Fig. 6.

H₂¹⁸O incorporation into PC lipids. The CDP-choline pathway [33], [34] is a representative metabolic pathway outlining reactions that allow for H₂¹⁸O incorporation into PCs and/or lyso-PCs. From the top, choline kinase (CK) phosphorylates choline to phosphocholine, which is converted to cytidine-diphosphocholine (CDP-choline) by CTP: phosphocholine cytidylyltransferase (CCT). Choline phosphotransferase (CPT) catalyzes PC synthesis. Fatty acyl chains are cleaved from the PC sn-2 position by phospholipase A₂ (PLA₂) to generate lyso-PC and free fatty acids. Free fatty acids are labeled (*), rapidly converted to acyl-CoA by acyl-CoA synthetase (ACS) and used for acylation of lyso-PL in a reaction catalyzed by acylglycerolphosphate acyltransferases (AGPAT). PC may be cleaved by PLC (phospholipase C) to yield diacylglycerides (DAG) and phosphocholine. Hydrolysis reactions in which incorporation of the ¹⁸O label may occur are shown in red, as are the indicated oxygens that may be labeled following PL remodeling.