Structural changes in human serum high density lipoprotein-3 attending incubation with blood leukocytes.

Preparations of human serum high density lipoprotein-3 (HDL3) were incubated in vitro with freshly isolated human leukocytes at various time intervals at either 4°C or 37°C in the presence or absence of lipoprotein-deficient serum (LPDS).HDL3 particles which were incubated at 37°C with cells in the absence of LPDS retained ultracentrifugal and spectral properties of untreated HDL3; their lysophosphatidylcholine content was relatively increased and these contained no apolipoprotein A-11 (apo A-11) as assessed by polyacrylamide gel electrophoresis and immunological techniques.The loss of apo A-11 was dependent on the leukocyte/HDL3 ratio and complete after 1 h of incubation.When HDL3 labeled in apo A-II was used, the loss of apo A-11 was associated with the generation of small radioactive peptides less than 10,000 daltons as assessed by electrophoretic methods.Loss of apo A-11 was prevented by carrying out the incubation at 4°C or by the presence in the medium of diisopropyl fluorophosphate.Most or all of the apo A-11 fragments were recovered in the HDL3 particle in the medium.Contrary to the HDL3 apo A-II hydrolysis, '261-ap0 A-I bound to HDL3 underwent a limited proteolysis upon incubation with leukocytes at 37°C.When the lipid-free HDL protein replaced HDL, in the medium, both apo A-I and apo A-11 underwent extensive hydrolysis although with a different time course.Selectivity of hydrolysis for apo A-11 was restored when cells were incubated with HDL3 protein relipidated with human HDL3 lipids.In all cases where proteolysis was observed, it was prevented by LPDS.