Journal of Lipid Research, Vol 22, 820-828, Copyright © 1981 by Lipid Research, Inc.
The content of apolipoprotein B in chylomicron particles
S Bhattacharya and TG Redgrave
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.