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

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 gamma- and alpha-phosphates but not the adenine and the sugar moieties of the ATP molecule to BSA. The observed stoichiometry was 0.75 mol Pi to mol BSA for the gamma-phosphate and 0.5 mol Pi to mol BSA for the alpha-phosphate of ATP. The presence of serine phosphate and threonine phosphate was detected in the hydrolysate of phosphorylated BSA by means of comparison of Rf 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.