THERMOXIDATION OF SUBSTRATE MODELS AND THEIR BEHAVIOR DURING HYDROLYSIS BY PORCINE PANCREATIC LIPASE

The behavior of thermoxidized triacylglycerols during hydrolysis catal yzed by porcine pancreatic lipase was evaluated using nonpolar triacyl glycerols isolated from palm olein (NPTPO), triolein, and sn-1,3 diole in substrates. Substrates were thermoxidized at 180 degrees C for 1 to 4 h. Owing to formation of polymers and dimers of triacylglycerols, t he molecular weight of the thermoxidized substrates increased. After 1 h heating, the concentration of polymers and dimers was similar for t he sn-1-3 diolein and triolein samples but higher in NPTPO samples. Co njugated double bonds were formed in all samples, and alpha,beta-unsat urated carbonyl compounds developed through allylic oxidations. These caused increased ultraviolet absorbance at 232 nm. The hydrolysis of h eated and unheated samples by the lipase can be described by a Michael ian equation. The enzyme showed a higher apparent V-max and K-M with h eated sn-1,3 diolein and triolein than with their unheated counterpart s. This was due to the generation of polar compounds which acted as em ulsifiers and which favored the formation of an oil/water microemulsio n. This behavior was not observed in NPTPO, where heating decreased th e apparent V-max and K-M over the first 2 h. Later, a tendency to incr ease these values was observed. The results could be explained by a ba lance between concentration of surfactants and of natural emulsifiers in the thermoxidized samples.