Lipogenesis by acini from mammary gland of lactating rats.

Intact acini were prepared from the mammary gland of lactating rats by treatment with collagenase. The acini were obtained in high yield. The preparation offers considerable advantage over the use of tissue slices. The acini were incubated with glucose, lactate, pyruvate, and acetate labeled with 14C and tritium, as single substrates or in combination with glucose. Synthesis of fatty acids from glucose was about 2500 patoms of carbon per g of dry fat-free tissue, or approximately 150 pmoles of acetyl units per g wet weight per hour. Lactate as the sole substrate is oxidized to a small extent and fatty acid synthesis is negligible. Acetate as the sole substrate is hardly utilized. Pyruvate is oxidized at a rapid rate, but lipogenesis is much lower than that from glucose. Addition of glucose stimulates lipogenesis from lactate and acetate, but not from pyruvate. Lipogenesis from glucose is stimulated slightly by acetate, depressed by lactate, and nearly completely inhibited by pyruvate at concentrations of 10 mM. Lipogenesis from lactate plus glucose, and acetate plus glucose, is greater than that from glucose alone. With 5 mM glucose and 5 mM lactate, lactate contributes threefourths of the fatty acid carbon; with 5 mM glucose and 2 mM lactate, the contribution of the two substrates is approximately equal. A balance between reducing equivalents and ATP production and utilization was calculated for glucose alone and for glucose in the presence of lactate and acetate. There is an excess of reducing equivalent production in the cytosol over the requirement for lipogenesis. In all of the conditions, there was a large apparent excess of ATP production over the requirements for biosynthesis.