Phospholipids. 2. Estimation of amino nitrogen in intact phospholipids

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Abstract
The ninhydrin procedure of Moore and Stein (Jour. Biol. Chem. 17(6): p.367.1948), modified to maintain the lipids in solution, was applied to estimation of phosphatidylethanolamine (1-5 [mu]g. N) in the presence of phosphatidylcholine (up to 1000 [mu]g. N) without preliminary hydrolysis of the phospholipid. Phosphatidylcholine as well as phosphatidylethanolamine liberates gas in the Van Slyke manometric procedure for estimation of amino N owing to reaction of its component unsaturated fatty acids with nitrous acid. This method can be applied only empirically to unhydrolyzed phospholipids using a reaction time chosen for the particular material to agree with a detn. on the fatty acid-free hydrolysate. It is much less sensitive than the ninhydrin method.