| Abstract: | Human high-density lipoprotein class-3 (HDL3) was incubated with freshly isolated blood polymorphonuclear leukocytes (PMN) at 37 and 4 degrees C. At both temperatures the release of proteolytic activity (PA) causing the specific hydrolysis of apo-A-II was dependent on the concentration of HDL3 in the medium. At 37 degrees C, the efflux of PA was linear and no saturation was reached up to an HDL3 protein concentration in the medium of 800 micrograms/ml. In turn, at 4 degrees C, maximal PA release was reached at a concentration below 600 micrograms/ml of HDL3 protein/ml in the medium. Canine HDL, which contains apo-A-I, but not apo-A-II, was as effective as human HDL3 in promoting the release of PA from PMN. This property was also exhibited by egg lecithin/cholesterol vesicles containing apo-A-I. At 4 degrees C, there was no strict correlation between efflux of PA affected by HDL3 and specific binding of 125I-apo-A-I (HDL3). In competitive binding experiments, a 50-fold excess of unlabeled HDL3 prevented more than 90% of the binding of 125I-apo-A-I (HDL3) to PMN, whereas an excess of unlabeled low-density lipoprotein exhibited no effect. When human HDL3 was incubated with PMN at 4 or 37 degrees C and then subjected to ultracentrifugation at d 1.21 g/ml, most of the PA that was initially associated with this lipoprotein was recovered in the bottom of the tube. By gel filtration, both PA and HDL3 were in the same peak in a low ionic strength buffer, but were dissociated from each other by a high-salt solution (d 1.21 g/ml). We conclude that both naturally occurring HDLs and apo-A-I-stabilized lipid vesicles favor the release from PMN of an enzymatic activity which cleaves human apo-A-II. This release appears to be dependent both on the interaction of the cells with the lipoprotein ligand and on the lipoprotein surface area acting as the acceptor for the enzyme, probably through electrostatic forces. |
