Reduction of phosphatidylcholine hydroperoxide by apolipoprotein A-I: purification of the hydroperoxide-reducing proteins from human blood plasma.

Plasma glutathione peroxidase (GSHPx) has been suggested to reduce submicromolar levels of free fatty acid hydroperoxides and phosphatidylcholine hydroperoxides (PC-OOH), and therefore these hydroperoxides are undetectable in human blood plasma. The capacity for the reduction should be about 2.5 microM as the level of glutathione in human plasma is about 5 microM. However, 2 h of aerobic incubation of 58 microM PC-OOH in human plasma at 37 degrees C resulted in the formation of 36 microM phosphatidylcholine hydroxide (PC-OH). The presence of PC-OOH-reducing protein other than plasma GSHPx was suggested by the results. a) The same rates of PC-OOH decay and PC-OH formation were observed in both sera from rats with selenium-deficient and selenium-supplemented diet; b) the PC-OOH-reducing activity was observed only in the high molecular weight fraction but not in the low molecular weight fraction; and c) albumin did not work as a reducing substrate of plasma GSHPx. We have isolated two hydroperoxide-reducing protein fractions from human plasma by a sequential purification scheme, comprising an ammonium sulfate precipitation followed by sequential chromatography on anion exchange, hydrophobic interaction, and heparin columns. One of the proteins was identified as apolipoprotein A-I by N-terminal amino acid sequence analysis. Moreover, the hydroperoxide-reducing activity of one of the fractions was inhibited almost completely by the addition of anti-apolipoprotein A-I antibody. These findings demonstrate that apolipoprotein A-I in high density lipoprotein can reduce PC-OOH to PC-OH.