver circulation: an experimental study in swine]

In this experimental study the authors report an experience in the evaluation of hepatic blood flow with intraoperative echo-Doppler during orthotopic liver transplantation and side-to-side or end-to-side portacaval shunt. Doppler ultrasonography studied the flow of portal vein, hepatic artery and inferior vena cava before the recipient hepatectomy, and after reperfusion during liver grafting. Furthermore echo-Doppler of the portal system was performed to confirm portacaval shunt efficacy. Usually intraoperative Doppler ultrasonography may give informations about the patency of the shunt and regarding the development of early hepatic artery thrombosis during liver transplantation, but often unclear is the exact evaluation of the velocity of the blood flow through the liver. Further experimental studies and clinical evaluations need to find safe parameters and markers of vascular alteration using this superior diagnostic procedure.

[Backache from exertion in health personnel of the Istituti Ortopedici Rizzoli in Bologna. A case-control study of the injury phenomenon in the 10-year period of 1987-1996]

Low-back (lumbosacral) injuries are known as one of more important occupational health problems in health care workers, because of high prevalence and impact of socioeconomic costs. To investigate the risk of low-back pain in hospital workers into the Istituti Ortopedici Rizzoli in Bologna (Italy), a retrospective study was carried out. The adapted study design was the matched (1:1 for age) case-control, enlisting the first injuries occurred in the hospital staff like cases, and personnel in force, matched for age and year of happened case, like controls. Information about diagnoses and occupational data was obtained from the current informative systems. Comparison with a control group suggests the validity of the work-relatedness of low-back pain in nursing and health aides (OR: 21.67; LC: 4.69-196.56), in nurses (OR: 20.21; LC: 4.81-177.95), in therapists (OR: 16.36; LC: 2.80-163.54) and in X-ray technicians (OR: 13.64; LC: 2.25-138.99). The risk of occupational injury is not homogeneously allocated into the hospital, and significatively prevails in the orthopaedic wards, in the plaster-rooms, in the operating blocks and in the sterilization plants. Specific manual handling were associated with an increased risk of back pain, while some non occupational factors like cigarette smoking, previous trauma leading to hospital admission, and number of children in female, were resulted weakly associated.

Functional and structural analysis of cis-proline mutants of Escherichia coli aspartate aminotransferase

To elucidate the role of the two conserved cis-proline residues of aspartate aminotransferase (AspAT), one double and two single mutants of the enzyme from Escherichia coli (EcAspAT) were prepared: P138A, P195A and P138A/P195A in which the two prolines were replaced by alanine. The crystal structures of P195A and P138A/P195A have been determined at 2.3-2.1 A resolution. The wild-type geometry, including the cis conformation of the 194-195 peptide bond is retained upon substitution of proline 195 by alanine, whereas the trans conformation is adopted at the 137-138 peptide bond. Quite surprisingly, the replacement of each of the two prolines by alanine does not significantly affect either the activity or the stability of the protein. All the three mutants follow the same pathway as the wild type for unfolding equilibrium induced by guanidine hydrochloride [Herold, M., and Kirschner, K. (1990) Biochemistry 29, 1907-1913]. The kinetics of renaturation of P195A, where the alanine retains the wild-type cis conformation, is faster than wild type, whereas renaturation of P138A, which adopts the trans conformation, is slower. We conclude that cis-prolines seem to have been retained throughout the evolution of aspartate aminotransferase to possibly play a subtle role in directing the traffic of intermediates toward the unique structure of the native state, rather than to respond to the needs for a specific catalytic or functional role.

[Unreliability of the lidocaine test as a prognostic indicator of acute liver failure: an experimental study in swine]

It has been reported, in the recent literature, that fifteen minutes lidocaine-MEGX (monoethylglycinexylidide) test can also be used in case of acute hepatic failure because in these conditions the test allows a rapid evaluation of the hepatic damage, uneffected by te infusion of liquid or fresh plasma which can alter conventional laboratory parameters. The Authors have demonstrated, in a experimental model, the unreliability of the lidocaine-MEGX test as measure of functional hepatic damage in the early stages of an acute liver failure. The slight decrease of the rate of the MEGX test in these animals submitted to one-stage hepatic devascularization suggests that the hepatic failure is not so early detected by this specific liver function test. Accordingly, the 15' MEGX test should not be used as isolated discriminatory measure to detect an early hepatic failure following fulminant hepatitis or an early hepatic insufficiency as a result of extensive liver resection.

The length of a single turn controls the overall folding rate of "three-fingered" snake toxins

Snake curaremimetic toxins are short all-beta proteins, containing several disulfide bonds which largely contribute to their stability. The four disulfides present in snake toxins make a "disulfide beta-cross"-fold that was suggested to be a good protein folding template. Previous studies on the refolding of snake toxins (Ménez, A. et al. (1980) Biochemistry 19, 4166-4172) showed that this set of natural homologous proteins displays different rates of refolding. These studies suggested that the observed different rates could be correlated to the length of turn 2, one out of five turns present in the toxins structure and close to the "disulfide beta-cross". To demonstrate this hypothesis, we studied the refolding pathways and kinetics of two natural isotoxins, toxin alpha (Naja nigricollis) and erabutoxin b (Laticauda semifasciata), and two synthetic homologues, the alpha mutants, alpha60 and alpha62. These mutants were designed to probe the peculiar role of the turn 2 on the refolding process by deletion or insertion of one residue in the turn length that reproduced the natural heterogeneity at that locus. The refolding was studied by electrospray mass spectrometry (ESMS) time-course analysis. This analysis permitted both the identification and quantitation of the population of intermediates present during the process. All toxins were shown to share the same sequential scheme for disulfide bond formation despite large differences in their refolding rates. The results presented here demonstrate definitely that no residues except those forming turn 2 accounted for the observed differences in the refolding rate of toxins.

[A facilitated technique for donor hepatectomy: experimental study in the pig]

A facilitated technique of porcine donor hepatectomy is reported underlying the possibility to make an easy procedure without in situ portal perfusion, so other abdominal organs supplied by superior mesenteric system can be harvested at the same time. The viability of those grafts procured without in situ portal perfusion has been compared with an other group of livers harvested with in situ aortic and portal flushing. The evaluation of the histology and early graft function two hours after riperfusion has shown no differences between both groups.

Lipoprotein(A) levels and apoprotein(a) phenotypes in a Sicilian population

The aim of this study was to evaluate the influence of lipoprotein(a) levels and apoprotein(a) isoform size in determining the low cardiovascular risk of a rural, inland Sicilian population. Plasma levels of total cholesterol, LDL-cholesterol, HDL-cholesterol, triglycerides, lipoprotein(a) and apoprotein B and AI were measured in a sample of 278 subjects (141 males, 137 females) representative of a population of 1351 subjects (622 males, 729 females). The apoprotein(a) isoforms were also identified. Results indicated that the levels of the common lipo-apoliproprotein parameters were lower than those of other populations, while lipoprotein(a) plasmatic levels and apoprotein(a) isoform distribution were similar to those of other Caucasian populations. The distribution of lipoprotein(a) levels was skewed to the right, with a higher prevalence of low levels, the apoprotein(a) isoforms most strongly represented in our sample were of intermediate size (25-27 kringles IV). Univariate analysis showed that lipoprotein(a) levels were correlated to apoprotein(a) isoform size (R = -0.48, p < 0.001) and to the age of the subjects (R = +0.13, p < 0.01). In a multiple regression analysis, lipoprotein(a) levels were correlated to the apoprotein(a) isoform size of the homozygous isoforms or smaller heterozygous isoforms, while larger heterozygous forms were not correlated. In conclusion, our study showed that in our population, lipoprotein(a) levels and apoprotein(a) isoforms are similar to those of other Caucasian populations. Other factors, such as the physical activity of a rural population or the Mediterranean diet, must be considered in order to explain the lower cardiovascular risk of this population.

[Urethral ultrasonography: a new diagnostic approach in the evaluation of stenosis of the male urethra]

Endoscopy and radiological investigations, using contrast medium, have been the only available methods in the study of urethral pathology. In this article we have suggested using the technique of urethral ultrasound to visual the urethra before using ultrasound. Our study included 16 patients ranging in age from 23 to 82 years with urethral stenosis already diagnosed or suspected on the basis of symptomatology at referral. When the urethra is distended by the gel it appears to be a black tube with a smooth surface which does not produce echoes. The gel determines the compression of the corpus spongiosa which is normally a rigid, inelastic tissue because of the prevalence of collagen fibres. The possibility of adding further gel inside the urethra to confirm the echo results even after the introduction of a standard quantity at the start of the procedure we feel is the second definitive advantage with this method. In fact it was possible in our survey by increasing the quantity of gel to currently differentiate between complete and partial stenosis. No patient suffered from a temperature or using retention after this test. No-one complained of discomfort during the procedure. The anterior and posterior urethra are separated by the external sphincter. Radiological images are better obtained from the anterior urethra than the posterior urethra. We have found that ultrasound is a reliable and possibly superior method. With this method it is possible to obtain more information about the peri-urethral scar tissue, the length of the stenosis, the degree of spongy fibrosis therefore allowing more precise urethrotomies to be performed. The test is comfortable for the patient, simple and reduces the risk of radiation. The disadvantages of this method are that the posterior urethra is poorly visualized. Examination of the posterior urethra using a trans-scrotal approach have given poor results. It is necessary to avoid excessive pressure on the probe which could give false positive results. To eliminate this problem it is necessary to scan longitudinally and transversally within the lumen at various stages of filling. It is important to note that air present within the gel could simulate urethral calculi, if the image has been frozen. During the test the presence of air helps to evaluate the degree of flow through the stenosis.

Structural characterization and independent folding of a chimeric glycoprotein comprising granulocyte-macrophage colony stimulating factor and erythropoietin sequences

MEN 11300 is a hybrid glycoprotein of 297 amino acids obtained by fusion of the cDNA encoding GM-CSF with the cDNA encoding EPO followed by transfection of the hybrid gene into CHO cells. The oligonucleotide construct incorporated a spacing sequence between the two individual cDNAs which encodes eight amino acids constituting a linker peptide intended to separate the GM-CSF and EPO moieties. The recombinant MEN 11300 protein was submitted to a detailed structural characterization including the verification of the entire amino acid sequence, the assignment of the disulfide bridges pattern, the identification of the glycosylation sites and the definition of the glycosidic moiety, including site-specificity. Partial processing of the C-terminal Arg residue and the occurrence of N-glycosylation sites at Asn27, Asn155, Asn169, Asn214 were established. Moreover, O-glycosylation at Ser257 and at the N-terminal region was also detected. A large heterogeneity was observed in the N-glycans due to the presence of differently sialylated and fucosylated branched complex type oligosaccharides whereas O-linked glycans were constituted by GalGalNAc chains with a different number of sialic acids. The disulfide bridges pattern was established by direct FABMS analysis of the proteolytic digests or by ESMS analysis of HPLC purified fractions. Pairing of the eight cysteine residues resulted in Cys54-Cys96, Cys88-Cys121, Cys138-Cys292, and Cys160-Cys164. This S-S bridges pattern is identical to that occurring in the individual natural GM-CSF and EPO, thus showing that the two protein moieties in MEN 11300 can independently acquire their native three-dimensional structure.

Modulation of gamma-aminobutyric acid (GABA) receptors and the feeding response by neurosteroids in Hydra vulgaris

Gamma-Aminobutyric acid (GABA) receptors are present in membrane preparations from Hydra vulgaris, one of the most primitive organisms with a nervous system. These receptors are sensitive to muscimol and benzodiazepines and appear to be important in the regulation of the feeding response. The effects of neurosteroids, general anaesthetics, and GABA antagonists on GABA(A) receptors in membranes prepared from Hydra and on the feeding response have now been investigated. The neurosteroids tetrahydroprogesterone and tetrahydrodeoxycorticosterone increased [3H]GABA binding to hydra membranes with nanomolar potency (EC50, 141+/-11 and 623+/-36 nM, respectively) and high efficacy (maximal increase 79+/-6.5 and 62+/-4%, respectively), whereas the 3beta-hydroxy epimer of tetrahydroprogesterone was ineffective. The benzodiazepine receptor ligands diazepam (100 microM), clonazepam (100 microM) and abecarnil (30 microM) enhanced [3H]GABA binding to Hydra membranes by 22, 20 and 24%, respectively; effects abolished by the specific benzodiazepine antagonist flumazenil (100 microM). On the contrary, the peripheral benzodiazepine receptor ligand 4'chlorodiazepam failed to affect [3H]GABA binding to Hydra membranes. The general anaesthetics propofol and alphaxalone similarly increased (+38% and +30% respectively) [3H]GABA binding. Moreover, [3H]GABA binding to Hydra membranes was completely inhibited by the GABA(A) receptor antagonist SR 95531, whereas bicuculline was without effect. The modulation of GABA(A) receptors in vitro by these various drugs correlated with their effects on the glutathione-induced feeding response in the living animals. Tetrahydroprogesterone and tetrahydrodeoxy-corticosterone (1 to 10 microM) prolonged, in a dose-dependent manner, the duration of mouth opening induced by 10 microM glutathione, with maximal effects of +33 and +29%, respectively, apparent at 10 microM neurosteroid. Alphaxalone (10 microM) similarly increased (+33%) the effect of glutathione. The effects of steroids on the feeding response were inhibited by SR 95531 in a dose-dependent manner; t-butylbyclophosphorothyonate (1 microM), a specific Cl- channel blocker, which per se, like picrotoxin but not bicuculline, shortened the duration of the response, also counteracted the steroids effects at 1 microM. These results suggest that the modulation of GABA(A) receptors by steroids is an ancient characteristic of the animal kingdom and that the pharmacological properties of these receptors have been highly conserved through evolution.

Topology of the calmodulin-melittin complex

The topology of the Ca2+-calmodulin-melittin ternary complex has been investigated by a combined strategy which integrates limited proteolysis and cross-linking experiments with mass spectrometric methodologies. The rationale behind the methods is that the interface regions of two interacting proteins are accessible to the solvent in the isolated molecules, whereas they become protected following the formation of the complex. Therefore, when limited proteolysis experiments are carried out on both the isolated proteins and the complex, differential peptide maps are obtained from which the interface regions can be inferred. Alternatively, cross-linking reactions performed under strictly controlled conditions lead to the identification of spatially closed amino acid residues in the complex. Mass spectrometry can be employed in both procedures for the definition of the cleavage sites and to identify covalently linked residues. Our results show that melittin interacts with calmodulin by adopting a parallel orientation, i.e. the N and C-terminal halves of the peptide are anchored to the amino and carboxy-terminal domains of the protein, respectively. This orientation is inverted with respect to all the peptide substrates examined so far. A model of the complex was designed and refined on the basis of the experimental results, supporting the above conclusions. This finding reveals a further dimension to the already remarkable capability of calmodulin in binding different protein substrates, providing this protein with the capability of regulating an even larger number of enzymes.

Monoethylglycinexylidide (MEGX) test in patients with different liver diseases

We have studied the levels of the MEGX test in a heterogeneous group of 50 patients with chronic liver disease and with hepatic tumours and we have compared it with the routine LFTS commonly used to assess liver function and with the Child-Pugh Classification system. Our results demonstrate a statistically significant relationship between MEGX levels and prothrombin levels, and between MEGX and alkaline phosphatase and a highly significant relationship between MEGX and cholinesterase. In the group of patients with cirrhosis we found a statistically significant difference amongst the MEGX levels in the 3 classes of the Child Classification system. The MEGX test is a good index in evaluating hepatic function and it is also quick and easy to perform and capable of determining residual liver function. The test can also be used for preoperative assessment in patients with focal hepatic lesions, especially in those with a previous history of cirrhosis, and in patients with functional hepatic disease.

[Primary carcinoma of the medium and distal third of the biliary tract. Personal experience in diagnosis and treatment]

A group of 36 patients, observed between 1985 and 1995, with primary carcinoma localized in the medium and distal third of biliary tract is presented. Sixteen patients (44.4%) have undergone radical surgical treatment and the other twenty (55.5%) received a palliative procedure with an average survival of 33.7 months and 8.8 months for the first and second group respectively. The actual location of the tumor was obtained by ERCP and its final stadiation occurred during operation mainly trough intraoperative ultrasonography. The preoperative biliary drainage led to a better postoperative outcome and to a better prognosis. In the case where the tumor was localized only in the third medium of the biliary tract, a limited resection of the biliary tract with Roux's reconstruction was carried out. Otherwise in the cases where the lesion was localized in the border between the medium and distal third and in the distal tract, a duodenocephalopancreasectomy was performed. Even if this is a group of lesions with a poor prognosis, when it is possible to have an early diagnosis it is necessary to carry out an aggressive surgical treatment considering the higher percentage of survival compared to any other palliative procedure.

Managed occupational health care in an HMO

This paper describes the efforts of an HMO to improve its delivery of occupational health services. Customer needs identification, occupational health structure, data systems, case management, clinical guidelines, and quality management are outlined. Our experience suggests that high-quality occupational health services can be integrated into managed care systems thereby offering cost-effective care to large numbers of workers. Comparing 1991 to 1995, physician authorization of total disability days was reduced 17.9% per disability case (p < .0001). Based on July 1994 to June 1995 Oregon State Accident Insurance Fund (SAIF Corporation) data, HMO average total claim cost was $916/claim representing respectively, a 21% and a 20% reduced cost compared to two PPO model programs (MCO 00 and MCO 01). Patient satisfaction data indicated that 90% of patients were satisfied or very satisfied with the physician they saw. The savings appear to be due to cost-effective treatment and rapid return to work.

Liver is not the unique site of synthesis of beta 2-glycoprotein I (apolipoprotein H): evidence for an intestinal localization

Apolipoprotein H is a protein of about 50 kilodaltons, structurally related to the regulators of the complement activation family. Its physiological function is poorly understood but it has been implicated in lipid metabolism and coagulative pathways. The major site of synthesis is thought to be the liver. Several reports indicate that apolipoprotein H is the antigen of the antiphospholipid antibodies and also behaves as an acute-phase reactant. Moreover, 40% of plasma apolipoprotein H is associated with very low-density lipoprotein, high-density lipoprotein, and postprandial chylomicrons. In this study we investigated other sites of synthesis by reverse transcription/polymerase chain reaction and we found apolipoprotein H mRNA expression in intestinal cell lines and tissues. Immunohistochemistry was performed on various fresh and paraffin-embedded tissues and apolipoprotein H was immunolocalized in the cytoplasm of hepatocytes and epithelial cells from colon and jejunum. This study indicates that apolipoprotein H is expressed at both mRNA and protein levels in enterocytes.

Endothelin-1 in diabetic and nondiabetic men with erectile dysfunction

PURPOSE

We evaluated plasma concentration of endothelin-1 in diabetic and nondiabetic men complaining of erectile dysfunction, and the variation of endothelin-1 in cavernous body blood during intracavernous injection of prostaglandin E1.

MATERIALS AND METHODS

We evaluated plasma concentrations of endothelin-1 in venous blood of 20 men with erectile dysfunction, 10 with and 10 without diabetes. Plasma concentration of endothelin-1 was also evaluated in the cavernous body blood of the 20 men with erectile dysfunction, during erection induced by intracavernous injection of 10 micrograms prostaglandin E1. A severe vasculogenic component of erectile dysfunction was excluded in all patients.

RESULTS

Basal plasma concentration of endothelin-1 in the cubital vein was increased in nondiabetic (1.13 +/- 0.4 pg./ml.) and in diabetic (1.80 +/- 0.2 pg./ml.) patients with erectile dysfunction, compared to control men (0.64 +/- 0.1 pg./ml.) (p < 0.0005 and p < 0.0001, respectively), and in diabetic compared with nondiabetic patients (p < 0.002). No difference and close correlation were observed in the concentration of endothelin-1 in the cavernous body blood evaluated 5 minutes and 30 minutes after injection of prostaglandin E1 (r = 0.89, p < 0.0001, y = 0.98 x + -0.066). The concentration of endothelin-1 in the cavernous body blood evaluated 30 minutes after injection of prostaglandin E1 did not show any difference compared to peripheral venous concentration of the peptide in the 2 patient groups. Concentrations of endothelin-1 in the peripheral vein and the cavernous body blood were not different in patients with a full erection compared with incomplete penis erection after injection of prostaglandin E1 in the cavernous body.

Effect of glutaredoxin and protein disulfide isomerase on the glutathione-dependent folding of ribonuclease A

Protein folding, associated with oxidation and isomerization of disulfide bonds, was studied using reduced and denatured RNase A (rd-RNase A) and mixed disulfide between glutathione and reduced RNase A derivative (GS-RNase A) as starting materials. Folding was initiated by addition of free glutathione (GSH + GSSG) and was monitored by electrospray mass spectrometry (ESMS) time-course analysis and recovery of the native catalytic activity. The ESMS analysis permitted both the identification and quantitation of the population of intermediates present during the refolding process. Refolding of rd-RNase A and GS-RNase A was also performed in the presence of glutaredoxin (Grx) and/or protein disulfide isomerase (PDI). All the analyses indicate a pathway of sequential reactions in the formation of native RNase A. First, the reduced protein reacts with a single glutathione molecule to form a mixed disulfide which then evolves to an intramolecular S-S bond via thiol-disulfide exchange. Only at this stage, the intermediate containing one intramolecular S-S reacts with a further glutathione molecule, reiterating the process. An analogous mechanism occurs in the refolding of GS-RNase A. The structural analysis of the intermediates formed during the refolding of RNase A showed for the first time that Grx is actually able to catalyze both formation and reduction of mixed disulfides involving glutatione. In both refolding processes, starting from either rd-RNase A or GS-RNase A, Grx displays a significant catalysis at the early stages of the process. Addition of PDI led to a net catalysis of the entire process without appearing to alter the refolding pathway. In the presence of both Grx and PDI, the two enzymes showed a synergistic activity either starting from rd-RNase A, as previously reported [Lundström, J., and Holmgren, A. (1995) J. Biol. Chem. 270, 7822-7828], or starting from GS-RNase A. Present data suggest that the synergistic effect can be explained assuming that Grx actually facilitates PDI action by catalyzing formation or reduction of mixed disulfides. The mixed disulfides are then rapidly converted into intramolecular disulfides in the presence of PDI. These steps are repeated sequentially throughout the whole refolding, resulting in an immediate formation of fully oxidized species even at the very beginning of the reaction. Finally, a Grx mutant, C14S Grx, in which one of the active site cysteine residues (Cys14) had been replaced by serine, had a similar effect on the distribution of folding intermediates, compared to the wild-type protein, thus demonstrating that Grx acts by a monothiol mechanism either in the reduction or in the oxidation step.

Surface topology of Minibody by selective chemical modifications and mass spectrometry

The surface topology of the Minibody, a small de novo-designed beta-protein, has been probed by a strategy that combines selective chemical modification with a variety of reagents and mass spectrometric analysis of the modified fragments. Under appropriate conditions, the susceptibility of individual residues primarily depends on their surface accessibility so that their relative reactivities can be correlated with their position in the tertiary structure of the protein. Moreover, this approach provides information on interacting residues, since intramolecular interactions might greatly affect the reactivity of individual side chains by altering their pKa values. The results of this study indicate that, while overall the Minibody model is correct, the beta-sheet formed by the N- and C-terminal segments is most likely distorted. This is also in agreement with previous results that were obtained using a similar approach where mass spectrometry was used to identify Minibody fragments from limited proteolysis (Zappacosta F, Pessi A, Bianchi E, Venturini S, Sollazzo M, Tramontano A. Marino G, Pucci P. 1996. Probing the tertiary structure of proteins by limited proteolysis and mass spectrometry: The case of Minibody. Protein Sci 5:802-813). The chemical modification approach, in combination with limited proteolysis procedures, can provide useful, albeit partial, structural information to complement simulation techniques. This is especially valuable when, as in the Minibody case, an NMR and/or X-ray structure cannot be obtained due to insufficient solubility of the molecule.

Stability of a thermophilic TIM-barrel enzyme: indole-3-glycerol phosphate synthase from the thermophilic archaeon Sulfolobus solfataricus

The stability and activity of indole-3-glycerol phosphate synthase from Sulfolobus solfataricus were studied as a function of pH and temperature. In this paper we focus on three points: (1) the long-term stability of the protein to irreversible denaturation at high temperature; (2) the short-term stability of the protein to reversible temperature-driven unfolding; and (3) the dependence of its activity on temperature. Results can be summarized as follows: (a) the same first-order kinetic constant (0.020+/-0.003 min-1) was determined at different pH values (6.5, 8.0 and 9.5) from long-term stability experiments at 80 degrees C; (b) short-term stability experiments revealed different behaviour in two different pH ranges (6.5-8.0, 8.5-9.5), suggesting that the melting temperature is higher at alkaline than at neutral pH; (c) the dependence of activity on temperature was investigated at pH 7.0 and 9.0, and a discontinuity was observed in the Arrhenius plot of kcat values at pH 9.0. We also investigated the stability in the presence of guanidinium chloride at 20 degrees C either at pH 7.0 or at pH 9.0, and we present data that indicate that the unfolding mechanism closely approaches a two-state model at pH 7.0 and a more complex mechanism at pH 9.0. Satisfactory fitting of the equilibrium unfolding transition obtained by fluorescence measurements at pH 9.0 required a model that involves a stable intermediate in addition to the native and unfolded forms. At 20 degrees C the folded conformation is more stable than the unfolded conformation by (14. 7+/-1.2) kJ/mol at pH 7.0 and by (25.5+/-1.8) kJ/mol at pH 9.0.