Reactions of diazomethane with glycerolipids in the presence of serum or inorganic salts.

Diazomethane is widely used for the selective methylation of nonesterified fatty acids in the presence of other lipids. However, when the reaction is carried out directly with plasma or serum, substantial methanolysis of phospholipid acyl groups occurs. Because of the importance of rigorous selectivity in the assay of unesterified fatty acids which are present only in trace amounts in cells and body fluids, we have investigated the diazomethane procedure in detail and reached the following conclusions: (i) When diazomethane reacts with lipid extracts in organic solvent, no ester hydrolysis occurs. (ii) In the presence of serum or plasma, diazomethane reacts with water and inorganic salts, causing the solution to become basic (CH2N2 + NaCl + HOH-->Ch3Cl + Na+ + OH- + N2); methoxide ions are formed from methanol (CH3OH + OH(-)-->CH3O- + HOH) causing extensive methanolysis (CH3O- + RO-CO-R'-->CH3O-CO-R' + RO-). An analogous reaction takes place with ethanol. All esters of glycerol are transesterified in aqueous salt solution by this mechanism. It is therefore essential to prepare a lipid extract prior to the assay of unesterified fatty acids when using the diazomethane procedure.