Characterization of sheep lung lymph lipoproteins: chemical and physical properties

We have determined the composition and distribution of plasma and lung lymph lipoproteins from unanesthetized ewes. Cholesterol, triglyceride, and phospholipid levels in lung lymph were 45%, 50%, and 50%, respectively, of those in plasma. Lipoproteins from both lymph and plasma were separated into two major fractions: d less than 1.063 g/ml or "LDL", and d 1.063-1.21 g/ml or HDL. HDL was the major lipoprotein species in the plasma and lymph. Gradient gel electrophoresis of HDL on 4-30% gels showed that, in lymph, HDL particles were shifted to larger sizes; in addition to a peak at 8.5 nm, which was similar to plasma HDL, there were two additional components of larger size, one at 9.2 nm and the other at 12 nm. Electron microscopy revealed that lymph HDL contained two new particles not seen in plasma: large, round particles, 13.6 nm diameter, and discoidal particles, 18.7 by 4.9 nm, long and short axis, respectively. Compositional analysis of lymph HDL revealed a relative enrichment in free cholesterol as well as an enrichment in apolipoprotein E. Lymph "LDL" on gradient gel electrophoresis was extremely heterogeneous. Several peaks were evident in the 23-30 nm size range (similar to plasma "LDL"), but a supplementary component at approximately 15-16 nm was also present. Whereas plasma "LDL" on electron microscopy contained only round particles 26 nm in diameter, lymph contained an additional, unusual particle which was close-packed, with square geometry, and was 15 nm in diameter. Lymph apolipoprotein composition differed from that of plasma by the appearance of apoE and A-I as well as apoB. Particles containing apoE and A-I were separated from apoB-containing particles in a fraction of d 1.047-1.063 g/ml by density gradient centrifugation. On electron microscopy, this fraction revealed square-packing particles; the density and apolipoprotein composition suggest that these unusual particles are a continuum of HDL. Changes in the physical and chemical properties of lung lymph lipoproteins suggest that these particles are metabolically modified.