The Localization and Multimeric Nature of Component PP3 in Bovine Milk: Purification and Characterization of PP3 from Caprine and Ovine Milks

The distribution of proteose-peptone component PP3 in bovine whey, milk fat globule membrane, and casein has been investigated with antibodies raised against highly purified PP3. Using Western blot analysis, we show that PP3 is present in the milk fat globule membrane and in whey but is absent in the casein fraction. The proposed multimeric structure of bovine PP3 was analyzed by mass spectrometry and gel filtration. Calibrated gel filtration of acidic whey showed that PP3 eluted at a volume corresponding to 190 kDa, indicating that PP3 exists as a multimeric aggregate in bovine milk. Western blot analysis with anti-bovine PP3 immunoglobulins was used to analyze caprine, ovine, and human milks, and immunoreactive proteins were detected in caprine and ovine milks. Finally, the immunoreactive proteins from caprine and ovine milks were purified and characterized as PP3 analogues by amino acid analysis and N-terminal sequence analysis.