Abstract
Lipoprotein lipase (LPL) releases fatty acids from triglyceride-rich lipoproteins for use in cellular metabolic reactions. How this hydrolysis, which occurs at the vascular endothelium, is regulated is poorly understood. A fatty acid feedback system has been proposed by which accumulation of fatty acids impedes LPL-catalyzed hydrolysis and dissociates the enzyme from its endothelial binding sites. We examined this hypothesis in humans who were subjected to an oral fat tolerance test of a mixed-meal type. Plasma triglycerides, free fatty acids, and LPL activity were measured before and repeatedly during a 12-h period after intake of the fat load. Since soybean oil with a high content of linoleic fatty acid was the source of triglycerides, a distinction could be made between endogenous free fatty acids (FFA) and FFA derived directly from lipolysis of postprandial triglyceride-rich lipoproteins. Mean LPL activity was almost doubled (P less than 0.01) 6 h after intake of the oral fat load. The rise in LPL activity was accompanied by an increase of plasma triglycerides and linoleic free fatty acids (18:2 FFA), but not of total plasma FFA, which instead displayed a heterogeneous pattern with essentially unchanged mean levels. The postprandial response of LPL activity largely paralleled the postprandial responses of 18:2 FFA and triglycerides. The highest degree of parallelism was seen between postprandial 18:2 FFA and LPL activity levels. Furthermore, the integrated response (area under the curve, AUC) for plasma measurements of LPL correlated with the AUC for 18:2 FFA (r = 0.40, P less than 0.05), but not with the AUC for plasma triglycerides (r = 0.21, ns). The high degree of parallelism and significant correlation between postprandial plasma LPL activity and 18:2 FFA support the hypothesis of fatty acid control of endothelial LPL during physiological conditions in humans.
Highlights
Lipoprotein lipase (LPL) releases fatty acids from triglyceride-rich lipoproteins for use in cellular metabolic reactions
Lipolysis of triglycerides from chylomicrons and very low density lipoproteins (VLDL) is catalyzed mainly by lipoprotein lipase (LPL). This enzyme is bound to the vascular endothelium by affinity for heparan sulfate, and only low levels of LPL are found in the circulation [1, 2]
T h e binding of LPL to the endothelium is thought to be weakened by local fatty acid accumulation that may result from lipolysis of plasma triglycerides
Summary
Lipoprotein lipase (LPL) releases fatty acids from triglyceride-rich lipoproteins for use in cellular metabolic reactions. The high degree of parallelism and significant correlation between postprandial plasma LPL activity and 18:2 FFA
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