The kinetics of the oxidation of human low density lipoprotein (LDL) can be measured continuously by monitoring the change of the 234 nm diene absorption. The time-course shows three consecutive phases, a lag-phase during which the diene absorption increases only weakly, a propagation phase with a rapid increase of the diene absorption and finally a decomposition phase. The increase of the dienes is highly correlated with the increase of MDA or lipid hydroperoxides. The duration of the lag-phase is determined by the endogenous antioxidants contained in LDL (vitamin E, carotenoids, retinylstearate). Water-soluble antioxidants (ascorbic acid, urate) added in micromolar concentrations prolong the lag-phase in a concentration-dependent manner. The determination of the lag-phase is a convenient and objective procedure for determining the susceptibility of LDL from different donors towards oxidation as well as effects of pro- and antioxidants.

Oxidized LDL is taken up by the macrophage scavenger receptor and leads to lipid laden foam cells, which are characteristic constituents of early atherosclerotic lesions. Oxidation of LDL is a lipid peroxidation process, in which the PUFAs contained in the LDL lipids (on average 1200 molecules PUFAs/LDL particle, 88 O/ o 18 : 2, 12 "10 20 : 4) are rapidly converted to lipid hydroperoxides and to aldehydic lipid peroxidation products. Aldehydes and lipid alkoxy radicals alter the property of the apo-B.The rapid oxidation of LDL is preceded by a lag phase during which the endogeneous antioxidants of LDL are consumed in the sequence: alphatocopherol, gamma-tocopherol, lycopine, retinyl derivative and betacarotene. LDL with artificially elevated alpha-tocopherol content is significantly more resistant to oxidation as measured by the duration of the lag phase. Urate and ascorbate can also retard LDL oxidation,

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The lipoprotein complexing activity of glycosaminoglycans (GAG) prepared from human aortas with iipoprotein Lp(a) in comparison to low density lipoproteins (LDL) was determined tubidimetrically in the presence of Ca ++ . In control experiments, purified chondroitin-6 sulfate and proteoglycans (PG) were used. Lp(a) exhibited approximately a threefold higher reactivity. Analyzing the chemical composition of the complexes, we found that Lp(a) had greater than fourfold higher binding capacity for GAG. The binding capacity of Lp(a) to PG was 3.4-fold higher as compared to LDL. The binding capacity of both lipoproteins for chondroitin-6 sulfate was only 50% in comparison to GAG, but again Lp Its plasma concentration is under genetic control, and evidence is accumulating that a plasma level above 25 to 30 mg/dl must be considered as an independent risk factor for atherosclerosis and myocardial infarction. 234 Size, morphology, and lipid composition of Lp(a) is similar to low density lipoprotein (LDL). 5 The most striking difference between the two lipoproteins is the presence of an additional apolipoprotein (apo) in the Lp(a) particle. This protein is called apo a or Lp(a) specific antigen. Apo a may be dissociated from Lp(a) by treating with disulfide reducing agents, 6 leaving "Lp(a-)", a lipoprotein that is chemically and immunochemically similar to LDL Apo a exhibits a considerable size heterogeneity ranging from 300 to 700 kD. 7 The carbohydrate content, notably that of M. Bihari Varga and E. Gruber are at the Second Department of Pathology, Semmelweis Medical University, Budapest, Hungary. M. Rotheneder, R. Zechner, and G. M. Kostner are at the Medical Biochemistry Department, University of Graz, Graz, Austria.These studies were partly supported by Grant P 5891 from the Austrian Research Foundation.Address for correspondence: Professor Dr. G.M. Kostner, Medical Biochemistry Department, University of Graz, A-8010 Graz, Austria.Received November 12,1987; revision accepted July 18,1988. sialic acids of apo a, is considerably higher than that of apo B. To shed more light on the possible molecular mechanisms of atherogenicity, Lp(a) and LDL from the same donor were made to interact with GAG and proteoglycans (PG), and their capabilities to induce cholesteryl ester accumulation in mouse peritoneal macrophages (MPM), were compared. Major emphasis was given to lipoprotein-GAG complexes, because free GAG, but little PG, is found in circulating plasma.7 . 1011 Methods Isolation and Characterization of LipoproteinsLDL and Lp(a) were prepared from pooled plasma of donors with Lp(a) levels of >40 mg/ld. In most cases, LDL and Lp(a) prepared from identical pools were compared. For a control, LDL was also prepared from individuals who were apparently Lp(a)-negative.

The influence of low- and high-density lipoproteins on the proliferation of human breast cancer cells in culture was studied. We compared total cell number after incubation for 48 hr in culture medium containing or lacking plasma lipoproteins. Marked differences were found between hormone-dependent (MCF-7, T-47-D, ZR-75) and hormone-independent (MDA-MB-231, HBL-100) mammary tumor cell lines. The cells also reacted differently on the different lipoproteins offered in the medium. Human low-density lipoproteins (LDL) exhibited a marked stimulation of the growth of hormone-independent cell lines but no or only toxic effects upon the hormone-sensitive lines. Human high-density lipoproteins (HDL) stimulated the proliferation of all cell lines in a dose-dependent manner but hormone-independent cells showed a higher response. These findings point towards different utilizations of nutrients in hormone-dependent and hormone-independent cells.