Identification by a proteomic approach of a plasma protein as a possible biomarker of illicit dexamethasone treatment in veal calves

Corticosteroids have become the most widespread illegal growth promoters in veal calves and beef cattle. Testing for corticosteroids relies on either direct detection of compounds or their metabolites or indirect detection to identify changes in biological pathways. We used a comparative proteomic approach, based on two-dimensional electrophoresis (2DE), to identify plasma protein markers after short-term dexamethasone administration in veal calves. Twenty-three male Friesian veal calves were treated experimentally with dexamethasone sodium phosphate: 10 received low-dose administration of the drug (0.4mgday(-1) per os) for 20 consecutive days (treatment group); 10 received the drug at therapeutic dosage (2-4mgkg(-1) i.m.) for 3 consecutive days (comparison group). Three animals were not treated (control group). Plasma samples were collected from each animal at six time points (T1-T6; treatment and control group) and at four time points (T1-T4; comparison group) and stored at -80 degrees C until analysis. Plasma proteins were quantified and analysed in triplicate by 2DE. The images were analysed with Bionumerics((R)) software. Comparison of 2DE maps obtained from blood samples at T1 (before treatment) and at T6 (final sampling) showed a significant disappearance (p<0.001) of two protein spots at T6 in the treatment group. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis and immunoblotting identified these isoforms as serum paraoxonase/arylesterase 1 precursor (PON1). Synthesised in the liver and released into the blood, PON1 has an important role in lipid metabolism. The absence of variation of this protein in the comparison group suggests that the marker has good specificity for detecting illicit corticosteroid treatment.