Association of Lp(a) rather than integrally-bound apo(a) with triglyceride-rich lipoproteins of human subjects

Abstract

The majority of apolipoprotein (a) [apo(a)] in plasma is characteristically associated with Lipoprotein (a) [Lp(a)], having a buoyant density (1.05-1.08 g/ml) intermediate between low density lipoproteins (LDL) and high density lipoproteins (HDL). In the fed (postprandial) state or in the presence of fasting (endogenous) hypertriglyceridemia, a small proportion of plasma apo(a) is found in the density < 1.006 g/ml fraction of plasma, associated with larger and less dense triglyceride-rich lipoproteins (TRL). In order to further characterize the presence of apo(a) in ultracentrifugally-separated TRL (UTC-TRL), this lipoprotein fraction was isolated from plasma obtained in the fed state (three hours after an oral fat load) from healthy normolipidemic subjects (Lp(a): 38 +/- 8 mg/dl (mean +/- S.E.), n = 4) and also from plasma obtained after an overnight fast from hypertriglyceridemic patients (plasma TG: 8.16 +/- 2.00 mmol/l, Lp(a): 41 +/- 3 mg/dl, n = 18). Apo(a) in 3 h-postprandial UTC-TRL (5 +/- 2% of total plasma apo(a)) and in hypertriglyceridemic UTC-TRL (8 +/- 2% total apo(a)) was separable by electrophoresis and/or gel chromatography (FPLC) from the majority of UTC-TRL lipid. Apo(a) in UTC-TRL fractions had slow pre-beta electrophoretic mobility and was isolated in a lipoprotein size-range smaller than VLDL and larger than LDL, consistent with it being Lp(a). Recentrifugation of UTC-TRL resulted in the majority of apo(a) being recovered in the density > 1.006 g/ml fraction. Addition of proline to plasma samples before ultracentrifugation (final concentration: 0.1 M) substantially reduced the amount of Lp(a) in UTC-TRL. TRL separated from plasma by FPLC contained less apo(a) (2-5% of total plasma apo(a)), but this apo(a) was also readily dissociable from TRL lipid, had slow pre-beta electrophoretic mobility, and was associated with a lipoprotein with the size of Lp(a). Our data suggest that apo(a) in the TRL fraction of subjects with postprandial triglyceridemia or endogenous hypertriglyceridemia is not an integral component of plasma VLDL or chylomicrons, but represents the presence of non-covalently bound Lp(a).