Destruction of endogenous low density lipoprotein in incubated intima.

The effect of incubation on the content of endogenous intact plasma lipoprotein (LP) has been examined in minced samples of normal intima and lesions from 38 patients. Both the electrophoretically mobile and the immobilized LP fractions decreased on incubation, and the rate of destruction was proportional to LP concentration (r=0.832, p less than 0.001). Mincing the intima with EDTA before incubation increased the rate of destruction about 4-fold in fibrous lesions but not in lesions containing numerous fat-filled cells. The destruction of LP was highly dependent on pH; the rate was highest below pH 5.5 and destruction was almost completely inhibited above pH 6.4. In standard cathepsin assays haemoglobin substrate was hydrolysed at a rate comparable to the rate of destruction of LP. The results suggest that LP may be degraded by a lysosomal cathepsin in intima.

The release of an immobilized lipoprotein fraction from atherosclerotic lesions by incubation with plasmin.

A large amount of plasma low density lipoprotein is present in human aortic intima, and this can be removed and measured by electrophoresis directly from the minced tissue into an antibody-containing gel. We now find that, in addition to this electrophoretically mobile lipoprotein, there is an immobilized lipoprotein fraction than can be released from lesions by incubation of the tissue sample with plasmin or other proteolytic enzymes after the mobile lipoprotein has been removed. The concentration of immobilized lipoprotein is highly correlated with the concentration of the residual cholesterol (not mobile on electrophoresis) that has accumulated in the tissue (r = 0.702; P less than 0.001). Thus, in normal intima and early gelatinous lesions it is about 15% of the concentration of mobile lipoprotein, whereas in the atheroma lipid layers of fibrous or gelatinous plaques it may be 2 or 3 times greater than the concentration of mobile lipoprotein. This suggests that immobilization of plasma lipoprotein is an intermediate step in the irreversible deposition of extracellular cholesterol in atherosclerotic lesions. Incubation with plasmin allowed maximum release of lipoprotein: plasmin = crude collagenase greater than trypsin greater than "pure" collagenase greater than chondroitinase ABC in order of their relative effectiveness. The concentration of immobilized lipoprotein was significantly correlated (r = 0.793; P less than 0.001) with the concentration in the tissue of fibrin or other insoluble derivatives of fibrinogen ("fibrin"). In aliquots of lesions incubated with varying amounts of plasmin for varying times there was a constant relation between release of lipoprotein and release of fibrin-degradation products. Together, these findings suggest that the lipoprotein is associated with insoluble "fibrin". This appears to be of considerable clinical interest, suggesting a synergism between lipoprotein and fibrinogen in the accumulation of lipid in lesions.

Insoluble "fibrin" in human aortic intima. Quantitative studies on the relationship between insoluble "fibrin", soluble fibrinogen and low density lipoprotein.

A quantitative assay for fibrin or other insoluble fibrin-like antigens ("fibrin") in small samples of intima is described. Tissue samples were subjected to electrophoresis directly from the intima into an antibody-containing gel to remove and measure fibrinogen and other soluble fibrin reactive antigens (FRA). The residual tissue was then exhaustively incubated with plasmin, and the soluble fragments generated from the insoluble "fibrin" were measured by quantitative immunoelectrophoresis. "Fibrin" accounted for about 2% of the tissue dry weight in normal intima and the ratio fibrinogen/"fibrin" was 1-1.5. In the gelatinous lesions, which seem to be the precursors of fibrous plaques, there was a small increase in "fibrin" but a substantial increase in fibrinogen and low density (LD)-lipoprotein, and the ratio fibrinogen/"fibrin" rose to about 3, which suggests that the increase in "fibrin" is secondary to increased permeation of fibrinogen. At the edges of large plaques there was also a threefold increase in fibrinogen, but "fibrin" increased fivefold, and accounted for 10% of the tissue dry weight. The same high concentration was found in the centres of large fibrous plaques with advanced atheroma lipid. Raised levels of "fibrin" were accompanied by raised levels of fibrinogen in most tissue samples. About 80% of the total soluble FRA could be clotted with thrombin; there was no significant difference between normal intima and lesions, and the proportion clotted was not related to "fibrin" content.