Photo-Oxidation of Therapeutic Protein Formulations: From Radical Formation to Analytical Techniques

UV and ambient light-induced modifications and related degradation of therapeutic proteins are observed during manufacturing and storage. Therefore, to ensure product quality, protein formulations need to be analyzed with respect to photo-degradation processes and eventually protected from light exposure. This task usually demands the application and combination of various analytical methods. This review addresses analytical aspects of investigating photo-oxidation products and related mediators such as reactive oxygen species generated via UV and ambient light with well-established and novel techniques.

Protective Effect of Dinitrosyl Iron Complexes Bound with Hemoglobin on Oxidative Modification by Peroxynitrite

Dinitrosyl iron complexes (DNICs) are a physiological form of nitric oxide (•NO) in an organism. They are able not only to deposit and transport •NO, but are also to act as antioxidant and antiradical agents. However, the mechanics of hemoglobin-bound DNICs (Hb-DNICs) protecting Hb against peroxynitrite-caused, mediated oxidative modification have not yet been scrutinized. Through EPR spectroscopy we show that Hb-DNICs are destroyed under the peroxynitrite action in a dose-dependent manner. At the same time, DNICs inhibit the oxidation of tryptophan and tyrosine residues and formation of carbonyl derivatives. They also prevent the formation of covalent crosslinks between Hb subunits and degradation of a heme group. These effects can arise from the oxoferryl heme form being reduced, and they can be connected with the ability of DNICs to directly intercept peroxynitrite and free radicals, which emerge due to its homolysis. These data show that DNICs may ensure protection from myocardial ischemia.

Kinetic Models for the Role of Protein Thiols during Oxidation in Beer

Thiol-containing proteins have been suggested to have antioxidative properties in beer. A kinetic model has been setup for the reactivity of thiols during early stages of oxidative degradation of beer. Kinetic analysis based on the proposed reaction mechanism allowed evaluation of the relative reactivity of beer components, such as bitter acids from hops and polyphenols. The rate constants for the reaction of 1-hydroxyethyl radicals, which are generated during radical mediated oxidation of ethanol in beer, with hop bitter acids and thiols were very similar, and the concentration of these compounds in beer is therefore essential for the relative reactivity. For a standard pilsner beer with 35 international bitter units with typical concentrations of thiols and hop bitter acids, thiols were found to react with ca. 9% of 1-hydroxyethyl radicals, while bitter acids from hops accounted for ca. 88% of the reaction with 1-hydroxyethyl radicals. Polyphenols were not found to account for any major part of the reaction with 1-hydroxyethyl radicals due to low reaction rates and low concentrations in pilsner beer compared to the other components. The kinetic model suggests that the concentration of thiols has to be increased in order to contribute with any significant antioxidative protection and that the fate of thiols during oxidation must be considered since some thiol oxidation products may induce further damage.

The Reducing Capacity of Thioredoxin on Oxidized Thiols in Boiled Wort

Free thiol-containing proteins are suggested to work as antioxidants in beer, but the majority of thiols in wort are present in their oxidized form as disulfides and are therefore not active as antioxidants. Thioredoxin, a disulfide-reducing protein, is released into the wort from some yeast strains during fermentation. The capacity of the thioredoxin enzyme system (thioredoxin, thioredoxin reductase, NADPH) to reduce oxidized thiols in boiled wort under fermentation-like conditions was studied. Free thiols were quantitated in boiled wort samples by derivatization with ThioGlo1 and fluorescence detection of thiol-derivatives. When boiled wort was incubated with all components of the thioredoxin system at pH 7.0 and 25 °C for 60 min under anaerobic conditions, the free thiol concentration increased from 25 to 224 μM. At pH values similar to wort (pH 5.7) and beer (pH 4.5), the thioredoxin system was also capable of increasing the free thiol concentration, although with lower efficiency to 187 and 170 μM, respectively. The presence of sulfite, an important antioxidant in beer secreted by the yeast during fermentation, was found to inactivate thioredoxin by sulfitolysis. Reduction of oxidized thiols by the thioredoxin system was therefore only found to be efficient in the absence of sulfite.

Rational Design of a Profluorescent Substrate for S-adenosylhomocysteine Hydrolase and its Applications in Bioimaging and Inhibitor Screening

S-Adenosylhomocysteine hydrolase (SAHase) is a cellular enzyme that plays a key role in the methylation process, and a potential drug target in the discovery of antiviral and anticancer agents. There is increasing interest in determining its activity in the biological and clinical fields with chemosensors but with limited success so far. Herein, we designed and developed for the first time an off/on-type of fluorogenic substrate (NADE) that is directly responsive to SAHase activity. NADE used 1,8-naphthalimide as the signal reporter and adenosine (Ade) as the reaction center; removal of the Ade moiety enhanced the fluorescence by >10-fold. Kinetic study showed that NADE followed a non-Michaelis-Menten pattern that corresponded to the allosteric behavior of SAHase. NADE showed excellent selectivity and functioned efficiently in cells, allowing the microscopic imaging of SAHase activity. NADE can also be used to identify and measure the effectiveness of inhibitors in a markedly superior way. In a word, NADE would be broadly useful in clinical applications and academic studies.

Determination of 3-mercaptopropionic acid by HPLC: A sensitive method for environmental applications

The organic sulfur compound 3-mercaptopropionic acid (3-MPA) is an important thiol intermediate in organic sulfur metabolism in natural environments. It is generated during degradation of sulfur-containing amino acids (e.g. methionine) and from demethylation of dimethylsulfoniopropionate (DMSP). This pathway is an alternative enzymatic process in the DMSP catabolism that routes sulfur away from the climatically-active dimethyl sulfide (DMS). 3-MPA detection and subsequent quantification in different matrices is difficult due to its extreme reactivity. We therefore developed a sensitive method for determination of 3-MPA based on pre-column derivatization with monobromobimane and analysis by high-performance liquid chromatography (HPLC) with fluorescence detection. This methodology was first tested with 3-MPA standards under low (0.005-0.2μmolL(-1)) and high (1-25μmolL(-1)) concentrations. For the optimization of the reaction, CHES and, alternatively, Tris-HCl buffers were evaluated in the derivatization step, with Tris-HCl showing more effective separation of thiol derivatives and a better 3-MPA peak shape. The detection limit was 4.3nmolL(-1) with a 10μL sample injection, and mean recoveries of 3-MPA ranged from 97 to 105% in estuarine waters with different salinities (0.17 and 35.9ppt). The linearity (r>0.99) and repeatability of detector response, with intra- and inter-day precision (% CV) of 2.68-7.01% and 4.86-12.5%, respectively, confirmed the reliability of the method. Previous 3-MPA analytical methods required immediate analysis due to unstable derivatives, but in this method we achieved high stability of the derivatized samples when stored at 4°C, with only a 3-5% loss after more than one year of storage. This method was successfully applied to measure 3-MPA concentrations and rates of 3-MPA production in a variety of intertidal estuarine sediment slurries. Dissolved 3-MPA concentrations in these sediment slurries varied between 2 and 237μmolL(-1) and, 3-MPA net fluxes ranged in wet sediments between -3.6±1.7 and 30±5μmolL(-1)g(-1)h(-1). Thus, the application of this optimized methodology showed an efficient performance for measuring 3-MPA in environmental samples, with a straightforward sample derivatization and a simple analysis of stable 3-MPA derivatives.

Multiparametric protocol for the determination of thiol redox state in living matter

Thiol redox state (TRS) evaluation is mostly restricted to the estimation of GSH and GSSG. However, these TRS parameters can estimate the GSSG/GSH potential, which might be useful for indicating abnormalities in redox metabolism. Nonetheless, evaluation of the multiparameric nature of TRS is required for a more accurate assessment of its physiological role. The present protocol extends the partial assessment of TRS by current methodologies. It measures 15 key parameters of TRS by two modular subprotocols: one for the glutathione (GSH)- and cysteine (CSH)-based nonprotein (NP) thiols/mixed disulfides (i.e., GSH, GSSG, GSSNP, CSH, CSSNP, NPSH, NPSSNP, NP(x)SH(NPSSNP), NP(x)SH(NPSH)), and the other for their protein (P) thiols/mixed disulfides (i.e., PSH, PSSG, PSSC, PSSNP, PSSP, NP(x)SH(PSSNP)). The protocol eliminates autoxidation of GSH and CSH (and thus overestimation of GSSG and CSSNP). Its modularity allows the determination GSH and GSSG also by other published specific assays. The protocol uses three assays; two are based on the photometric reagents 4,4'-dithiopyridine (DTP) and ninhydrin (NHD), and the third on the fluorometric reagent o-phthaldialdehyde (OPT). The initial assays employing these reagents have been extensively modified and redesigned for increased specificity, sensitivity, and simplicity. TRS parameter values and their standard errors are estimated automatically by sets of Excel-adapted algebraic equations. Protocol sensitivity for NPSH, PSH, NPSSNP, PSSP, PSSNP, CSH, CSSNP, PSSC, NP(x)SH(NPSSNP), and NP(x)SH(NPSH) is 1 nmol -SH/CSH, for GSSNP 0.2 nmol, for GSH and GSSG 0.4 nmol, and for PSSG 0.6 nmol. The protocol was applied on human plasma, a sample of high clinical value, and can be also applied in any organism.

Simultaneous determination of glutathione, cysteine, homocysteine, and cysteinylglycine in biological fluids by ion-pairing high-performance liquid chromatography coupled with precolumn derivatization

Biologically active low-molecular-mass thiols, mainly including glutathione (GSH), cysteine (Cys), homocysteine (Hcy), and cysteinylglycine (Cys-Gly), are important physiological components in biological fluids, and their analytical methods have gained continuous attention over recent years. We developed and validated a novel HPLC method for the quantification of GSH, Cys, Hcy, and Cys-Gly in human plasma, urine, and saliva using 4-chloro-3,5-dinitrobenzotrifluoride as the derivatization reagent. Analyses were linear from 0.15 to 500 μM with the coefficient regression range of 0.9987-0.9994. Detection limits ranged from 0.04 to 0.08 μM (S/N=3). The developed method was applied to quantification of four thiols in human biological fluids collected from five donors with the concentration range of 2.50-124.25 μM, 0-72.81 μM, and 0-4.25 μM for plasma, urine, and saliva, respectively. The present method seemed to be an attractive choice for the determination of thiols in plasma, urine, and saliva.

Beer thiol-containing compounds and redox stability: kinetic study of 1-hydroxyethyl radical scavenging ability

The 1-hydroxyethyl radical is a central intermediate in oxidative reactions occurring in beer. The reactivity of thiol-containing compounds toward 1-hydroxyethyl radical was evaluated in beer model solutions using a competitive kinetic approach, employing the spin-trap 4-POBN as a probe and by using electron paramagnetic resonance to detect the generated 1-hydroxyethyl/4-POBN spin adduct. Thiol-containing compounds were highly reactive toward the 1-hydroxyethyl radical with apparent second-order rate constants close to the diffusion limit in water and ranging from 0.5 × 10⁹ L mol⁻¹ s⁻¹ for the His-Cys-Lys-Phe-Trp-Trp peptide to 6.1 × 10⁹ L mol⁻¹ s⁻¹ for the reduced lipid transfer protein 1 (LTP1) isolated from beer. The reactions gave rise to a moderate kinetic isotope effect (k(H)/k(D) = 2.3) suggesting that reduction of the 1-hydroxyethyl radical by thiol-containing compounds takes place by hydrogen atom abstraction from the RSH group rather than electron transfer. The content of reduced thiols in different beers was determined using a previously established method based on ThioGlo-1 as the thiol derivatization reagent and detection of the derivatized thiols by reverse-phase liquid chromatography coupled to a fluorescence detector. The total level of thiol in beer (oxidized and reduced) was determined after a reduction step employing 3,3',3″-phosphanetriyltripropanoic acid (TCEP) as the disulfide reductant. A good correlation among total protein and total thiol content in different beers was observed. The results suggest a similar ratio between reduced thiols and disulfides in all of the tested beers, which indicates a similar redox state.