Ligand binding to the FA3-FA4 cleft inhibits the esterase-like activity of human serum albumin

Probing the effect of glycation on the pseudo-esterase activity of Human Serum Albumin

Human Serum Albumin (HSA), a major protein in blood plasma, exhibits pseudo-enzymatic activity such as pseudo-esterase activity. This study explores the impact of glycation on the pseudo-esterase activity of HSA. Glycation, a non-enzymatic reaction between reducing sugars and HSA was induced using glucose, fructose, and ribose. Glycated HSA samples (gHSA, fHSA, and rHSA) were analyzed for their pseudo-esterase activity using p-nitrophenyl acetate (PNPA) as a substrate. The kinetics of the enzyme-substrate reaction were studied, and structural changes monitored through UV-Vis, fluorescence, and circular dichroism (CD) spectroscopy. Glycation was confirmed using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-ToF) mass spectrometry. Results indicated a significant reduction in the pseudo-esterase activity of glycated HSA compared to native HSA. Ribose-induced glycation exhibited the highest reduction in activity, attributed to the higher glycation efficiency of ribose. Fluorescence studies revealed changes in Trp fluorescence, suggesting conformational alterations in glycated as well as acetylated HSA. CD spectroscopy confirmed secondary structural changes upon glycation, followed by acetylation. A binding study was conducted to deduce the effect of glycation and acetylation of HSA on its binding activity. These findings highlight the significance of understanding how glycation modifies the biochemical properties of HSA, impacting its physiological roles and therapeutic applications in metabolic disorders. Our studies specifically reveal that glycation impairs the pseudo-esterase activity and alters the drug-binding properties of HSA.

A radical containing micellar probe for assessing esterase enzymatic activity with ultra-low field Overhauser-enhanced magnetic resonance imaging

The ability to track altered enzyme activity using a non-invasive imaging protocol is crucial for the early diagnosis of many diseases but is often challenging. Herein, we show that Overhauser magnetic resonance imaging (OMRI) can be used to monitor enzymatic conversion at an ultra-low field (206 μT) using a highly sensitive "off/on" probe with a nitroxide stable radical containing ester, named T2C12-T80. This TEMPO derivative containing probe forms stable electron paramagnetic resonance (EPR) silent micelles in water that are hydrolysed by esterases, thus yielding narrow EPR signals whose intensities correlate directly with specific enzymatic activity. The responsiveness of the probe to tumours, facilitated by increased esterase activity, was initially determined by comparing EPR signals measured upon incubation with 3T3 (healthy fibroblasts used as control), HepG2 (human hepatoma) and Hs766T (human pancreatic cancer cells) cell lysates and then with Hs766T and 3T3 living cells. Next, Overhauser MR images were detected on a phantom containing the probe and the esterases to show that the approach is well suited for being translated to the in vivo detection at the earth's magnetic field. Regarding detection sensitivity, ultra-low field OMRI (ULF-OMRI) is beneficial over OMRI at higher fields (e.g. 0.2 T) since Overhauser enhancements are significantly higher and the technique is safe in terms of the specific absorption rate.

Hydrolytic Metabolism of Withangulatin A Mediated by Serum Albumin Instead of Common Esterases in Plasma

BACKGROUND AND OBJECTIVES

The oral bioavailability of withangulatin A (WA) is low and may undergo first-pass metabolism because of the presence of two esters bonds. This study aimed to identify the hydrolysis behavior and mechanism of WA, thus enriching its structure-pharmacokinetic relationship.

METHODS

The in vivo pharmacokinetic studies of WA in rats were first investigated, followed by in vitro assays including metabolic stability, phenotyping identification and metabolic kinetics assays. After screening out the responsible enzymes with higher catalytic capacity, molecular docking study was performed to demonstrate the interaction mode between WA and metabolic enzymes. Then, metabolites in human serum albumin (HSA) were identified by LC-TOF-MS/MS.

RESULTS

In rats, the oral bioavailability of WA was only 2.83%. In vitro, WA was hydrolyzed in both rat and human plasma and could not be inhibited by selective esterase inhibitors. Physiologic concentration of HSA not recombinant human carboxylesterases (rhCES) could significantly hydrolyze WA, and it had a similar hydrolytic capacity with human plasma to WA. Furthermore, WA could stably bind to HSA by forming hydrogen bonds with Lys199 and Arg410, accompanied by the metabolic reaction of the lactone ring opening.

CONCLUSION

The study showed that WA underwent obvious hydrolysis in rat and human plasma, which implied a strong first-pass effect. Serum albumin rather than common esterases primarily contributed to the hydrolytic metabolism of WA in plasma.

Serum Albumin: A Multifaced Enzyme

Human serum albumin (HSA) is the most abundant protein in plasma, contributing actively to oncotic pressure maintenance and fluid distribution between body compartments. HSA acts as the main carrier of fatty acids, recognizes metal ions, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays esterase, enolase, glucuronidase, and peroxidase (pseudo)-enzymatic activities. HSA-based catalysis is physiologically relevant, affecting the metabolism of endogenous and exogenous compounds including proteins, lipids, cholesterol, reactive oxygen species (ROS), and drugs. Catalytic properties of HSA are modulated by allosteric effectors, competitive inhibitors, chemical modifications, pathological conditions, and aging. HSA displays anti-oxidant properties and is critical for plasma detoxification from toxic agents and for pro-drugs activation. The enzymatic properties of HSA can be also exploited by chemical industries as a scaffold to produce libraries of catalysts with improved proficiency and stereoselectivity for water decontamination from poisonous agents and environmental contaminants, in the so called “green chemistry” field. Here, an overview of the intrinsic and metal dependent (pseudo-)enzymatic properties of HSA is reported to highlight the roles played by this multifaced protein.

Pseudosterase activity-based specific detection of human serum albumin on gel

Human serum albumin (HSA) has pseudoesterase activity. So far on gel specific detection of such property of HSA is never reported. Moreover, protein binding dyes are non-specific for albumin. However, many of such dyes are used for HSA detection. So, dye-based albumin detection on the gel is expected to generate false-positive results for HSA. In this context, we have discovered that Fast Blue BB (FBBB, 0.12%) stains specifically HSA pseudoesterase activity with 2 Naphthyl acetate (2NA) as an ester substrate. Further, neostigmine has not inhibited the pseudoesterase activity associated with HSA. Neostigmine is a known inhibitor of many true esterases like acetylcholinesterase. So, neostigmine addition offers specificity to the method developed for staining of HSA. Additionally, 2NA stains HSA better than bovine serum albumin (BSA). Exploring all these novel findings, we have devised a simple method of HSA detection on the gel, accurately where other esterases are not detected. To the best of our knowledge, our method is the first to detect HSA pseudoesterase activity specifically on gel without getting interfered by any other esterase activity. The method detects HSA better than BSA. We feel that this method will go a long way for the specific detection of HSA on the gel. It is also relevant for understanding the purity of donor human milk matrix and pharmaceutical preparation of HSA. Our method can detect 7 μM of added HSA in human urine. Therefore, our method can be proceeded further for microalbuminuria detection in days to come.

Pseudoesterase activity of albumin: A probable determinant of cholesterol biosynthesis

The association between hypoalbuminemia and coronary artery disease is known from some time. However, the reason as to how such phenomenon is correlated remains unknown. We have observed from published scientific literature that HSA has the potential to control cholesterol biosynthesis due to its pseudoesterase activity. In-silico observations have supported our view since acetyl coA, the precursor molecule of cholesterol biosynthesis is shown to bind with Tyr 411 of HSA. Incidentally, Tyr411 is a critical moiety for pseudoesterase activity of albumin. With this frame of reference in mind we hypothesize that pseudoesterase activity of HSA is an important determinant of lipid metabolism including cholesterol biosynthesis. Therefore, albumin has the potential to influence the atherosclerotic state important for pathogenesis of coronary artery diseases.