Mitochondrial and microsomal phospholipid phosphorus metabolism during postnatal growth in rat heart and liver

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Abstract
The relative specific activities of phospholipids of hearts and livers of newborn and older rats were measured 2 hr after intraperitoneal32P‐orthophosphate. There was a close linear correlation between the specific activity of cardiolipin and the net increase in the amount of this phospholipid. In one‐day‐old animals, the relative specific activities in the heart and liver were highest and exceeded by 7.3‐ and 3‐fold, respectively, the corresponding activities in the adult. The apparent half‐lives of cardiolipins were calculated on the basis of the linear correlation that was found between the net increase in cardiolipin and the rate of incorporation. These half‐lives were 4.7 days in liver and 6.4 days in heart. Though the changes in the phospholipid composition of the organelles during neonatal development were small, the relative specific activities of the individual phospholipids varied considerably. In addition to cardiolipin, there was a good correlation between the specific activity and the net increase in mitochondrial phosphatidylinositol. At birth, the specific activity of mitochondrial phosphatidylcholine in liver was 2.9 times that in microsomes. During the 12 neonatal hours, the specific activity of microsomal phosphatidylcholine increased 6.3‐fold and exceeded the corresponding mitochondrial activity. The relation between the activation of microsomal phosphatidylcholine synthesis and the induction of serum lecithin synthesis in newborn liver is discussed. The finding that at birth the specific activity of mitochondrial phosphatidylcholine unexpectedly was higher than that of microsomal phosphatidylcholine points out the difficulties in interpreting the in vivo evidence for precursor‐product relationship.

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