Turnover of mammalian phospholipids. Rates of turnover and metabolic heterogeneity in cultured human lymphocytes and in tissues of healthy, starved and vitamin A-deficient rats.

1. Choline- and inositol-labelled phospholipids of human cultured lymphocytes turn over in a biphasic manner; phytohaemagglutinin activation stimulates turnover. 2. Choline-labelled phospholipids of rat liver and kidney, but not of blood, turn over in vivo as fast as those of duodenum, ileum or colon. Turnover in the intestinal tissues is greater in fed than in starved or vitamin A-deficient rats. In each case phosphatidylcholine turns over relatively faster than sphingomyelin or lyso-phosphatidylcholine. 3. It is concluded that phospholipid turnover of the type described is a common feature of viable cells, and that metabolically favourable conditions increase, rather than decrease, turnover.