Interaction of Uranium(VI) with α-Amylase and Its Implication for Enzyme Activity

Abstract

Because of its chemo- and radiotoxicity, the incorporation of uranium into human body via ingestion potentially poses a serious health risk. When ingested, the gastrointestinal fluids are the primary media to interact with uranium, eventually influencing and even determining its biochemical behavior in the gastrointestinal tract and thereafter. The chemical interactions between uranium and the components of gastrointestinal fluids are, however, poorly understood to date. In this study, the complexation of uranium(VI) (as the uranyl ion, UO) with the protein α-amylase, one of the major enzymes in saliva and pancreatic juices, was investigated over a wide range of pH or uranium/α-amylase concentrations covering physiological conditions. Macroscopic sorption experiments suggested a strong and fast complexation of UO to α-amylase between pH 5 and 7. Potentiometric titration was employed to determine the complex stability constants for the relevant UO α-amylase complexes, which is crucial for reliable thermochemical modeling to assess the potential health risk of uranium. Extended X-ray absorption fine structure (EXAFS) spectroscopy revealed that α-amylase is interacting with UO primarily via its carboxylate groups presumably from the aspartic acid and glutamic acid side chains. The effect of UO on the enzyme activity was also investigated to understand the potential implication of uranium for the in vivo functions of the digestive fluids, indicating that the presence of uranium inhibits the enzyme activity. This inhibitory effect can be, however, suppressed by an excess of calcium.

Strong Uranium(VI) Binding onto Bovine Milk Proteins, Selected Protein Sequences, and Model Peptides

“…The uranyl ion is a Lewis acid metal ion with a trans -oxo group being formally trifold, having an effective charge of +3.2 and thus constituting a strong acid within the HSAB concept . It forms stable complexes only in the equatorial plane and namely with Lewis bases such as hydroxide, carbonate, or phosphate. − This includes reactions with organic ligands possessing strong binding functions such as carboxyl, hydroxyl, amino, or phosphoryl groups. − These groups are abundant in peptides and proteins; their multifaceted reactions with the uranyl ion are discussed in the literature to an increasing extent. − Especially phosphoryl-containing proteins with strong binding properties for heavy metal ions including the uranyl ion are in the focus of interest. − …”