Diagnosis of acute coronary syndrome by immunoassay of ischemia-modified human serum albumin in blood

Abstract

Human serum albumin (HSA) is the most abundant circulating protein in human blood. In myocardial ischemia, the albumin-cobalt binding (ACB) assay is the only clinical test approved by the U.S. Food and Drug Administration for myocardial ischemia. The ACB assay has been thought that the ischemia-driven modification of HAS is localized within the NH2-Asp-Ala-His-Lys sequence of HSA. The high negative predictive value of ischemia-modified albumin (IMA) might be useful as an “elimination” marker in the emergency department. Despite the widespread use of the ACB assay, no absolute cutoff value has been established for the diagnosis of myocardial ischemia, and some controversial results have been reported. Recently, several studies have revealed a weak binding affinity of the N-terminus for cobalt compared to other sites of HSA, strongly implying that the primary binding site for cobalt is not the N-terminal region of HSA. Thus, an explanation for the low specificity of IMA in the ACB assay requires a greater understanding of the details of cobalt binding to IMA, which remain unclear. In the proposed study, we developed a standardized assay compared with commercially available laboratory results and a multiple-well plate-based ACB assay. An immunoassay for N-terminus of IMA for diagnosis of acute coronary syndrome was done by an enzyme-linked immunosorbent assay (ELISA) in patients admitted to the emergency department with acute chest pain. Compared with a conventional ACB assay, the ELISA for N-terminal-modified HSA differentiated less accurately acute coronary syndrome from non-ischemic chest pain. In acute chest pain patients, the N-terminal site of HSA has a negligible or limited role in cobalt binding in the ACB assay. The inhibition of cobalt binding was studied with drugs which selectively bind to Cys34 of HAS. Among these drugs, captopril showed inhibited cobalt binding to HSA. Captopril neither directly bound CoCl2 nor increased free cobalt in the ACB as...