Lactonase and lactonizing activities of human serum paraoxonase (PON1) and rabbit serum PON3

Human paraoxonase (PON1) was previously shown to hydrolyze over 30 different lactones (cyclic esters). In the present study purified human PON1 was found to catalyze the reverse reaction (lactonization) of a broad range of hydroxy acids. Hydroxy acid lactonization or lactone hydrolysis is catalyzed until equilibrium between the open and closed forms is reached. Lactonization by PON1 was calcium-dependent, had a pH optimum of 5.5-6 and could be stimulated with dilauroylphosphatidylcholine. Rabbit serum PON3 and a serine esterase in mouse plasma, presumably a carboxylesterase, also catalyzed hydroxy acid lactonization. Two endogenous oxidized unsaturated fatty acids, (±)4-hydroxy-5E,7Z,10Z,13Z,16Z,19Z-docosahexaenoic acid (4-HDoHE) and (±)5-hydroxy-6E,8Z,11Z,14Z-eicosatetraenoic acid (5-HETE) lactone, were very efficiently lactonized and hydrolyzed, respectively, by PON1. Human and mouse plasma samples also catalyzed 4-HDoHE lactonization and 5-HETE lactone hydrolysis. Studies with the PON1 inhibitor EDTA and the serine esterase inhibitor phenylmethylsulfonylfluoride suggest that about 80-95% of both activities can be attributed to PON1 in the human samples. In the mouse sample, PON1 accounted for about 30% of the 4-HDoHE lactonizing activity and 72% of the 5-HETE lactonase activity. Our results demonstrate that PON1 can lactonize the hydroxy acid form of its lactone substrates and that reversible hydrolysis of lactones may be a property of lactonases that is not generally considered. Also, the high activity of PON1 towards 4-HDoHE and 5-HETE lactone suggests that oxidized eicosanoids and docosanoids may be important physiological substrates for PON1.