Phospholipid metabolism in rat submandibular gland. Positional distribution of fatty acids in phosphatidylcholine and microsomal lysophospholipid acyltransferase systems concerning proliferation

Rat submandibular gland phosphatidylcholine mainly consisted of the 1-saturated acyl-2-unsaturated acyl type. The high occupancy of unsaturated fatty acid at the C-2 position is in part explained by the preference of microsomal acyl-CoA:1-acyl-sn-glycero-3-phosphocholine (1-acyl-GPC) acyltransferase for unsaturated fatty acyl-CoAs. This enzyme activity was partially inhibited by divalent cations. Ca²⁺ may be important for regulation of a deacylation-reacylation cycle, suggested because Ca²⁺ is also known to activate the deacylation enzyme, phospholipase A₂. Although the presence of 1-acyl-GPC acyltransferase activity is also observed in plasma membrane of the submandibular gland, the microsomal enzyme showed properties different from the enzyme in plasma membrane in terms of its susceptibility to neutral salts and detergents. Cell proliferation caused by chronic administration of isoproterenol resulted in an increase of linoleic acid at the C-2 position of phosphatidylcholine. However, this alteration did not correlate with the changes of activity and substrate specificity of 1-acyl-GPC acyltransferase and the other C-2 acylation enzyme, 1-acyl-sn-glycero-3-phosphate acyltransferase, which suggests that the alteration of fatty acid by isoproterenol treatment is due to a change of supply of substrates or specific acyl breakdown of phosphatidylcholine.