the cdp-choline pathway begins with the uptake of exogenous choline into the cell. the first enzymatic reaction is catalyzed by choline kinase (ck) and involves the phosphorylation of choline to form phosphocholine. phosphocholine is then activated by the addition of ctp catalyzed by the rate-limiting enzyme, ctp:phosphocholine cytidylyltransferase to form cdp-choline. the final step of the pathway involves the addition of the choline headgroup onto a diacylglycerol (dag) backbone to form pc, catalyzed by choline/ethanolamine phosphotransferase (cept).