Phosphatidylserine is produced in bacteria like E. coli by displacing cytidine monophosphate (CMP) via a nucleophilic attack from the hydroxyl group of serine. CMP originates from CDP-diacylglycerol through the action of PS synthase. In mammals, phosphatidylserine is obtained from phosphatidylethanolamine or phosphatidylcholine via one of two calcium-dependent head-group exchange reactions in the endoplasmic reticulum. The average daily intake of phosphatidylserine in a Western diet is estimated at 130 mg, as referenced in 'Food Composition and Nutrition Tables', published in Stuttgart by Medpharm Scientific Publishers in 2000.