MORIYA, T. and KANOH, H. In Vivo Studies on the de Novo Synthesis of Molecular Species of Rat Lung Lecithins. Tohoku J. exp. Med., 1974, 112 (3), 241-256In vivo incorporation of intrafemorally injected [9, 10-3112]-palmitic acid and [2-3H]-glycerol into molecular species of rat lung glycerolipids was studied in relatively early periods after injection (from 2 to 30 min). The pool size of 1, 2-diacylglycerols was also determined in the tissue. When [9, 10-3H2]-palmitic acid was injected, approximately 60% of the radioactivity was distributed in 1, 2-disaturated species of phosphatidic acids and 1, 2-diacylglycerols in 2 min, and this labeling pattern was reflected on the synthesis of lecithin species, although the calculated turnover of molecular species of lecithins and the results obtained with [2-3H]-glycerol injection suggested that the pathway of synthesis de novo could not be regarded as being fully responsible for the formation of dipalmitoyl lecithin in rat lung. Analysis of the distribution of radioactivity in the 1 and 2-positions of the formed glycerolipids after administration of [9, 10-3H2]-palmitic acid showed that approximately 75% of the incorporated radioactivity was located in the 2-position of dipalmitoyl lecithin during the present experimental periods, while the ratio of the distribution was almost 1: 1 in disaturated species of 1, 2-diacylgly cerols. This phenomenon could reasonably explain the discrepancy observed between the experimental results with [9, 10-3112]-plamitic acid and [2-3H] glycerol, suggesting that there exists other unknown mechanism than de novo synthesis to introduce palmitic acid of a higher specific radioactivity to the 2-position of dipalmitoyl lecithin. 1, 2-diacylglycerols; dipalmitoyl lecithin

Naturally occurring lecithins in most animal tissues have been found to be esterified principally with a saturated fatty acid in the 1-position and with an unsaturated in the 2-position, while the mammalian lung tissues have been con firmed to contain typically large amounts of dipalmitoyl lecithin (Kanoh 1967; Veerkamp et al. 1962; Montfoort et al. 1971). Various metabolic routes leading to the formation of this specific lecithin species have been suggested by many investigators in the lung tissues. Although the recent progress in the studies of the metabolism of molecular species of glycerophospholipids has revealed distinct characteristics of various biosynthetic pathways to form specific phospholipid