Lipoprotein-apoprotein exchange in aqueous systems: Relevance to the occurrence of APOA-I and APOC proteins in a common particle

Human plasma high density lipoproteins (HDL) were labeled in vitro with [¹²⁵I]apoA-I. Chromatography of the [¹²⁵I]HDL on Sephacryl S-200 revealed that a certain fraction of [¹²⁵I]apoA-I readily dissociates from the intact particle over a wide range of HDL concentrations. The relatively constant value of the dissociated apoA-I concentration observed at various HDL concentrations suggests that a "critical monomer concentration" of apoA-I is in equilibrium with the parent lipoprotein. Addition of [¹³¹I]apoC proteins to HDL induces additional dissociation of oligomeric apoA-I with the concomitant incorporation of apoC into a new particle of about 460,000 daltons.