摘要：Platelet activating factor (1- O -alkyl-2-acetyl- sn -glycero-3-phosphocholine; PAF) acetylhydrolase (PAF-AH) activity has been identified in bovine colostrum and high levels of this activity are found in early colostrum (within 24 h after parturition). In this study, PAF-AH in early colostrum was purified by ammonium sulfate precipitation, and sequential use of butyl-Toyopearl 650M, DEAE-Sepharose, heparin-Sepharose, hydroxyapatite, chelating-Sepharose and Mono Q HPLC column chromatography. This enzyme is a monomeric polypeptide with a molecular weight of approximately 45 kDa on 12.5% SDS-PAGE. The V max and K m for PAF-AH were 87.6 μ M and 7.96 nmol/min/mg respectively. This enzyme was inhibited by phenylmethylsulfonyl fluoride, iodoacetamide and p -bromophenacylbromide, suggesting that both serine and histidine residues are required for enzyme activity. It was not inactivated by NaF or dithiothreitol. The purified enzyme did not degrade phospholipids with a long chain fatty acyl group at the sn -2 position. Accordingly, this enzyme is distinct from phospholipase A2. In addition, PAF-AH selectively hydrolyzed oxidatively modified phosphatidylcholine. Furthermore, this enzyme was shown by Western blot analysis using antibody to human plasma PAF-AH to be plasma type PAF-AH. These results clearly demonstrate that 45 kDa plasma type PAF-AH activity exists in bovine colostrum.
