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Contribution of the Fatty Acids of Three Low Density Serum Lipoproteins to Bovine Milk Fat

Biochemistry Department, National Institute for Research in Dairying, Shinfield, Reading, RG2 9AT, England

ABSTRACT

Tritim-labeled palmitic acid was put into the rumen of a lactating cow and the precipitable lipopretiens, consisting of a mixture of one lipoprotein of very low density and two others of low density, were separated from serial smaples of arterial, jugular, and mammary venous serum.

Specific radioactivities of lipids in arterial and jugular venous serum were equivalent. There were arteriovenous differences in the fatty acid composition of the triglycerides of the lipoproteins. Specific radioactivity-time curves showed that the triglyceride fatty acids of two of the lipoproteins, which together accounted for 90% of the triglycerides carried by all three, behaved as a single pool to which those of the third did not belong. From mean concentrations and specific radioactivities, 45% and 47% of milk glyceride palmitic and mixed fatty acids, respectively, had been derived from precipitable lipoproteins. The relative contribution of each subtraction was also calculated. A net transfer of radioactivity from triglycerides to nonesterified fatty acids, in which there was no arteriovenous difference in concentration, showed that fatty acids had exchanged between the two lipids during passage through the udder.

The specific radioactivity-time curve of the triglyceride palmitic acid of the two lipoprotcins which formed a single pool could be described by the sum of four exponential functions. This palmitic acid must have been in equilibrium with a larger pool containing at least 9 g.