Observations on changes in lipid composition and lecithin-cholesterol-acyl transferase reaction of bovine plasma induced by heat exposure

A study has been made with steers maintained at 22 C and 35 C on the lipid composition of the plasma, with particular reference to the role and specificity of the lecithin-cholesterol-acyl transferase enzyme in the formation of the cholesteryl esters. Exposure to a temperature of 35 C produced a steady decline in the total lipid concentration of the plasma, which reached a new equilibrium after 7–8 days. The concentrations of the lecithin, free cholesterol and cholesteryl ester fractions in the plasma of the steers at 35 C were reduced to ca. 60% of the corresponding concentrations found in the animals maintained at 22 C. The concentration of 18∶2 circulating in both the cholesteryl ester and lecithin fractions was reduced particularly by exposure to the elevated temperature. No significant change in total blood volume could be detected in the animals between the two environmental temperatures. Incubation of the plasma at 38 C for 20 hr resulted in an increase in the concentrations of the fatty acids contained in the cholesteryl ester and lysolecithin fractions and a decrease in the concentration of the fatty acids contained in the lecithin fraction for the animals at both environmental temperatures. The net esterification of cholesterol by the acyl transferase enzyme in the plasma from the animals at 35 C was very much less than that found in the plasma from the animals at 22 C. Under the conditions of the experiment, the esterification process was shown to have a high specificity for 18∶2. Evidence is available to suggest that the relationship between the acyl transferase activity of the plasma and the polyunsaturated fatty acids of the circulatory lipids may be severely disturbed by high environment temperatures. These results are discussed in relation to several known metabolic effects observed in animals exposed to high environmental temperatures.