Abstract After preincubation of human lactate dehydrogenase (EC 1.1.1.27) isoenzymes 1 through 5 with lactate at pH 9.4, 9.8, 10.25, and 10.6 at 30 degrees C for 10 min, the reduction of NAD+ was measured at the same pH values and temperature during the interval 1-2 min after adding the coenzyme. Relative to the reference activities measured at pH 8.7 by the method of Buhl et al. (Clin Chem 23: 1289-1295, 1977), the respective activities of isoenzymes 1 through 5 thus measured were 111, 104, 96, 68, and 0% at pH 9.4; 123, 108, 88, 0, and 0% at pH 9.8; 140, 100, 0, 0, and 0% at pH 10.25; and 138, 0, 0, 0, and 0% at pH 10.6. These relations allow selective assay of the respective isoenzymes in samples containing mixtures of them. Optimal conditions for such selective assay at 37 degrees C are reported.
Abstract Acrolein, which is an irritating and off‐flavor compound formed during heating of vegetable oils, was estimated by the gas–liquid chromatography (GLC). Several vegetable oils such as high‐oleic sunflower, perilla, rapeseed, rice bran, and soybean oils were heated at 180 °C for 480 min and then the concentration of acrolein in the head space gas was determined by GLC. The formation of acrolein was greatest in perilla oil among the tested oils, while it was much lower in rice bran oil and high‐oleic sunflower oil. There was a good correlation between the level of acrolein and linolenate (18:3n‐3) in the vegetable oils. To investigate the formation of acrolein from linolenate, methyl oleate, methyl linoleate, and methyl linolenate were also heated at 180 °C, and the amounts of acrolein formed from them were determined by GLC. The level of acrolein was the greatest in methyl linolenate. Acrolein was also formed from methyl linoleate, but not from methyl oleate. Acrolein in vegetable oils may be formed from polyunsaturated fatty acids, especially linolenic acid but not from glycerol backbone in triacylglycerols.
Effects of rice bran oil on the oxidative and flavor stability of fish oil were investigated by the gas liquid chromatography-head space method. When fish oil blending with different ratio of rice bran oils was oxidized at room temperature in the dark, volatile compounds produced during autoxidation was measured by gas liquid chromatography. The amounts of volatile compounds were decreased with increased the ratio of blended rice bran oil as well as peroxide value. The level of propanal and acrolein which gave unpleasant flavor was also decreased with increased the ratio of blended rice bran oil. Especially, the level of propanal and acrolein and peroxide value were remarkably decreased when blending more than 75% of rice bran oil. Blending of rice bran oil improved the oxidative and flavor stabilities of fish oil.
