Hydroxopentaamminechromium(III) promoted phosphorylation of bovine serum albumin: its potential implications in understanding biotoxicity of chromium

Abstract

Evidence for chromium(III) induced phosphorylation of a biomarker protein bovine serum albumin (BSA) is presented. Radiolabelled adenosine 5'-triphosphate (ATP) was reacted with BSA in the presence of various Cr(III) salts. While [Cr(NH₃)₅(H₂O)]³⁺ brought about phosphorylation of BSA, several Cr(III) complexes, viz. [Cr(bpy)₃]³⁺, [Cr(phen)₃]³⁺, [Cr(en)₃]³⁺, [Cr(salen)(H₂O)₂]⁺ and [Cr(salprn)(H₂O)₂]⁺, did not phosphorylate BSA. The Cr(III) mediated the transfer of γ- and α-phosphates but not the adenine and the sugar moieties of the ATP molecule to BSA. The observed stoichiometry was 0.75 mol P_i to mol BSA for the γ-phosphate and 0.5 mol P_i to mol BSA for the α-phosphate of ATP. The presence of serine phosphate and threonine phosphate was detected in the hydrolysate of phosphorylated BSA by means of comparison of R_f values with authentic samples of phosphoserine and phosphothreonine after chromatographic separation and autoradiography. [Cr(NH₃)₅(H₂O)]³⁺ at pH 7.4 is known to exist as the conjugate base [Cr(NH₃)₅(OH)]²⁺ and is capable of ligand substitution involving metal-oxygen bond retention. Such anation reaction of [Cr(NH₃)₅(OH)]²⁺ with ATP subsequently leads to the esterification of alcoholic hydroxyl groups of serine and threonine of BSA. Possible consequences of chromium(III) induced in vivo phosphorylation of proteins are discussed.