LOW-DENSITY-LIPOPROTEIN IS THE MAJOR CARRIER OF LIPID HYDROPEROXIDES IN PLASMA - RELEVANCE TO DETERMINATION OF TOTAL PLASMA-LIPID HYDROPEROXIDE CONCENTRATIONS

High-density lipoprotein (HDL) has been proposed as the principal carr ier of hydroperoxides in plasma, based upon data gathered with an HPLC -chemiluminescence technique. To test this hypothesis we have measured total lipid hydroperoxides in native plasma using the ferrous oxidati on in Xylenol Orange (FOX) assay and then fractionated plasma into ver y-low-density lipoprotein, low-density lipoprotein (LDL) and HDL fract ions. Hydroperoxides were found to accumulate principally (more than 6 5%) in LDL, as judged by hydroperoxide content per amount of protein o r cholesterol, or expressed as a proportion of total hydroperoxide in plasma. Plasma was also incubated at 37 degrees C in the presence and absence of 2,2'-azo-bis-(2-amidinopropane) hydrochloride (AAPH), an at e-initiator of lipid peroxidation. The majority of hydroperoxides gene rated in plasma were recovered in the LDL fraction. Furthermore, when isolated lipoproteins were subject to oxidation initiated by AAPH, ver y low-density lipoprotein and LDL showed the greatest propensity for h ydroperoxide accumulation, whereas HDL seemed relatively resistant. Es timates for plasma and LDL peroxidation based upon techniques which me asure total lipid hydroperoxides suggest that levels of hydroperoxides in plasma and LDL are far higher than that those estimates generated by ostensibly more selective techniques. Higher levels of hydroperoxid es in LDL than those reported by HPLC-chemiluminescence also seem in g reater accordance with other available data concerning LDL oxidation.