Abstract
Elevated circulating Retinol-binding protein 4 (RBP4) has been associated with insulin resistance, dyslipidemia, and hypertension. However, many commonly used RBP4 ELISAs have limited dynamic range. We therefore developed an enzyme-linked immunosorbent sandwich assay (ELISA) employing a novel immunoglobulin A (IgA)-type capture mAb called AG102 instead of IgG subtypes, which was selected for its stability, capture efficiency, and specificity for human RBP 4. These features of RBP4 have hampered the development of quantitative immunological assays. Molecular analysis of AG102 revealed IgA heavy and light chains and a J chain, as expected. AG102 demonstrated notable detection of both bacterial- and HEK293-expressed RBP4 in Western blots. Serial and internal deletion experiments suggested that a putative epitope may be located in the first 35 amino acids of the mature RBP4. Compared with commercial ELISAs, the AG102-based system exhibited more significant recovery of RBP4 from serum or urine at any given dilution factor. To substantiate its quantitation capacity, comparison between RBP4 measurements from quantitative western blots and the AG102-based ELISA demonstrated a significant correlation (R(2) = 0.859). After measurement for those analytes, our data suggested that IgA-based ELISA could be adapted for quantitative measurement of those analytes existing as major serum proteins or as multi-protein complexes like RBP4.