The use of zonal centrifugation to study membrane formation during the life cycle of mammalian cells. Synthesis of 'marker' enzymes and other components of cellular organelles.

1. The activity of enzymes characteristic of microsomes (NADPH-cytochrome c reductase and uridine diphosphatase) and of inner mitochondrial membranes (cytochrome c oxidase and succinate-cytochrome c reductase) increases during the cell cycle of P815Y neoplastic mast cells in concert with total protein. The activity of glutamate dehydrogenase, an enzyme of the mitochondrial matrix, increases in a somewhat different manner. 2. The specific activity of mitochondrial structures involved in energy-coupling measured with a fluorescent probe remains constant during the cell cycle. 3. Mitochondrial and microsomal protein increases during the cycle at the same time as total protein; nuclear protein increases rather more sharply. 4. The rate of incorporation of labelled choline or inositol into nuclear, mitochondrial or microsomal phospholipid during the cell cycle follows the rate of incorporation into total phospholipid. 5. It is concluded that the major components of cellular membranes are synthesized, like total protein or phospholipid, throughout most of the intermitotic period.