Phospholipid pools in insect muscle: labelling by 32P in subcellular fractions in vivo.

The time course of the labelling of phospholipids by 32P, followed for 14 days in vivo, shows that there exists a fraction of PC (phosphatidylcholine) and PE (phosphatidyl ethanolamine) in mitochondria which exchanges phosphate very slowly; this fraction comprises about 25% of the total mitochondrial PC and about 50% of PE. In muscles followed during maturation these fractions were considerably smaller. For PI (phosphatidyl inositol), isotopic equilibrium is established rapidly between mitochondria and the microsomal fraction. During anaerobiosis the labelling of microsomal PC and PE is slowed down, yet the labelling of the mitochondrial phospholipids continues at a rate strongly suggesting that a transport from microsomal membranes to mitochondria takes place in vivo. In the 105 000 x g supernatant small amounts of phospholipids were found the specific activity of which was (for PC and PE) higher than that of microsomes after short incorporation times. On the basis of these observations a tentative scheme of the intracellular organization of phospholipid metabolism in insect flight muscle is being proposed.