Significantly improved detection performances of immunoassay for ractopamine in urine based on highly urea-tolerant rabbit monoclonal antibody

Highly sensitive and accurate screening of ractopamine (RAC) residue in animal urine is greatly needed to ensure food security. The detection performance of immunoassay for RAC was always seriously harmed by the antibody inactivation derived from urea. Here, we first discovered one rabbit monoclonal antibody (RmAb) to RAC with a high affinity of 0.007 ng mL-1 and a surprising urea tolerance of 3 M urea, which is beneficial for developing robustly developed immunoassay in urine without sample pretreatment. The limits of detection of developed indirect competitive enzyme-linked immunosorbent assay based on RmAb1 for RAC were 0.0042-0.014 μg L-1 with the coefficient of variation below 11.7% in swine, sheep, and cow urine, significantly improved 10-100-fold in sensitivity. Moreover, the urea-tolerant mechanism of RmAb1 showed that more non-polar amino acids, more hydrogen bond donors on the surface, and preponderant Pi interaction of antibody-RAC all contributed to the stability of the RmAb1 in a high concentration of urea.