Changes of high density lipoprotein subfraction concentration and composition by intralipid in vivo and by lipolysis of intralipid in vitro.

Serum lipoproteins were measured during a single infusion of intralipid and during parenteral nutrition with intralipid and glucose. Postheparin plasma lipolytic enzymes and plasma LCAT activity were assayed before and after the parenteral nutrition. Both single and repeated infusions of intralipid were followed by a significant rise of HDL2 concentration (p less than 0.01), whereas the HDL3 decreased. The composition of HDL subclasses altered. The HDL2 triglyceride and phospholipids increased, while the HDL3 esterified cholesterol and protein decreased. In vitro incubation of serum with intralipid alone caused no changes in the zonal profile of HDL subclasses, but hydrolysis of intralipid by lipoprotein lipase was followed by conversion of HDL3 into lighter particles floating in the density range of HDL2. The present results provide additional evidence for a precursor-product relationship between the HDL2 and HDL3. During 4 days of parenteral nutrition with intralipid, the basal (morning) values of serum total and VLDL triglyceride did not change. The LDL phospholipids increased progressively (from 67 to 98 mg/dl, p less than 0.05). The total HDL cholesterol decreased and this change was due to the fall of HDL3 cholesterol esters (from 19 to 12 mg/dl, p less than 0.05). Also the basal values of apo A-I and A-II in HDL3 decreased. The basal level of the HDL2 remained constant. Postheparin plasma LPL activity increased by 52% (p less than 0.01) but hepatic lipase activity fell by 49% (p less than 0.05). These changes may account for the maintenance of plasma HDL2, whereas the progressive fall of the basal HDL3 is probably due to the lack of intestinal apoprotein synthesis during absent intestinal absorption.