The content of apolipoprotein B in chylomicron particles.

The number of particles and the mass of apoprotein B were measured in samples of rat lymph chylomicrons in order to assign the mass of apoprotein B per particle. Chylomicron numbers were estimated by electron microscopy after negative staining with 2% potassium phosphotungstate at pH 4.8. The distribution of particle numbers was plotted against particle volume. An exponential relationship was found. Integration of the function gave an estimate of the total number of particles for the total chylomicron volume, which was calculated from the chemical composition and the partial specific volumes of the constituents. Apoprotein B was measured by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Two procedures were employed to allow for the influence of artefactual flattening of the chylomicrons on the electron microscope grid. First, it was reasoned that the flattening artefact would be greatest for the largest particles, so measurements of subfractions of smaller chylomicrons would be less affected by flattening. The mass of apoprotein B per particle was 0.48 x 10(6) daltons in the smallest subfraction. Second, a correction was applied such that the apparent volume of the oblate spheroid produced by flattening was converted to the true volume of the native spherical particle. The flattening artefact increased exponentially with particle size and was an inverse power function of the surface tension. At an assumed value of chylomicron surface tension of 2 mN/m, the apoprotein B per particle was 0.47 x 10(6) daltons.