STUDIES ON THE SUBCELLULAR-DISTRIBUTION OF 1-O-ALKYL-2-ACETYL-SN-GLYCERO PHOSPHOCHOLINE (PAF) AND ON THE ENZYMATIC-ACTIVITIES INVOLVED IN ITS BIOSYNTHESIS WITHIN THE CILIATE TETRAHYMENA-PYRIFORMIS

The ciliated protozoan Tetrahymena pyriformis contains platelet-activa ting factor (PAF) as a physiological minor lipid. Its subcellular loca lization was found as follows: 13.7% in the pellicles, 24.9% in mitoch ondria, 56.5% in microsomes and 7.1% in the cytosol. Succinate dehydro genase was used as marker enzyme. PAF remains cell-associated unless b ovine serum albumin is included in the extracellular medium, In this c ase 15% of total PAF, portion comparable to that found in the pellicle s, is released. Investigation of the principal enzymic activities invo lved in PAF formation showed that PAF-acetyltransferase (2.3.1.67) is totally absent from the protozoan. This means that the 'remodelling' p athway occurring in pro-inflammatory cells does not contribute in PAF formation in our system. A dithiothreitol (DTT)-insensitive CDPcholine phosphocholinetransferase activity involved in PAF biosynthesis is sh own for the first time to be responsible for PAF production in T pyrif ormis. It uses exogenous alkyl-acetyl-glycerol as substrate and is sat urated over substrate concentration 250 muM. It can also use endogenou s lipids as substrate. It is distributed mainly in mitochondria and mi crosomes, much less is found in the pellicles and it is totally absent from the cytosol. Its insensitivity to DTT, its selectivity to alkyl- acetyl-G and its different distribution compared to the enzymic activi ty involved in PC formation (EC 2.7.8.2) suggest that a different enzy me, specific for PAF formation (EC 2.7.8.16) via the de novo pathway e xists in the protozoan.