SEPARATION AND CHARACTERIZATION OF HUMAN SERUM CHYLOMICRONS

Abstract

Chylomicrons were separated by low and high speed ultracentriiugation from lipemic sera of human subjects in the absorptive phase. The final chylomicron preparation was free of other serum components and contained a small constant amount of protein, approximately 2% of the chylomicron fraction. Electrophoresis, im-munochemical analysis and absorption experiments identified it as the protein part of a mixture of [beta] and [alpha]-1 serum lipoproteins. Large ali-quots of an emulsion of serum freed of chylomicrons and coconut oil were incubated at 37[degree]C for 2 hours and ultracentrifuged as in the preparation of chylomicrons. The fat particles now showed the presence of minute amounts of [beta] and [alpha]-1 serum lipoproteins in almost the same proportion as found in chylomicrons. "Finger prints" of delipidized samples of chylomicrons and particles from serum-coconut oil emulsion gave similar, although not identical patterns. The data on "clearing factor" activity corroborated the finding that serum -1 lipoproteins are contained in chylomicrons and particles from serum-coconut oil emulsion. These 2 lipid particles, partially delipidized, were both able to activate a "clearing factor" system in vitro, a property exhibited only by intact or partially delipidized [alpha]-1 serum lipoproteins. Clearing activity was satisfactorily determined by using an emulsion of coconut oil mixed in agar as a substrate to give an opaque gel, in which the diffusing enzyme showed its activity by areas of clearing. The results obtained by this technique were in agreement with those based on falliin optical density and nonesterified fatty acid production. Chemical analysis of serum chylomicrons showed a concentration of cholesterol and phos-pholipids higher than could be accounted for by the attached [beta] and [alpha]-1 serum lipoproteins. On the basis of these results the assumption is made that in the blood stream small amounts of serum lipoproteins, by a process of absorption, form a complex with the absorbed triglycerides, cholesterol and phospholipids, to produce chylomicrons.