SECRETORY PAF-ACETYLHYDROLASE OF THE RAT HEPATOBILIARY SYSTEM - CHARACTERIZATION AND PARTIAL-PURIFICATION

Hepatocytes and Kupffer cells in primary culture both secrete plasma-t ype platelet-activating factor-acetylhydrolase (pPAF-AH) into serum-fr ee culture medium. The rate of secretion of pPAF-AH by Kupffer cells w as 20 to 25 times higher than from hepatocytes, and Kupffer cells expr essed a higher level of pPAF-AH mRNA than did hepatocytes. Purified li ver cell-secreted pPAF-AH exhibited a major protein band of 65-67 kDa on SDS-PAGE; this was the band predominantly labeled when the enzyme c atalytic center was reacted with [H-3]diisopropylfluorophosphate ([H-3 ]DFP). Rat bile collected from cannulated bile ducts contained signifi cant PAF-AH activity, and bile samples possessed a prominent band at 3 0-32 kDa, which was the exclusive target for [H-3]DFP. Experiments usi ng tunicamycin, an inhibitor of N-linked glycosylation, and endoglycos idase H suggested that pPAF-AH secreted constitutively by cultured hep atocytes and Kupffer cells is glycosylated. The present study supports the notion that hepatic secretion of pPAF-AH into the blood contribut es to the regulation of PAF and oxidized phospholipid levels in the ci rculation, whereas secretion of PAF-AH into the bile may allow hepatic control of these phospholipid signaling molecules in the gastrointest inal tract.