| Summary: | Protein composition was determined in mesenteric lymph chylomicrons from fat-fed rats. Among the proteins of intermediate molecular weight, apoproteins A-I and the arginine-rich apoprotein accounted for 31% and 4% of the total protein mass, respectively. Apoprotein B and apoprotein A-IV each accounted for about 10% and proteins of low molecular weight (C apoproteins and apoprotein A-II) accounted for most of the remainder. Apoprotein A-I also accounted for more than 30% of the protein mass of mesenteric lymph lipoproteins of density less than 1.006 g/ml (“small chylomicrons”) obtained from rats fed glucose. Aproprotein A-I was partially dissociated from chylomicrons during brief ultracentrifugation. Both the arginine-rich apoprotein and the C apoproteins in rat blood serum were transferred to lymph chylomicrons from fat-fed rats during incubation in vitro. Content of arginine-rich apoprotein, determined immunochemically, increased six-fold when chylomicrons were diluted to a final concentration of 500 mg/dl in blood serum. Upon incubation of chylomicrons in equivalent volumes of ultracentrifugal fractions of serum, the increase of the arginine-rich apoprotein was: very low density lipoproteins, 1.5-fold; high density lipoproteins, 1.8-fold; density fraction greater than 1.006 g/ml, 5.0-fold; density fraction greater than 1.21 g/ml, 11-fold. Content of apoprotein A-I, also determined immunochemically, was not altered appreciably by exposure to serum or its ultracentrifugal fractions, whereas content of C apoproteins, estimated from intensity of staining of the low molecular weight protein component in polyacrylamide gel electropherograms, increased in all cases except for the density fraction greater than 1.21 g/ml. The fractional content of apoprotein A-I in the protein of chylomicrons fell after incubation, whereas that of the arginine-rich apoprotein remained constant or rose substantially. The fractional content of apoprotein A-IV in chylomicron-protein tended to follow that of apoprotein A-I, as judged from polyacrylamide gel electropherograms. Transfer of the arginine-rich and C apoproteins to chylomicrons from blood serum was directly related to the volume of serum in which the chylomicrons were diluted and occurred rapidly at room temperature or at 4 degrees C.
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