Development of an immunoassay for the beta-adrenergic agonist ractopamine

Antibody generated from ractopamine-hemiglutarate-KLH was used to develop a ractopamine ELISA. The antibody showed good sensitivity in phosphate buffer, with an IC50 of 4.2 ng/ml (ppb) toward ractopamine and 16.2 ng/ml toward glucuronides of ractopamine conjugated to the phenethanolamine phenol of ractopamine. Phenylbutylamine phenol glucuronides of the (RS, SR) ractopamine diastereoisomers showed about 4% cross-reactivity, but the glucuronide of the (RR, SS) diastereoisomers conjugated at the same phenolic group showed no detectable reactivity with the antibody. The antibody generally had cross-reactivity towards compounds with bis-phenylalkyl amine structures rather than compounds with simple branched N-alkyl substituents. For example, the antibody showed little or no cross reactivity towards clenbuterol, isoproterenol, metaproterenol, and salbutamol, but cross-reacted with dobutamine. The system demonstrated a matrix effect similar to other enzyme immunoassays, dilution of urine decreased but did not eliminate the matrix effect.