The Formation of Phospholipid by the Hepatecto- Mized Dog as Measured with Radioactive Phosphorus

Hevesy (1,Z) and Artom (3) have inferred from studies on the incorporation of labeled inorganic phosphate into phospholipid that plasma phospholipid is formed largely in the liver. This inference was based on a comparison of the specific activities of phospholipid phosphorus of plasma with those of other organs. Similar observations have been made in this laboratory (Table I). The phospholipid Ps2 per gm. of total phospholipid of several tissues of the dog was determined at 6, 18, 36, and 98 hours after the intraperitoneal injection of inorganic Ps2 (Table I). At the first interval the phospholipid Ps2 per gm. of phospholipid of the liver was higher than that of any other tissue. At the 18 hour interval the values for liver and plasma were about the same and higher than for small intestine, kidney, and muscle. At 36 hours the values for plasma and liver were still about the same, but higher than those for other tissues. At 98 hours the values for kidney and small intestine approximated those for liver and plasma. Although the above observations do not contradict the view that the liver is the source of plasma phospholipids, interpretations based on comparisons of specific activities of phospholipid phosphorus are open to the criticism that they do not deal with a single substance but with a mixture of several different compounds. The following limitation should be noted even in the case of a single substance: the finding that the specific activity of an organ’s lecithin P is lower than that of plasma serves to exclude that organ as the principal source of the plasma lecithin, but the observation that the specific activity of an organ’s lecithin P is higher than that of plasma means only that the organ may be the source of the plasma lecithin. More definite criteria by which an organ can be determined as a source of plasma lecithin or cephalin, etc., were pointed out by Zilversmit et al. (4);