Low Molecular Weight End-Functionalized Poly(N-Vinylpyrrolidinone) for the Modification of Polypeptide Aminogroups

New monofunctionalized amphiphilic oligomers, poly ( N-vinyl pyrrolidinones) with an hydroxyl end group, were prepared by radical polymerization with 2-isopropoxyethanol, fractionated by both gel-permeation chromatography and fractional precipitation. The terminal hydroxyl group oligomers was activated and reacted with the amino groups of a model peptide, and a protein. These hydroxylated oligomers were also converted to carboxylate end group which were also activated and used as protein and peptide modifying agents.

Influence of PEGylation on the Release of Low and High Molecular-Weight Proteins from PVA Matrices

Model proteins of therapeutic interest, ribonuclease, superoxide dismutase, catalase and albumin, extensively PEGylated with linear 5,000 Mw PEG, were entrapped in PVA hydrogels (31,000-50,000 Mw) by means of a cryogel procedure and the rate of release was evaluated. All of the PEGylated proteins were released from the gel, but at a lower rate than the unmodified proteins. The rate for both native and PEGylated species decreases as the molecular weight of the protein increased. Moreover, for both native and PEGylated proteins, the release rate was reduced when the gel was lyophilized before the release evaluation. Release was also dependent on the PVA Mw, in fact, for Mw's of 124,000-186,000, permanent entrapment was achieved. This behaviour was verified with PEGylated glucose oxidase and coline oxidase which were permanently retained in high Mw PVA. This study indicates the potential for the combination of PEGylation and cryogel entrapment of enzymes for the biotechnological and therapeutic applications.

Middle-term expansion of hematopoietic cord blood cells with new human stromal cell line feeder-layers and different cytokine cocktails

Cord blood (CB) is a source of hematopoietic stem cells (HSCs) and is an alternative to bone marrow for allogenic transplantation in patients with hematological disorders. The improvement of HSC in vitro expansion is one of the main challenges in cell therapy. Stromal components and soluble factors, such as cytokines, can be useful to induce in vitro cell expansion. Hence, we investigated whether feeder-layers from new stromal cell lines and different exogenous cytokine cocktails induce HSC expansion in middle-term cultures. CB HSC middle-term expansion was carried out in co-cultures on different feeder-layers exposed to three different cytokine cocktails. CB HSC expansion was also carried out in stroma-free cultures in the presence of different cytokine cocktails. Clonogenic tests were performed, and cell growth levels were evaluated. Moreover, the presence of VCAM-1 mRNA was assessed, and the mesenchymal cell-like phenotype expression was detected. All feeder-layers were able to induce a significant clonogenic growth with respect to the control culture, and all of the cytokine cocktails induced a significant increase in CB cell expansion indexes, even though no potential variation dependent on their composition was noted. The modulative effects of the different cocktails, exerted on each cell line used, was dependent on their composition. Finally, all cell lines were positive for CD73, CD117 and CD309, similar to mesenchymal stem cells present in adult bone marrow and in other human tissues, and negative for the hematopoietic markers. These data indicate that our cell lines have, not only a stromal cell-like phenotype, but also a mesenchymal cell-like phenotype, and they have the potential to support in vitro expansion of CB HSCs. Moreover, exogenous cytokines can be used in synergism with feeder-layers to improve the expansion levels of CB HSCs in preparation for their clinical use in allogenic transplantation.

PEG-Ara-C conjugates for controlled release

The antitumour agent 1-beta-D arabinofuranosilcytosyne (Ara-C) was covalently linked to poly(ethylene glycol) (PEG) in order to improve the in vivo stability and blood residence time. Eight PEG conjugates were synthesised, with linear or branched PEG of 5000, 10000 and 20000 Da molecular weight through an amino acid spacer. Starting from mPEG-OH or HO-PEG-OH, conjugation was carried out to the one or two available hydroxyl groups at the polymer's extreme. Furthermore, to increase the drug loading of the polymer, the hydroxyl functions of PEG were functionalised with a bicarboxylic amino acid yielding a tetrafunctional derivative and, by recursive conjugation with the same bicarboxylic amino acid, products with four or eight Ara-C molecules for each PEG chain were prepared. A computer graphic investigation demonstrated that aminoadipic acid was a suitable bicarboxylic amino acid to overcome the steric hindrance between the vicinal Ara-C molecules in the dendrimeric structure. In this paper we report the optimised conditions for synthesis and purification of PEG-Ara-C products with a low amount of remaining free drug, studies toward the hydrolysis of PEG-Ara-C and the Ara-C deamination by cytidine deaminase, pharmacokinetics in mice and cytotoxicity towards HeLa human cells were also investigated. Increased stability towards degradation of the conjugated Ara-C products, in particular for the highly loaded ones, improved blood residence time in mice and a reduced cytotoxicity with respect to the free Ara-C form was demonstrated.

Pegylated enzyme entrapped in poly(vinyl alcohol) hydrogel for biocatalytic application

A procedure for enzyme entrapment into matrices suitable for biocatalytic applications is reported. The method, which takes advantage of the stable formation of polyvinyl alcohol (PVA) hydrogels by freezing and thawing PVA aqueous solutions, was assayed using lipase as model enzyme. The leakage of lipase was minimised by using high molecular weight PVA and by previous conjugation of the enzyme to PEG. The immobilised PEG enzyme maintained its catalytic activity in organic solvents also, thus allowing enzymatic activity towards water insoluble substrates. The activity was largely increased reducing the diffusional constrain by cutting the matrices into slices of micron size. Matrix-entrapped lipase-PEG, when used in the hydrolysis of acetoxycoumarins, showed a conversion rate of about 10 times lower than the enzyme-PEG in the free form, and maintained regioselectivity when a diacetylated product was used as substrate.

Immunological properties of uricase conjugated to neutral soluble polymers

For a comparative study of immunological properties of protein-polymer conjugates, uricase was modified with (a) poly(N-vinylpyrrolidone) 6000 Da, (b) poly(N-acriloylmorpholine) 6000 Da, (c) branched monomethoxypoly(ethylene glycol) 10000 Da, and (d) linear monomethoxypoly(ethylene glycol) 5000 Da. Spectroscopic studies performed by UV, fluorescence, and circular dichroism did not show any relevant difference in protein conformation among the native and the conjugates. Immunological studies showed that both uricase antigenicity and immunogenicity were altered by polymer conjugation to an extent that depended upon the polymer composition; in particular, monomethoxypoly(ethylene glycol) 10000 Da remarkably reduced the protein antigenicity, while unexpectedly, the poly(N-vinylpyrrolidone) derivative presented higher antigenicity than the native protein. In Balb/c mice, the native protein elicited a rapid and intense immunoresponse whereas all the conjugates induced a lower production of anti-native uricase antibodies. The rank order of immunogenicity was native uricase > uricase-poly(N-vinylpyrrolidone) > or = uricase-poly(N-acriloylmorpholine) > uricase-monomethoxypoly(ethylene glycol) 5000 Da > uricase-monomethoxypoly(ethylene glycol) 10000 Da. The four conjugates also induced anti polymer immunoresponse. Anti poly(N-vinylpyrrolidone) and anti poly(N-acriloylmorpholine) antibodies were generated from the first immunization while low levels of anti polymer antibodies were found with both poly(ethylene glycol) conjugates only after the second immunization.

New PEGs for peptide and protein modification, suitable for identification of the PEGylation site

New PEG derivatives were studied for peptide and protein modification, based upon an amino acid arm, Met-Nle or Met-beta Ala, activated as succinimidyl ester. PEG-Met-Nle-OSu or PEG-Met-beta Ala-OSu react with amino groups in protein-yielding conjugates with stable amide bond. From these conjugates PEG may be removed by BrCN treatment, leaving Nle or beta Ala as reporter amino acid, at the site where PEG was bound. The conjugation of PEG and its removal by BrCN treatment was assessed on a partial sequence of glucagone and on lysozyme as model peptide or protein. Furthermore, insulin, a protein with three potential sites of PEGylation, was modified by PEG-Met-Nle, and the PEG isomers were separated by HPLC. After removal of PEG, as reported above, the sites of PEGylation were identified by characterization of the two insulin chains obtained after reduction and carboxymethylation. Mass spectrometry, amino acid analysis and Edman sequence, could reveal the position of the reporter norleucine that corresponds to the position of PEG binding.

Therapeutic proteins: a comparison of chemical and biological properties of uricase conjugated to linear or branched poly(ethylene glycol) and poly(N-acryloylmorpholine)

Uricase from Bacillus fastidiosus (UC) was covalently linked to linear PEG (PEG-1) (Mw 5 kDa), branched PEG (PEG-2) (Mw 10 kDa) and to poly(N-acryloylmorpholine) (PAcM) (Mw 6 kDa). The conjugation of UC with linear PEG and PAcM was accompanied by complete loss of enzymatic activity but, if uric acid as site protecting agent was included in the reaction mixture, the conjugate protein retained enzymatic activity. On the other hand, the modification with PEG-2 gave a conjugate that also maintained enzymatic activity in the absence of any active site protection. This behaviour must be related to hindrance of the branched polymer in reaching the enzyme active site. The UC conjugates exhibited increased resistance to proteolytic digestion while minor variations in the inhibitory constant, optimal pH, heat stability, affinity for substrate, were observed. Pharmacokinetic investigations in mice demonstrated increased residence time in blood for all the conjugates as compared with native uricase. Uricase conjugated with linear PEG was longer lasting in blood UC derivative, followed by branched PEG and the PAcM conjugates. Unconjugated uricase was rapidly removed from circulation. All these data are in favour of the use of the less known amphiphilic polymer PAcM as an alternative to PEGs in modification of enzymes devised for therapeutic applications.

Biopharmaceutical properties of uricase conjugated to neutral and amphiphilic polymers

A comparative pharmacokinetic and biodistribution investigation of polymer-protein conjugates prepared with various amphiphilic polymers was carried out using uricase as a model. Four polymer-uricase derivatives have been obtained by covalent binding of a similar number of polymer chains of (a) linear poly(ethylene glycol) (Mw 5000 Da); (b) branched poly(ethylene glycol) (Mw 10 000 Da); (c) poly(N-vinylpyrrolidone) (Mw 6000 Da); (d) poly(N-acryloilmorpholine) (Mw 6000 Da). By intravenous administration to Balb/c mice, the conjugates displayed different pharmacokinetic and organ distribution behaviors. (1) The unmodified enzyme and the poly(N-vinylpyrrolidone) conjugate were the enzyme forms with the shortest and the longest permanence in blood respectively (mean residence time 45 and 4378 min). (2) Native uricase was found to localize soon after administration significantly in heart, lungs, and liver from where it was also rapidly cleared. (3) The poly(N-acryloilmorpholine) derivative showed the highest concentration levels in liver (up to 25.5% of the dose) and considerable accumulation took also place in the other considered organs. (4) Poly(N-vinylpyrrolidone)-uricase displayed a relevant tropism for liver but low uptake indexes were found for the other organs. (5) The branched poly(ethylene glycol) derivative accumulated preferentially in liver and spleen. (6) The linear poly(ethylene glycol) conjugate was, among the various uricase forms, the species with the lowest distribution levels in all the examined organs. (7) Finally, all the enzyme forms slowly disposed in kidneys with higher levels for the poly(N-acryloilmorpholine) derivative (15% after 2880 min) and unmodified uricase (14% after 1440 min).

Protective effect of superoxide dismutase and polyethylene glycol-linked superoxide dismutase against renal warm ischemia/reperfusion injury

The protective effect of oxygen free radical scavenger superoxide dismutase (SOD) against the warm ischemic damage that occurs in kidneys harvested from non-heart-beating donors is controversial because of its short half-life. In this model, we compared the protective effect of SOD and two longer lasting polyethylene glycol (PEG)-linked forms of SOD in a model of renal ischemia induced by 60 min of arterial clamping in rats. Rats treated with PEG1-SOD and PEG2-SOD had a better renal function than controls, with significantly lower serum creatinine levels throughout the follow-up period and a significantly higher creatinine clearance on postoperative days 1, 2, and 4. In native SOD treated-rats, serum creatinine was lower than in controls, though not significantly so, and creatinine clearance was significantly higher on postoperative day 4. Our results indicate that the protective effect of SOD against renal warm ischemia can be enhanced by prolonging its half-life by binding the enzyme to PEG.

Covalent modification of mushroom tyrosinase with different amphiphic polymers for pharmaceutical and biocatalysis applications

Two different poly(ethylene glycol) derivatives (linear, mol wt 5000 and a branched form, mol wt 10000) and a new polymer (poly-[acryloylmorfoline], mol wt 5500) were covalently bound to the enzyme tyrosinase. The polymer-protein conjugates were studied with a view to their potential pharmaceutical application and to their use for the bioconversion of phenolic substrates in organic solvents. Vmax and Km for the dopa-dopaquinone conversion, thermostability, stability toward inactivation by dopa oxidation products, half-life in blood circulation, and behavior in organic solvents for the different adducts were investigated. Arrhenius plots for the dopa-dopaquinone conversion were also obtained in order to study the effects of temperature on the different enzyme forms. Covalent attachment of the polymers increased enzyme stability in aqueous solution and the solubility in organic solvents. However, organic solvent solubilization brought about loss of enzyme conformation as assessed by CD measurements, which is accompanied by a nonreversible loss of catalytic activity.

A branched monomethoxypoly(ethylene glycol) for protein modification

Procedures are described for linking monomethoxypoly(ethylene glycol) (mPEG) to both epsilon and alpha amino groups of lysine. The lysine carboxyl group can then be activated as a succinimidyl ester to obtain a new mPEG derivative (mPEG2-COOSu) with improved properties for biotechnical applications. This branched reagent showed in some cases a lower reactivity toward protein amino groups than the linear mPEG from which it was derived. A comparison of mPEG- and mPEG2-modified enzymes (ribonuclease, catalase, asparaginase, trypsin) was carried out for activity, pH and temperature stability, Km and Kcat values, and protection to proteolytic digestion. Most of the adducts from mPEG and mPEG2 modification presented similar activity and stability toward temperature change and pH change, although in a few cases mPEG2 modification was found to increase temperature stability and to widen the range of pH stability of the adducts. On the other hand, all of the enzymes modified with the branched polymer presented greater stability to proteolytic digestion relative to those modified with the linear mPEG. A further advantage of this branched mPEG lies in the possibility of a precise evaluation of the number of polymer molecules bound to the proteins; upon acid hydrolysis, each molecule of mPEG2 releases a molecule of lysine which can be detected by amino acid analysis. Finally, dimerization of mPEG by coupling to lysine provides a needed route to monofunctional PEGs of high molecular weight.

Pharmacokinetics and distribution of ribonuclease and its monomethoxypoly(ethylene glycol) derivatives in rats

The changes of pharmacokinetics and distribution of 3H radio-labelled ribonuclease (RNase) and RNase adduct with monomethoxypoly(ethylene glycol) (MPEG) were studied after intravenous administration in the dose of 25 mg kg-1 to rats. Whereas the value of total plasma clearance of RNase is close to the value of glomerular filtration rate, the value of that for MPEG-RNase is about three hundred times lower. The half-life of elimination is 79 min, 181 min and 65 h for RNase, MPEG and MPEG-RNase, respectively. The main elimination pathway of compounds under study was elimination into urine and no evident specific distribution in the examined organs (liver, kidney, muscle, lung, brain, spleen and heart) was found. The study indicates that conjugation of RNase with MPEG can improve its pharmacokinetic properties.

[The modification of Cu, Zn-superoxide dismutase by monomethoxypolyethylene glycol improves the indices of the experimental therapy of the ischemic myocardium in rats]

Mice were used to make a comparative study of the biological distribution of intravenous preparations of native and monomethoxypolyethylene glycol-modified superoxide dismutase isolated from bovine liver, as well as native and aldehyde dextran. The study demonstrated that the biodistribution of the native enzymes from various sources was, however, equal, but in the mouse liver there was a higher accumulation of SOD isolated from the rat liver. AD-SOD was found to have a longer half-life in the blood and in the liver of mice, in particular, while MPEG-SOD showed 10, 15, and 16 times longer in the lungs, blood and heart of the animals examined, respectively. The elevated accumulation of MPEG-SOD in some organs was used for their treatment, particularly for experimental therapy of rat myocardial ischemia. A rat model of ischemia demonstrated that the intravenous bolus administration of MPEG-SOD reduced the size of a myocardial necrotic area by 40% as compared to a 13% decrease when the other compounds were assayed. The findings suggest that the MPEG-SOD preparation is promising for decreasing reperfusion injuries of the cardiovascular system and the lungs.

Preparation and properties of monomethoxy poly(ethylene glycol) doxorubicin conjugates linked by an amino acid or a peptide as spacer

Polymeric doxorubicin prodrugs were prepared linking monomethoxy poly(ethylene glycol), 5000 D molecular weight, to the doxorubicin amino group, using an amino acid or a peptide as a spacer arm. As spacers glycine, L-phenylalanine, L-tryptophan and glycil-L-valil-L-phenylalanine were used. The conjugates showed enhanced stability to alkaline degradation compared to the free doxorubicin. Towards Ehrlich solid tumor in mice the glycin spaced derivative was devoid of activity, whereas the phenylalanine and tryptophan derivatives were 20% and 16% active and the tripeptide one 50% active with respect to free doxorubicin. On the other hand the derivatization was accompanied by a great decrease of toxicity in mice with respect to the free drug. Doxorubicin was not released from conjugates by chymotrypsin incubation or in plasma.

Controlled release of proteins and peptides from hydrogels synthesized by gamma ray-induced polymerization

Hydrogels prepared by radiation-induced polymerization at a low temperature have been used as carriers for the controlled release of peptides and proteins. It was found that polymerization of 2-hydroxyethyl methacrylate in the presence of poly (ethylene glycol) methyl ether (MPEG) enabled the more porous and swellable matrics to be obtained, the higher the molecular weight of MPEG. As a consequence, protein release took place at an increasing extent and, provided that MPEG molecular weight was high enough, high molecular weight proteins could also be released. Such a state of affairs was not met in the case of hydrogels based on poly (2-hydroxyethyl acrylate). SEM analysis revealed that even high molecular weight MPEG did not give rise to any porosity, even though the degree of swelling was very high. As a result, no protein release was observed. It was therefore concluded that control of hydrogel porosity for the controlled release of large proteins is of overwhelming importance.

Accurate evaluation method of the polymer content in monomethoxy(polyethylene glycol) modified proteins based on amino acid analysis

To overcome the uncertainty of the colorimetric or fluorimetric method so far employed for the evaluation of monomethoxy(polyethylene glycol) (MPEG) covalently bound to protein, a direct method based on amino acid analysis is proposed. The method exploits the use of MPEG, which was bounded with the unnatural amino acid norleucine (MPEG-Nle). MPEG-Nle was activated at its carboxylic group to succinimidyl ester for the binding to the amino groups of protein. After acid hydrolysis, the amino acid content is evaluated by conventional amino acid analyzer or by reverse-phase HPLC as phenylthiocarbamyl derivative. The number of bound MPEG chains is calculated from the amino acid composition, since one norleucine residue is released from each bound polymer chain. The method was verified with several proteins in comparison with colorimetric ones, also in the case of proteins that contain chromophores in the visible range, such cytocrome C. It was observed that in most of the cases, the colorimetric methods give an overestimation of the degree of protein modification.

In vitro and in vivo behaviour of narciclasine released from matrices based on poly (2-hydroxyethyl methacrylate)

Narciclasine (1,2,3,7-tetrahydroxy-8,9-methylendioxy-1,2,3,4-tetrahydrophena ntridone) is a natural substance with strong antimitotic effects on cells and potential antitumor activity. Its release form a hydrogel matrix was studied with the purpose of avoiding the concentration spikes of the parenteral administration. The matrix prepared by gamma ray polymerization of a mixture of 2-hydroxyethyl methacrylate (85%) and trimethylolpropane trimethacrylate (15%) was found to release narciclasine for several days, according to a diffusion controlled mechanism. In agreement with its antimitotic effect, narciclasine inhibited the growth rate of healthy mice, when the drug-loaded matrix was introduced subcutaneously. Antitumor effect was observed in an experimental model of Erlich ascitic tumor when low amounts of tumor cells were inoculated. No effect was observed at high concentrations of inoculum or towards solid tumors (Sarcoma 180). This behaviour was related to the rapid clearance of narciclasine from the body which prevented the reaching of sufficient therapeutical concentrations. A pharmacokinetic investigation carried out by an original method of assay demonstrated that narciclasine was accumulated in significant amounts in the kidney only and eliminated in urine with a half time of less than 20 min.

Surface modification of enzymes for therapeutic use: monomethoxypoly (ethylene glycol) derivatization of ribonuclease

Bovine pancreatic ribonuclease A (RNase) was modified at various extent at the lysine residues by monomethoxypoly(ethylene glycol) (MPEG) activated as active ester. For pharmacokinetic experiments a radioactive adduct was also prepared with tritiated amino acid as spacer between polymer and protein. The modification reduced only slightly the RNase catalytic activity and Km towards the substrate cytidine-2',3'-cyclic monophosphate. On the other hand extensively modified MPEG-RNase samples, showed significant decrease in activity towards ribonucleic acid. The polymer modification did not change the pH activity profile, increased the stability to proteolytic digestion, while the behaviour towards denaturants and heat was not modified. The native and MPEG-RNase administered IV, IM and SC to rats, showed impressive differences in pharmacokinetics: the half-life of the modified enzyme, evaluated in blood by radioactivity, was increased of 40-50 folds with respect to the native form.

Anti-inflammatory activity of monomethoxypolyethylene glycol superoxide dismutase on adjuvant arthritis in rats

In previous studies we observed an enhanced anti-inflammatory activity of MPEG-SOD derivatives in acute inflammation in rat. To assess the activity in chronic inflammation we tested the compound with longer half-life (MPEG-SOD 18) in complete adjuvant arthritis in rat. According to the prophylactic schedule of treatment (i.m. administration at alternative days of 10 mg/kg from day 3 to day 21), the MPEG-SOD derivative reduced arthritic lesions in a significant way (P less than 0.01 at 14th, 21st, 28th day). Indomethacin, administered i.m. daily at the dose of 1.5 mg/kg according to the same schedule, significantly inhibited adjuvant arthritis each time it was considered (% of inhibition are 66.5% at 14th day, 58.3% at 21st day and 50.8% at 28th day). Native SOD and inactivated enzyme, administered from day 3 to day 21 did not show any anti-inflammatory properties. According to the therapeutic schedule of treatment (from day 14 to day 28), neither MPEG-SOD nor native SOD showed antiarthritic activity.

Enzyme modification by MPEG with an amino acid or peptide as spacer arms

A method for the modification of enzymes by MPEG carrying an amino acid or peptide as a spacer arm is described and tested with aliphatic or aromatic side chains amino acids. The procedure involves MPEG activation by p-nitrophenylchloroformate for the amino acid or peptide coupling that is in turn activated for the protein binding. The advantage of the method resides in the possibility to introduce proper reporter groups between the polymer and the protein as norleucine for a direct evaluation of the bound polymer chains, tryptophan for structural studies of the polymer-protein adduct, and radioactive amino acid for pharmacokinetic investigations. The method was positively tested with arginase, ribonuclease, and superoxide dismutase as enzymes of therapeutic value.

A direct assay for evaluation of polyethylene glycol in enzyme adducts used as drugs or biocatalysts

A specific and direct method for the evaluation of monomethoxypolyethylene glycol (MPEG) in enzyme adducts has been developed. The method is based on extensive modification by trinitrobenzensulfonic acid (TNBS) of the MPEG-enzyme, to form an acid-stable TNBS adduct with the available amino groups of lysine and alpha-amino acid. The MPEG-modified trinitrophenylated enzyme was hydrolyzed in 6N HCl and amino acid composition evaluated by a standard AA analyzer in comparison with the analysis of a MPEG enzyme sample. The number of bound MPEG polymers was calculated from the difference in amino acid composition of the two samples.

Spectroscopic characterization of polyethyleneglycol modified superoxide dismutase: 1H NMR studies on its Cu2Co2 derivative

Spectroscopic methods have been employed in order to understand the molecular basis of the decrease in enzymatic activity of the antiinflammatory enzyme copper-zinc superoxide dismutase (SOD) following the covalent binding of polyethyleneglycol (PEG) chains to the protein amino-groups. The PEG modification is a general method recently proposed to improve the therapeutic index of enzymes. 1H NMR spectra on the cobalt substituted PEG-modified SOD, Cu2Co2-PEG-SOD, have been recorded. The signals are quite broad with respect to the unmodified enzyme. This has been interpreted on the basis of the effect of molecular weight on the linewidth. The analysis has shown that the histidine hydrogens involved in metal binding at the enzyme active site are the same in both native and PEG-modified SOD. Similarly, circular dichroism and absorption spectra indicate that the overall conformation of the metal clusters is not perturbed upon modification. On the other hand, azide titration shows that the affinity constant of N-3 for SOD is largely reduced upon PEG modification (K = 154 M-1 and 75 M-1 for the native and modified SOD, respectively). These results indicate that the decrease in enzymatic activity upon surface modification with PEG is not caused by a perturbation of the active site geometry, but to a decrease in the channeling of the O2- ion towards the enzyme active site.

Effects of monomethoxypoly(ethylene glycol) modification of ribonuclease on antibody recognition, substrate accessibility and conformational stability

The effects of modification of bovine pancreatic ribonuclease A by monomethoxypoly(ethylene glycol) (MPEG) were examined for changes in recognition by antiRNase antibodies, enzymatic activity against low and high molecular weight substrates and conformational stability to temperature elevation. Modified forms of RNase were prepared containing an average of 4, 9, and 11 mol of MPEG/mol protein, by amino group modification. These were analysed by binding to RNase antibodies crosslinked to solid phase-immobilized protein A. The affinity column was incorporated into a high performance liquid chromatograph and the RNase species were studied by both zonal and frontal analytical affinity chromatography. An antibody dissociation constant of 7.6 x 10(-8) M was found for unmodified RNase, as compared to values of 1.3 x 10(-7) and 1.2 x 10(-6) M for RNase with 4 and 9 covalently bound MPEG chains, respectively. Modification also led to progressive loss of enzymatic activity against RNA, down to 3% for the most highly modified enzyme. In contrast, enzymatic activity against cytidine-2',3'-cyclic monophosphate was suppressed to a maximum of only 33% at the highest modification level, and the stability to temperature, as followed by circular dichroism, was reduced only partially, from 67 degrees C for native protein to 57 degrees C for RNase with 11 mol equivalents MPEG incorporated. The above differential effects on enzymatic activity, antibody binding and temperature effects are consistent with the view that MPEG modification has relatively small effects on conformational stability and small molecule accessibility, but more dramatic effects on large molecule (substrate as well as antibody) accessibility.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation of proteolytic enzymes in effervescent tablets

A procedure for the evaluation of proteolytic enzymes in effervescent tablets is reported. A proper chromatographic step followed by enzymatic activity assay allows to overcome the interferences due to the high salt content of such formulations as well as to chromophores eventually present in the tablet components.

Evaluation of monomethoxypolyethyleneglycol derivatized superoxide dismutase in blood and evidence for its binding to blood cells

A method to evaluate in blood free superoxide dismutase (SOD) and SOD covalently bound to monomethoxypolyethylene glycol (MPEG-5000) is reported. The method takes advantage of gel filtration and cationic exchange chromatography to remove substances present in plasma which can interfere with the SOD enzymatic assay. Using this methodology, it was demonstrated that, contrary to free SOD, MPEG-SOD, when added to blood, was not completely recovered in plasma. Binding of MPEG-SOD to blood cell components takes place involving interactions of MPEG chains with cell membranes.

A high molecular weight form of human urinary urokinase

A very high molecular weight form of urokinase, of about 100,000 D (VHMr-UK), was isolated from human urine in trace amounts as compared to the well characterized two forms of urokinase, HMr-UK (Mr of about 50,000) and LMr-UK (Mr of about 30,000). This form was demonstrated to be a dimer of HMr-UK, in which no covalent bond is involved, on the basis of the following evidence: by SDS electrophoresis it is dissociated, to the 50,000 D form; by electrophoresis in SDS under reducing conditions it is dissociated, as HMr-UK, to two polypeptide chains of about 30,000 and 20,000 D; by short heating at pH 5 it is quantitatively converted to the 50,000 D form; the kinetic constants towards the alpha-carbobenzoxy-L-lysine-p-nitro-phenylester substrate are the same for HMr-UK and VHMr-UK.

PEG superoxide dismutase derivatives: anti-inflammatory activity in carrageenan pelurisy in rats

Two derivatives of superoxide dismutase (SOD) with monomethoxypolyethyleneglycol, having different molecular weights (46,000 and 121,000 D) and different plasma half-lives are tested in carrageenan pleurisy in rats after single i.v. injection. The compounds were prepared following an original method where the polymer is activated by trichlorophenylchloroformate to give a phenylcarbonate which is reactive towards the protein amino groups. The anti-inflammatory activity of derivatives results correlated to their kinetics, lasting longer for the derivative with the longest half-life (SOD-PEG 18). SOD activity, in plasma of rats treated with SOD-PEG 18, is still present 24 hour after carrageenan, in agreement with the prolonged anti-inflammatory activity. Moreover SOD-PEG 18 diffuses well in pleural exudate, as it is evidentiated from the increase with time of exudate/plasma concentration ratio.

Surface modification of proteins. Activation of monomethoxy-polyethylene glycols by phenylchloroformates and modification of ribonuclease and superoxide dismutase

A single-step method of activation of monomethoxypolyethylene glycols suitable for its binding to polypeptides and proteins is proposed. Based on the reaction with 2,4,5-trichlorophenylchloroformate or p-nitrophenylchloroformate, it gives reactive PEG-phenylcarbonate derivatives. The PEG intermediate is stable on storage, the activating group is easily quantified,and the reaction with amino acid and proteins proceeds rapidly at pH near neutrality. The PEG derivatization of enzymes with this procedure is less inactivating than those previously reported. Ribonuclease and superoxide dismutase were modified and the effect of (a) bound polymer on clearance time in rats, (b) antibody recognition, and (c) on the enzymatic activity toward low and high molecular weight substrates were studied.

Enzymatically active subunits of Bacillus stearothermophilus enolase bound to Sepharose

The octameric enolase from Bacillus stearothermophilus was immobilized onto Sepharose 4B activated by the cyanogen bromide reaction under conditions for achieving essentially a single-point attachment. The immobilized enzyme was dissociated with guanidine hydrochloride to yield bound monomeric enolase. The Sepharose-bound subunit regained activity upon removal of the denaturant. It was also possible to rehydribize immobilized monomers to native octamers. Of note, the thermal stability of the immobilized enolase subunit does not appreciably differ from that of the parent soluble octameric enzyme. Thus, these results indicate that single subunits of thermophilic enolase are active and that oligomerization is not a prerequisite for the enzymic activity as well as for thermal stability.

General stability of thermophilic enzymes: studies on 6-phosphogluconate dehydrogenase from Bacillus stearothermophilus and yeast

The thermophilic enzyme 6-phosphogluconate dehydrogenase (6-phospho-D-gluconate:NADP oxidoreductase, decarboxylating, EC 1.1.1.44) from Bacillus stearothermophilus was much more resistant to inactivation under different conditions of temperature, pH, guanidine-hydrochloride, and organic solvents (dioxane, dimethylformamide, acetone) than its mesophilic counterpart from yeast. In addition, the thermophilic enzyme largely withstands proteolysis with trypsin, chymotrypsin, and elastase when compared with the yeast enzyme. It is proposed that thermophilic enzymes are not only thermostable, but also generally more stable to most common protein denaturants than their mesophilic counterparts. Because of their remarkable stability, enzymes isolated from thermophilic microorganisms may be ideally suited for technological applications.

Drug protein interaction: displacement of albumin bound 8-methoxypsoralen by drugs

The displacement of 8-methoxypsoralen (8-MOP) bound to albumin by a number of drugs commonly used in therapy was investigated by equilibrium dialysis, using tritium labelled furocoumarin. Furocoumarin binding was found to be influenced by several compounds. However because of the concentrations of furocoumarin and displacers reached in plasma, only tolbutamide could modify the 8-MOP binding to a pharmacokinetically significant extent.

Drug-protein interaction: plasma protein binding of furocoumarins

The binding of six furocoumarins (angelicin, psoralen, 8-methoxypsoralen, 5-methoxypsoralen, 8-methylpsoralen, 4,5'8-trimethylpsoralen) to human serum and human serum albumin was studied by equilibrium dialysis using tritium labelled compounds. The results indicated that in serum all furocoumarins are bound mostly by albumin, the extent of binding being related to the structure of the furocoumarins; at any rate, high values of the bound drug, ranging from 84 to 97% were observed. The percentage of binding is strictly related to the water solubility of the compounds. A limited number of binding sites, n = 1-2.4, were detected in the albumin molecule, indicating a high specificity in the binding process. The association constants of the furocoumarins to albumin. Ka, ranged from 1.2 X 10(4) M-1 (8-methoxypsoralen) to 1.9 X 10(5) M-1 (4,5'8-trimethylpsoralen).

Drug-protein interaction: 8-methoxy psoralen as photosensitizer of enzymes and amino acids

For a better insight of the molecular basis of the photobiological effects of furocoumarins, the relevance of proteins oxydation by singlet oxygen produced by these substrates under irradiation with long u. v. light was studied. Complex oligomeric as well as simple monomeric purified enzymes with high or low molecular weight and different properties and simple amino acids were irradiated under oxygen in presence of 8-methoxy-psoralen. The effects on both proteins and amino acids were compared with those obtained under similar conditions with typical photosensitizers as methylene blue and Rose Bengal. The results indicated that the photooxydation of proteins, although possible, appears to play a minor role, if any, in the mechanism of action of furocoumarin.

The binding of 8-methoxypsoralen by human serum albumin

The ability of 8-methoxypsoralen (8-MOP) to bind human serum albumin has been investigated in vitro through equilibrium dialysis and fluorescence quenching. By means of the first technique it was observed that, at concentrations presumably close to those obtainable in vivo following its administration in the photochemotherapy of psoriasis, over 80% of 8-MOP was bound to serum albumin. In human serum albumin fluorescence techniques revealed a preferential site of binding for 8-MOP with a high binding constant (Ka = 0.7 X 10(5) M-1) and precise steric requirements, since small conformational variations of the protein molecule were able to abolish its affinity for furocoumarin.