Mechanism for adaptive modification during cold acclimation of phospholipid acyl chain composition in Tetrahymena. I. Principal involvement of deacylation-reacylation.

The purpose of the present studies with in vivo isotope labeling was to explore the mechanism by which the fatty acyl composition of membrane phospholipids was altered quickly after a downward temperature-shift in Tetrahymena pyriformis. 1. When 39 degrees C-grown Tetrahymena cells were shifted to 15 degrees C, unsaturated fatty acids, especially gamma-linolenic acid, were increased, with compensating decrease of palmitic acid in phospholipids. 2. However, in the [32P]Pi-prelabeled cells, the specific radioactivities of major membrane phospholipids, phosphatidylethanolamine, phosphatidylcholine and 2-aminoethylphosphonolipid, were not changed within 10 h after shift-down. Furthermore, in the [14C]palmitic acid-prelabeled cells, the specific radioactivities toward phospholipids also did not change after temperature-shift. 3. After the shift, the rate of incorporation of [14C]acetate into fatty acids was reduced to less than one-tenth and the principal fatty acid newly synthesized was linoleic acid, and palmitic and gamma-linolenic acids were also formed to almost same degree. 4. In contrast, when cells prelabeled with both [32P]Pi and [14C]palmitic acid were shifted to 15 degrees C and then linoleic and gamma-linolenic acids, which are known to increase during cold acclimation, were added to the medium, the 14C/32P ratios of major phospholipids were progressively decreased until 5 h after the shift. These results suggest the mechanism for the alteration of phospholipid fatty acyl composition at lowered growth temperature by which preexisting fatty acids of membrane phospholipids would be deacylated and then, after modification to adequate unsaturated fatty acids by desaturation and/or elongation, be reacylated again into lysophospholipids.