Lactating mammary glands of sheep and goats were perfused for several hours in the presence of [1-14C]palmitate or [1-14C]oleate. Adequate quantities of acetate, glucose, amino acids and chylomicrons were added to the perfusate.
The fall in specific activity of [1-14C]palmitic acid or [1-14C]oleic acid across the gland and the labelling of milk triglyceride fatty acids indicates an extensive transfer of radioactivity from plasma free fatty acids (FFA). The plasma triglycerides showed large arterio-venous differences in concentration. The small [14C] incorporation in plasma triglycerides decreased across the gland. In a control experiment triglycerides were also slightly labelled.
There were no significant arterio-venous differences in cholesterol esters and their fatty acid composition showed only slight changes during passage through the gland. Their specific activity showed a small rise across the gland.
In milk components, the [14C] was mainly localized in the triglycerides. An appreciable proportion of the palmitoleate is derived from palmitate by dehydrogenation within the gland, while there is no evidence for the hydrogenation of oleic acid to stearic acid. Elongation of palmitic acid to C18-acids does not occur to any important extent. FFA are catabolized to a variable extent by the gland.
The role of FFA in labelling of milk and blood plasma fatty acid fraction is discussed.