Composition and concentration of lymph and serum lipoproteins during fat and cholesterol absorption in the dog.

The term chylomicron was introduced by Gage in 1920 (1) to refer to particles, mostly 0.5 to 1 p in diameter and visible with the high power, dark-field microscope, found in the intestinal lacteals and plasma during the absorption of fats. A detailed report on the nature of these particles was published by Gage and Fish in 1924 (2). Chylomicrons, as defined above, have not as yet been isolated in pure form. It is now clear, from the work of a number of investigators, that absorbed long chain fatty acids and cholesterol are transported in lymph primarily in a very low density lipoprotein fraction which includes these chylomicrons (3, 4). Several reports dealing with the chemical composition of this fraction have appeared (5-10). The present report provides information on the composition of the chylomicron-containing, very low density fraction, as well as of the other lipide-carrying fractions of lymph of the dog. Five fractions were separated from lymph ultracentrifugally by combining two procedures. One of the procedures, developed by Lindgren and reported by Pierce (II), permitted separation of lipoproteins of extremely low density (less than 1.006) ; the other, described elsewhere, separation of four lipoprotein fractions from serum (12). Since the lipides contained in these five lipoprotein fractions accounted for the total lipide carried in lymph, we were able to obtain a relatively complete picture of the changes induced in the concentration and composition of all lipide-bearing fractions of lymph during fat and cholesterol absorption in the dog. The changes in serum lipoproteins during fat absorption were also studied.