Thiol disulfide exchange reactions in human serum albumin: the apparent paradox of the redox transitions of Cys₃₄

Abstract

Human serum albumin (HSA) is characterized by 17 disulfides and by only one unpaired cysteine (Cys ), which can be free in the reduced albumin or linked as a mixed disulfide with cysteine, or in minor amount with other natural thiols, in the oxidized albumin. In healthy subjects, the level of the oxidized form is about 35%, but it rises up to 70% after oxidative insults or in patients with kidney diseases. Oxidized albumin is therefore considered a short-term biomarker of oxidative stress as its level may increase or decrease under appropriate redox inputs in discrete temporal spans. This paper defines, for the first time, the kinetic properties of reduced and oxidized Cys of HSA in their reactions with natural disulfides and thiols. Kinetic constants support the evidence that the Cys redox oscillations observed in vivo are mainly due to the interaction with cysteine and cystine without the involvement of any enzymatic support. This study gives also a plausible explanation for the absence of involvement of the 17 disulfides naturally present in HSA in these redox transitions. This inert behavior toward cysteine is marginally due to solvent accessibility or flexibility factors of these bonds but mainly to their strong thermodynamic stability, which is caused essentially by a proximity effect. A similar mechanism is likely at play in the many proteins that maintain disulfide bridges in a reducing medium like the cytosol.

Kinetic studies reveal a key role of a redox-active glutaredoxin in the evolution of the thiol-redox metabolism of trypanosomatid parasites

“…2), which is 1 order of magnitude higher than that determined previously (28). The rate of Grx1 in these reactions is comparable with that of EcGrx1 with glutathionylated RNase (k cat /K m ϭ 3 ϫ 10 6 M Ϫ1 s Ϫ1 (35)) and somewhat higher than the previously reported for Grx from yeast ( (41)). The time course of the reaction of Grx1 with excess GSSG shows a very fast phase, lasting less than 4 ms, followed by an exponential decay in emission ( Fig.…”

Methanethiosulfonate Derivative of OX063 Trityl: A Promising and Efficient Reagent for Side‐Directed Spin Labeling of Proteins

“…Modification by attachment of a spin label is no exception: we can think of a spin‐labeled cysteine as like a mutation to an unnatural amino acid. We consider this possible effect a potential concern, because modifications of HSA are well known to tune the physical properties and physiological functions of HSA . Therefore, we tested for aggregation of HSA and other gross changes using a gel retardation assay.…”

The Universal Soldier: Enzymatic and Non-Enzymatic Antioxidant Functions of Serum Albumin

“…In the case of pathology, when the percentage of cystine increases, albumin acts as a redox buffer, maintaining a safe Cys-SH/Cys-S-S-Cys ratio for the body. Moreover, according to the data obtained in this work, the remaining 34 albumin cysteines (forming 17 disulfide bridges) barely undergo cysteinylation even with high concentrations of free cysteine, 150-fold higher than its normal concentration in the blood plasma [ 89 ].…”

“…In theory, the side radical of cysteine can be irreversibly oxidised to sulfonic acid (Cys34-S(O)O − O − ); however, according to the literature, the percentage of Cys34 in the form of sulfonic acid in blood plasma is extremely low [ 87 ]. Sulfenic acid can also be converted to disulfide (HSA-S-S-R) by interacting with low-molecular-weight blood plasma thiols (GSH, homocysteine, free cysteine), and then reduced to HSA-SH [ 88 , 89 , 90 ].…”

“…Bocedi et al [ 89 ] wanted to answer the question of why a healthy person has only 30 percent of oxidised albumin, while for low-molecular-weight thiols this value is from 70 to 80 percent. The HSA-cysteine conjugate (HSA-Cys34-S-S-Cys) was used as a model of oxidised human albumin, since this disulfide is the main form of oxidised albumin in blood plasma.…”