Lipoprotein(a) is an LDL-like lipoprotein whose concentration in plasma is correlated with atherosclerosis. The characteristic protein component of lipoprotein(a) is apolipoprotein(a) which is disulphide-linked to apolipoprotein B-100. Sequencing of cloned human apolipoprotein(a) complementary DNA shows that it is very similar to human plasminogen. It contains a serine protease domain and two types of plasminogen-like kringle domains, one of which is present in 37 copies.

Lipoprotein(a) (Lp(a)) has been strongly linked with atherosclerosis and is an independent risk factor for myocardial infarction. Distinguishing Lp(a) from other low-density lipoprotein particles is its content of a unique apoprotein, apo(a). The recently described sequence of apo(a) indicates a remarkable homology with plasminogen, the zymogen of the primary thrombolytic enzyme, plasmin. Lp(a) may contain 37 or more disulphide-looped kringle structures, which are 75-85% identical to the fourth kringle of plasminogen. Plasminogen receptors are widely distributed on blood cells and are present at extremely high density on endothelial cells. These receptors promote thrombolysis by accelerating plasminogen activation and protecting plasmin from inhibition. If, by molecular mimicry, Lp(a) competes with plasminogen for receptors, then thrombolysis would be inhibited and thrombosis promoted. Here we provide support for such a mechanism being responsible for the thrombotic risks associated with elevated Lp(a) by demonstrating that Lp(a) inhibits plasminogen binding to cells.

COMPOSITIONAL ANALYSIS The basic compositional data used in our analysis were taken from a recent critical compilation by Scanu and Kruski (6). The reported data are based on dry weight and thus they do not take into consideration the possible presence of water in the lipoprotein particle. In our analysis we also assumed the absence of significant amounts of structural water inside lipoproteins. Minor components, such as free fatty acid, carbohydrates, vitamins, and unidentified compounds, were ignored because, at best, they amount to not more than 5% of the total weight (6) and are likely not to play an essential structural role. For very low density lipoprotein (VLDL) and chylomicrons Scanu and Kruski presented only average compositional data. VLDL and chylomicrons are, in fact, particles of rather broad size distribution, and several authors reported compositional data on their subfractions (2, 7). These additional data on subfractions were also used wherever appropriate. Table 1 summarizes the weight percentage compositions of the major constituents of all lipoprotein classes.From the molecular weight and the weight percentage composition, assuming no structural water, one can calculate the number of molecules of each component in a lipoprotein particle ( Table 2). The following molecular weights were used for the calculations: triglycerides, 850; phospholipid, 775; cholesterol ester, 650; and cholesterol, 387. For the protein content it was not useful to calculate the number of protein molecules per particle because the various lipoprotein classes contain a variety of apoproteins of different molecular weights. For comparison it was more suitable to calculate the number of amino acid residues per particle, assuming an average residue weight of 100. Assuming that all lipoproteins are spherical particles, one can calculate from the molecular weight and the density of the particle the radius of an equivalent sphere r in units of A, also shown in Table 2. Since for the average VLDL and chylomicrons no reliable molecular 'weight data were available, we calculated the molecular weight from the experimentally determined average particle radii, 200 A and 600 A, respectively. Triglyceride and cholesterol ester compositionThe major components of lipoproteins can be divided into nonpolar lipids (triglycerides and cholesterol esters) and polar constituents (phospholipids, cholesterol, and protein). If hydrophobic interactions play an important part in organizing the components, then one would expect the existence of a hydrophobic phase, as it has already been suggested for VLDL and 837

Apolipoprotein(a) [apo(a)] is a glycoprotein with Mr %280,000 that is disulfide linked to apolipoprotein B in lipoprotein(a) particles. Elevated plasma levels of lipoprotein(a) are correlated with atherosclerosis. Partial amino acid sequence of apo(a) shows that it has striking homology to plasminogen. Plasminogen is a plasma serine protease zymogen that consists of five homologous and tandemly repeated do-

Lipoprotein(a), Lp(a), an athero-thrombotic risk factor, reacts with EO6, a natural monoclonal autoantibody that recognizes the phophorylcholine (PC) group of oxidized phosphatidylcholine (oxPtdPC) either as a lipid or linked by a Schiff base to lysine residues of peptides/proteins. Here we show that EO6 reacts with free apolipoprotein(a) apo(a), its C-terminal domain, F2 (but not the N-terminal F1), kringle V-containing fragments obtained by the enzymatic digestion of apo(a) and also kringle V-containing apo(a) recombinants. The evidence that kringle V is critical for EO6 reactivity is supported by the finding that apo(a) of rhesus monkeys lacking kringle V did not react with EO6. Based on the previously established EO6 specificity requirements, we hypothesized that all or some of the six lysines in human kringle V are involved in Schiff base linkage with oxPtdPC. To test this hypothesis, we made use of a recombinant lysine-containing apo(a) fragment, rIII, containing kringle V but not the protease domain. EO6 reacted with rIII before and after reduction to stabilize the Schiff base and also after extensive ethanol/ ether extraction that yielded no lipids. On the other hand, delipidation of the saponified product yielded an average of two mol of phospholipids/mol of protein consistent with direct analysis of inorganic phosphorous on the nonsaponified rIII. Moreover, only two of the six theoretical free lysine amino groups per mol of rIII were unavailable to chemical modification by 2,4,6-trinitrobenzene sulfonic acid. Finally, rIII, like human apo(a), stimulated the production of interleukin 8 in THP-1 macrophages in culture. Together, our studies provide evidence that in human apo(a), kringle V is the site that reacts with EO6 via lysine-oxPtdPC adducts that may also be involved in the previously reported pro-inflammatory effect of apo(a) in cultured human macrophages.