1. 1. A comparison of 2-hexadecanoylthio-ethane-1-phosphocholine and 3-hexadecanoylthio-propane-1-phosphocholine and their oxyester counterparts as substrates for some lipolytic enzymes was made.\ud \ud 2. 2. The critical micelle concentration and the transition temperature of the synthetic substrates were compared with the values for 1-hexadecanoyl-sn-glycero-3-phosphocholine.\ud \ud 3. 3. All above-mentioned compounds were deacylated by lysophospholipases. Phospholipase A2 hydrolyzed only the acyl- sulfur- and oxygenester bond in 2-hexadecanoyl-ethane-1-phosphocholine.\ud \ud 4. 4. Kinetic parameters, Km and V, for hydrolysis of these substrates were determined. Km values for thioester substrates were 5–10-fold lower than for the corresponding oxyesters. Maximal hydrolysis rates were 2–5 times higher for the thioesters.\ud \ud 5. 5. Hydrolysis of thioesters by phospholipase A2, lipase and lysophospho-lipase was shown to proceed by an S-acyl cleavage mechanism.\ud \ud 6. 6. Beef liver lysophospholipase II was rapidly and stoichiometrically inactivated by diisopropylfluorophosphate and bis(p-nitrophenyl) phosphate. Inactivation by the latter inhibitor showed burst-like kinetics.\ud \ud 7. 7. Attempts to show burst-kinetics during the pre-steady state hydrolysis of 2-hexadecanoylthio-ethane-1-phosphocholine by lysophospholipase II were negative. These results are interpreted to indicate that a step prior to deacylation of the enzyme is rate-determining
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