Structure and synthesis of milk fat XI. Effects of heparin on paths of incorporation of glucose and palmitic acid into milk fat.

Intravenous injection of heparin increases lipoprotein lipase activity of circulating serum presumably by removing the enzyme from its location on the capillary endothelium. The incorporation of carbon-14 uniformly labeled glucose and carbon-14 1-labeled palmitic acid into fractionated milk fat triglycerides was studied in both normal and heparin treated lactating goats. The objective was to remove lipoprotein lipase from the mammary gland capillaries and to contrast normal milk fat synthesis with a situation presumed to cause the gland to be solely dependent on the phosphatidic acid pathway. The studies with labeled glucose indicated that under normal conditions there are two sources of milk glyceride glycerol; while following heparin injections, there is a single glycerol pool providing most of the glyceride glycerol. The investigations with labeled palmitic acid indicated that under normal conditions there are two sources of palmitic acid coming from the blood which enter nonequilibrating cellular pools. Palmitic acid from both pools is available for triglyceride synthesis. Following heparin injections there appears to be a common intracellular pool of pre-formed palmitic acid derived from the blood. The data indicate that lipoprotein lipase operating on blood triglycerides yields a 2-mono-glyceride which subsequently enters the gland and is utilized for milk fat synthesis.