Discordant results for determinations of triglycerides in pig sera

We recently determined triglyceride concentrations in pig sera by three fully enzymatic methods (Kodak Ektachem 700, Hitachi 707, and Abbott EPx) and obtained significantly lower values than those obtained with chemical or enzymatic methods based on chemical hydrolysis. All methods used involve microbial lipases for liberating glycerol from glycerides and glycerol phosphate dehydrogenases or oxidases for subsequent oxidation. The methods were validated against reference methods by using fresh human sera and survey materials. The discordant results were not from matrix sample-method interaction but from incomplete hydrolysis of pig serum triglycerides by the lipolytic enzymes. When serum triglycerides from 10 pigs showing the highest biases were hydrolyzed by microbial lipases and the reaction mixture was subjected to thin-layer and gas-liquid chromatography, the predominant end products were palmitoyl monoglyceride and a mixture of free fatty acids with the following composition (fatty acid as percent of total +/- SD): 16:0, 7.8 +/- 2; 18:0, 5.4 +/- 2.2; 18:1, 53 +/- 12; 18:2, 31 +/- 4.6; and 18:3, 2.5 +/- 1. Assuming that the lipases exhibit the usual specificity toward the 1 and 3 positions of the triglyceride, the data suggest that, in pig, triglycerides 18:1 and 18:2 occupy the 1 and 3 positions and 16:0 (palmitic acid) predominantly occupies the 2 position. Triglycerides of this structure may not be well hydrolyzed by the typical lipolytic enzymes in clinical assays.