Detection of alkaline phosphatase by competitive indirect ELISA using immunoglobulin in yolk (IgY) specific against bovine milk alkaline phosphatase

Immunoglobulin in yolk (IgY) (with a titer of 1.3 × 10⁶) specific against bovine milk (BM) alkaline phosphatase (ALP) was obtained by intramuscularly immunizing hens on the thigh and was used as the primary antibody to conduct competitive indirect enzyme-linked immunosorbent assay (CI-ELISA) to determine BMALP in ALPs from BM and Escherichia coli sources. A relationship between the ELISA value and the BMALP level (0.01-10 μg/mL) in whole milk (R² = 0.9019) or in skimmed milk (R² = 0.9402) was observed. The maximal inhibition (%) of BMALP on the microtiter plate by free BMALP at 10 μg/mL whole milk (3.89 mU/μg BMALP) was about 50%, while no inhibition (%) of BMALP by free E. coli ALP at concentrations between 0.01 to 10 μg/mL (60 mU/μg E. coli ALP) was determined. At BMALP levels higher than 0.1 μg/mL, CI-ELISA was proved to be effective in differentiating between BMALP and E. coli ALP and quantifying BMALP in whole milk or skimmed milk in the presence of E. coli ALP with an activity of 0.6 U/mL. Higher inhibition (about 70%) of BMALP on the microtiter plate by free BMALP in diluted (10¹-10⁴ fold) milk samples was observed. The optimal conditions for CI-ELISA in determining BMALP (0.1-10 μg/mL) from ALPs in milk samples were using 10³-fold diluted crude IgY specific against BMALP as primary antibody and 10³-fold diluted goat anti-chicken IgG-ALP conjugate as the secondary antibody.