Jelena M. Aćimović

Influence of the microenvironment of thiol groups in low molecular mass thiols and serum albumin on the reaction with methylglyoxal.

Methylglyoxal (MG), a reactive alpha-oxoaldehyde that is produced in higher quantities in diabetes, uremia, oxidative stress, aging and inflammation, reacts with the thiol groups (in addition to the amino and guanidino groups) of proteins. This causes protein modification, formation of advanced glycated end products (AGEs) and cross-linking. Low molecular mass thiols can be used as competitive targets for MG, preventing the reactions mentioned above. Therefore, this paper investigated how the microenvironment of the thiol group in low molecular mass thiols (cysteine, N-acetylcysteine (NAcCys), carboxymethylcysteine (CMC) and glutathione (GSH)) and human serum albumin (HSA) affected the thiol reaction with MG. The SH group reaction course was monitored by (1)H-NMR spectroscopy and spectrophotometric quantification. Changes in the HSA molecules were monitored by SDS-PAGE. The microenvironment of the SH group had a major effect on its reactivity and on the product yield. The reactivity of SH groups decreased in the order Cys>GSH>NAcCys. CMC did not react. The percentages of the reacted SH groups in the equilibrium state were almost equal, regardless of the ratio of thiol compound/MG (1:1, 1:2, 1:5): 38.1 + or - 0.9%; 38.2 + or - 0.7% and 39.0 + or - 0.8% for Cys; 26.5 + or - 0.6%; 26.6 + or - 2.6% and 27.4 + or - 2.5% for GSH; 10.8 + or - 0.9%; and 11.2 + or - 0.7% and 12.2 + or - 0.9% for NAcCys, respectively. Our results explain why substances containing alpha-amino-beta-mercapto-ethane as a pharmacophore are successful scavengers of MG. In equilibrium, HSA SH reacted in high percentages both with an insufficient amount and with an excess of MG (55% and 65%, respectively). An analysis of the hydrophobicity of the microenvironment of the SH group on the HSA surface showed that it could contribute to high levels of SH modification, leading to an increase in the scavenging activity of the albumin thiol.

Method for monitoring of the protein amino group changes during carbonylation

OBJECTIVES Carbonylation of the protein amino, guanidino and thiol groups is one of the important causes of vascular complications in diabetes. We developed a simple spectrophotometric method for monitoring of the changes in the protein amino group contents during carbonylation. DESIGN AND METHODS The method is based on the reaction of amino group with p-benzoquinone in the slightly alkaline media. It was applied during carbonylation in vitro with methylglyoxal and in vivo in 13 patients with type 2 diabetes and 20 healthy persons. RESULTS The method is simple, fast, precise (RSD in the range of 1.2-1.8%) and accurate (recovery about 100%). The content of amino groups in human serum albumin isolated from diabetics was significantly lower (p<0.01) in comparison with a control group. CONCLUSION The method developed is suitable for quantification of protein amino groups during in vitro carbonylation as well as for clinical practice.

Reactivity of IGF binding protein‐3 isoforms towards concanavalin A in healthy adults and subjects with cirrhosis

The capacity of the liver to synthesize insulin‐like growth factors (IGFs) and their binding proteins (IGFBPs) may be compromised by alcohol. The characteristics of IGFBP‐3 variants obtained from healthy individuals and patients with alcoholic cirrhosis (ALC) were compared. Concanavalin A (Con A) affinity electrophoresis and ligand blotting demonstrated that there was a gradual change in carbohydrate properties of putative IGFBP‐3 with progression of ALC from stages A to C. As many as 12 ionic species of IGFBP‐3 could be distinguished, corresponding probably to variously glycosylated and/or phosphorylated isoforms of the core protein. Three of them reacted significantly with the immobilized Con A, the pattern being altered in patients with ALC. Patients with ALC in stage B exhibited the presence of clearly differentiated IGFBP‐3 variants less and more Con A reactive, suggesting this stage to be a turning point with the most intensive changes in the IGF‐IGFBP system. Because the glycosylation pattern is tissue specific, pathological post‐translational modifications found for one glycoprotein (IGFBP‐3) are probably shared by others of the same tissue origin. This may affect their susceptibility to proteolysis and subsequently their function.

The influence of fatty acids on determination of human serum albumin thiol group.

During investigation of the changes of the Cys34 thiol group of human serum albumin (HSA) (isolated by affinity chromatography with Cibacron Blue (CB)) in diabetes, we found that the HSA-SH content was higher (11-33%) than the total serum thiol content. The influence of fatty acids (FA) binding to HSA on this discrepancy was investigated in vitro (using fluorescence and CD spectroscopy and GC) and with HSA samples from diabetic (n=20) and control groups (n=17). HSA-bound FA determine the selection of HSA molecules by CB and enhance reactivity and/or accessibility of the SH group. A high content of polyunsaturated FA (35.6%) leads to weaker binding of HSA molecules to CB. Rate constants of DTNB reaction with the SH group of HSA applied to a CB column, bound-HSA and unbound-HSA fractions, were 4.8×10(-3), 21.6×10(-3), and 11.2×10(-3) s(-1), respectively. The HSA-SH group of diabetics is more reactive compared with control individuals (rate constants 20.9×10(-3)±4.4×10(-3) vs 12.9×10(-3)±2.6×10(-3) s(-1), P<0.05). Recovery values of the SH group obtained after chromatography of HSA with bound stearic acid ranged from 110 to 140%, while those for defatted HSA were from 98.5 to 101.7%. Thus, HSA-bound FA leads to an increase of HSA-SH content and a contribution to total serum thiols, which make the determination of the thiol group unreliable.

Monitoring of the human serum albumin carbonylation level through determination of guanidino group content

Carbonylation of the protein amino, guanidine, and thiol groups with α-oxoaldehydes (which are produced in higher quantities in diabetes, uremia, oxidative stress, aging, and inflammation) is one of the important causes of vascular complications. For monitoring of the human serum albumin (HSA) carbonylation level, a spectrophotometric method based on the formation of colored adduct between guanidine group and thymol-sodium hypobromite reagent in the alkaline medium was investigated. Beers law is obeyed in the concentration range of Arg and protein guanidine groups from 1 to 40 mM. Precision of the method (relative standard deviation) was in the range of 0.9 to 2%. Accuracy was examined by the standard addition method (recovery ~100%). The method was applied for monitoring of the carbonylation level of HSA with methylglyoxal in vitro and of HSA isolated (using affinity chromatography) from sera of 21 patients with type 2 diabetes and 12 healthy persons. The content of guanidine groups in HSA isolated from diabetics (19.64 ± 1.07 mM/mM albumin) was significantly lower (P < 0.001) in comparison with a control group (21.87 ± 1.02 mM/mM albumin). The method is simple and fast, has good accuracy and precision, and is suitable for clinical practice as well for in vitro protein carbonylation experiments.

Quantification of total content of non-esterified fatty acids bound to human serum albumin

Non-esterified fatty acids bound to the human serum albumin (HSA) contribute to several HSAs properties of special concern in pathologies, for instance to the reactivity of the free HSA-Cys34 thiol group (important antioxidative thiol pool in plasma), and to the affinity for binding of molecules and ions (for example cobalt as a prominent biomarker in heart ischemia). Therefore, the method for determination of FAs bound to HSA was developed. FAs were released from HSA (previously isolated from serum by ammonium sulfate precipitation) using acidic copper(II) sulfate in phosphoric acid, extracted by n-heptane-chloroform (4:1, v/v) mixture, spotted on TL silica-gel and then developed with n-heptane-chloroform-acetic acid (5:3:0.3, v/v/v). Common office flatbed scanner and software solution for densitometric image analysis, developed in R, were used. The linearity of calibration curve in concentration range from 0.1 to 5.0mmol/L stearic acid was achieved. The method was proved to be precise (with RSD of 1.4-4.7%) and accurate. Accuracy was examined by standard addition method (recoveries 97.2-102.5%) and by comparison to results of GC. The method is sample saving, technically less demanding, and cheap, and therefore suitable for determination of FAs/HSA ratio when elevated concentrations of free FAs are reliable diagnostic/risk parameter of pathological states.