ECTOPIC EPIDIDYMAL EXPRESSION OF GUINEA-PIG INTESTINAL PHOSPHOLIPASE-B - POSSIBLE ROLE IN SPERM MATURATION AND ACTIVATION BY LIMITED PROTEOLYTIC DIGESTION

Guinea pig intestinal phospholipase B is a calcium-independent phospho lipase hydrolyzing sequentially the acyl ester bonds at sn-2 and sn-1 positions of glycerophospholipids, promoting the formation of sn-glyce ro-3-phosphocholine from phosphatidylcholine. This 140-kDa glycoprotei n from the brush border membrane of differentiated enterocytes contrib utes to lipid digestion as an ectoenzyme, The cDNA coding for guinea p ig phospholipase B was revealed to be the homologue of AdRab-B, an mRN A appearing in rabbit upon intestine development. The sequence predict s a polypeptide of 1463 amino acids displaying four homologous repeats , two of them containing the lipase consensus sequence GXSXG, A 5-kilo base transcript was particularly abundant in mature ileal and jejunal enterocytes but was also detected in epididymis, where phospholipase B displayed a higher molecular mass (170 kDa versus 140 kDa in intestin e), with no obvious evidence for enzyme activity. Trypsin treatment of phospholipase B immunoprecipitated from epididymal membranes reduced its size to 140 kDa, coinciding with the appearance of a significant p hospholipase A, activity. The same results were obtained in COS cells transfected with phospholipase B cDNA. Since sn-glycero-3-phosphocholi ne present at high concentrations in seminal plasma mainly stems from epididymis, this suggests a possible role of phospholipase B in male r eproduction, This novel localization also unravels a mechanism of phos pholipase B activation by limited proteolysis involving either trypsin in the intestinal lumen or a trypsin-like endopeptidase in the male r eproductive tract.