We monitored peroxidative stress in the surface monolayer as compared with the outer core of large, buoyant (d 1.025-1.032 g/ml) and small, dense (d 1.040-1.054 g/ml) low density lipoprotein (LDL) subfractions using the oxidation-labile fluorescent probes parinaric acid (PnA) and parinaric acid methyl ester (PnME), which partition preferentially into these respective regions of LDL. Oxidation was initiated either with CuSO4 (5 microM) or the iron (Fe3+)-containing lipophilic complex hemin (1.0 microM) plus cumene hydroperoxide to facilitate heme degradation. In the presence of Cu2+, PnA was depleted significantly more rapidly than PnME in dense (P = 0.039) but not in buoyant LDL, suggesting that surface vulnerability is enhanced in dense LDL particles. With hemin, PnA and PnME were similarly susceptible within both subfractions, although there was a trend toward slower loss of PnA in buoyant LDL (P = 0.10), consistent with the internal site of initiation and a greater surface resistance in buoyant particles. As indicated by conjugated diene lag times, dense LDL was more susceptible than buoyant LDL to oxidation by Cu2+ (P = 0.03) but not hemin (P = 0.68). These results suggest that the increased susceptibility of dense LDL to oxidation by external agents such as Cu2+ is at least partially mediated by an enhanced vulnerability of the surface compartment.