Effect of fasting on two postheparin plasma triglyceride lipases and triglyceride removal in obese subjects

A new method was used for selective measurement of lipoprotein lipase and hepatic lipase in human postheparin plasma. Hepatic lipase was assayed in 1.0 M NaCl withour addition of serum, and the activity of lipoprotein lipase was determined in 0.1 M NaCl after immunoprecipitation of hepatic lipase with specific antiserum. The activity of both these enzymes and the total lipolytic activity were measured in plasma samples taken during a 4-h infusion of heparin. Each of the activities was related to basal serum triglyceride concentration and to the fractional removal constant (K) of Intralipid in 13 obese subjects before and after prolonged fasting. During a normal isocaloric diet the lipolytic activities showed a biphasic response to heparin infusion in all subjects. A peak activity was reached within 30 minutes ("early response") and thereafter the lipase activities decreased to a constant level maintained during the rest of the heparin infusion ("late response"). The early response of lipoprotein lipase showed a significant inverse correlation with the basal serum triglyceride level (r = -0.85) and a significant positive correlation with the fractional removal rate of Intralipid (r = 0.84). The late response of lipoprotein lipase was not related to either of these parameters. The early response of hepatic lipase was not correlated with basal triglyceride concentration or Intralipid removal, whereas the late response of this enzyme showed a significant negative correlation with the removal rate of Intralipid (r = -0.82). After fasting for several days the acute response of all lipolytic activities to heparin was markedly decreased or totally abolished, but the magnitude of the late response was similar to that seen in the fed state. The fractional removal rate of Intralipid was slightly increased by starvation. All correlations between postheparin plasma lipases and serum triglyceride concentration and removal disappeared in fasting subjects. It is concluded that the rapidly releasable lipoprotein lipase probably reflects the activity of the tissue enzyme(s) which is responsible for the primary removal of very low density lipoprotein (VLDL) triglycerides and chylomicrons. It is probable that this component of the postheparin plasma lipolytic activity is derived from the endothelial lipoprotein lipase pool. This enzyme plays a key role in the efflux of plasma triglycerides under normal conditions, and it is thus one determinant of plasma triglyceride level. Prolonged fasting obviously changes the triglyceride removal sites and mechanism but does not impair the removal efficiency.