Further Studies on Milk Fat Synthesis. ∗

Abstract

We have reported evidence(l) for the de novo synthesis of milk fat by mammary tissue based on a study of the radioactivity found in milk fat glycerol following intramammary infusion of various C¹⁴ labeled metabolites. The synthesis of glycerol from glucose(2) and subsequent incorporation of the newly synthesized glycerol into milk fat suggested that milk fat was synthesized within the udder from “pools” of fatty acids and glycerol. An alternate pathway, equally possible but lacking experimental proof, envisions milk fat synthesis as a process of fatty acid transacylation onto preformed triglyceride molecules which were previously absorbed from the blood(3). The present study was undertaken to extend our earlier work and to test our hypothesis of de novo fat synthesis by mammary tissue.

Experimental. Details pertaining to the method and site of infusion, the isotope infused, and the milk yield of the various cows used in these studies were reported earlier for all trials except stearate C¹²(l). The cows were milked at 3 and 8-10 hours following intramammary infusion of the isotope, and the milk was stored at 4°C until the cream separated from the skim milk. Milk fat was extracted from the cream fraction and purified by the Roese-Gottleib technic. Ten grams of each milk fat sample were then saponified with alcoholic KOH and the constituent fatty acids were isolated from the reaction mixture by ether extraction. The fatty acids were dried, combusted in a microfurnace, and the resulting CO₂ precipitated as BaCO₃. Planchets of “infinite thickness” were prepared for radiochemical assay in a standard G.M. counter.

In the stearate 1-C¹⁴ trials, 1.6 me sodium stearate were blended with 100 ml of “HP” vehicle† prior to infusion through the teat canal into the right half of the udder. Milk fats obtained after infusion of stearate C¹⁴ were subjected to silicic acid chromatography prior to hydrolysis. This technic removes free fatty acids and was employed to prevent inadvertent contamination of milk fatty acids with unabsorbed stearic acid.