Role of plasma lipids in transport of fatty acids for butterfat formation

¹ Department of Animal Husbandry, University of California, Davis, California

Cow plasma containing P³²- and C¹⁴-labeled lipids and proteins was infused into a lactating cow. The specific activity of the lipids and volatile acids of the plasma and of the expired CO₂ and milk constituents was determined. The turnover rate of phospholipid P and of plasma lipid volatile acids was 0.1 hours^–1, this means a half-time of 7 hours. Little radioactivity was found in the expired CO₂ and none was found in plasma volatile acids. These findings and the distribution of C¹⁴ in milk constituents indicate that plasma proteins do not contribute carbon to butterfat and that the catabolism of plasma lipids and proteins does not lead to a significant formation of nonlipid butterfat precursors. These observations permit a further calculation which indicates that 50% of the butterfat C originates from plasma lipid C.