| The presence of organic fluorine compounds in human plasma is presumed to result from exposure to synthetic environmental contaminants. NMR characterization of organic fluorine isolated from human plasma had identified perfluorinated fatty-acyl compounds six to eight carbons in length. Reports that a perfluorocarbon compound could uncouple monooxygenase electron transport, i.e., act as a microsomal substrate and stimulate the transport of electrons from NADPH-cofactor to oxygen while not being metabolized, suggested that such compounds might stimulate the peroxidation of microsomal membranes by the production of activated oxygen molecules. Since peroxidation of subcellular organelles has been implicated as a mechanism of cell injury in both chemically induced toxicities and disease processes, I examined the effects of perfluoro-octanoic acid (PFOA) on microsomal membrane peroxidation. PFOA was a relevant choice because it has an NMR spectrum similar to that of the organic fluorine fraction isolated from human plasma.;Enzymatic peroxidation stimulated by CCl(,4), a well-known pro-oxidant agent, was used as a positive control in these experiments. Maximum stimulation by PFOA required less than one-third the concentration necessary for the maximum CCl(,4) effect, although both compounds stimulated peroxidation to a similar extent. Addition of EDTA reduced PFOA peroxidation to a level indistinguishable from control, but 30% of the CCl(,4) stimulation was insensitive to EDTA addition. These data suggested that PFOA peroxidation, unlike that of CCl(,4), was initiated by divalent metal cations, possibly introduced as contaminants in the PFOA. However, PFOA contamination was ruled out by direct analysis, purification, and use of different PFOA preparations.;PFOA or ferrous iron, but not CCl(,4), stimulated peroxidation in the presence of ascorbic acid, a non-enzymatic reducing agent. Simultaneous addition of PFOA and ferrous iron to this non-enzymatic system had a synergistic effect upon the rate of peroxidation.;Since PFOA is a powerful surface active agent, I postulated that it stimulated peroxidation by partially solubilizing microsomal membranes. Consistent with this hypothesis, Tween 20, a surface active agent, exhibited effects upon non-enzymatic peroxidation of microsomal membranes that were qualitatively similar to the effects of PFOA. Furthermore, at concentrations used in peroxidation experiments, both PFOA and Tween 20 partially solubilized microsomal membranes.;PFOA stimulated NADPH-dependent (enzymatic) peroxidation of microsomal membranes, as measured by malondialdehyde formation. Consistent with the hypothesis that PFOA uncoupled monooxygenase electron transport, less than 0.02% of PFOA was metabolized during the lipo-peroxidative process. However, concentrations of PFOA that produced maximum stimulation of enzymatic peroxidation significantly reduced microsomal electron transport rates, indicating that PFOA was not an uncoupler of monooxygenase electron transport.;It was concluded that PFOA and CCl(,4) stimulate peroxidation by different mechanisms. The effects of PFOA are probably due to partial solubilization of the microsomal membranes, allowing greater accessibility of either intrinsic or extrinsic peroxidation initiators to the polyunsaturated fatty acids. |
