The postprandial lipoprotein metabolism is important since it determines the circulation of potentially atherogenic particles and influences the metabolism of high-density lipoproteins (HDL) in a complex manner that is at present not completely understood. Therefore, the short-term (24-h) changes in postprandial lipoprotein metabolism, including retinyl palmitate (RP), apolipoprotein A-I (apo A-I), and apolipoprotein B, were studied in relation to postheparin lipolytic activities in six healthy normolipidemic men after an oral RP fat tolerance test. The fat load (98 g) was cleared in 7 h, because the triglyceride (TG) concentrations had returned to initial values (0.72 +/- 0.31 mmol/l) at that time. RP showed a peak in plasma at 4 and 5 h but remained present in chylomicron (remnants) in low concentrations after 8 and 24 h. After the fat load, HDL cholesterol and HDL-associated apo A-I showed a significant decrease in concentration of 35 and 29%, respectively. The decrease coincided with the increase in chylomicron remnants and the transient appearance of TG-enriched HDL. Hepatic lipase was correlated to both the initial HDL cholesterol concentration as well as the peak concentration of TG in chylomicron remnants, suggesting that it could be one of the regulating common physiological pathways in postprandial HDL and TG metabolism. In the subjects studied, the atherogenic potential of plasma increased in response to an oral fat load, characterized by a decrease in HDL cholesterol and HDL-associated apo A-I.
SummaryLipoprotein(a) [Lp(a)] is recognized as an independent risk factor for atherosclerosis. Lp(a) consists of a LDL-like moiety with an additional glycoprotein, apo(a), linked to apolipoprotein B-100. Apo(a) has a high homology with plasminogen (Pg). In vivo, Pg is activated on a fibrin surface by tissue Pg activator (tPA). We prepared Lp(a) from plasma by sequential ultracentrifugation followed by lysine-sepharose affinity chromatography. We found that a changing (donor dependent) fraction of the Lp(a) did not bind to lysine-sepharose. This fraction, designated Lp(a)lys–, was further purified using gel filtration. Bound Lp(a) [Lp(a)lys+] was eluted with 0.2 M EACA. Apo(a) isoforms in both fractions were identical. In contrast Lp(a)lys+ inhibited Pg activation by tPA in vitro (IC50% 20 mg/1), whereas Lp(a)lys– did not. In addition Lp(a)lys– did not bind to CNBr-digested fibrinogen whereas Lp(a)lys+ did (K
d, app = 0.2 nM). Therefore we conclude that a changing donor dependent fraction of human plasma Lp(a) does not inhibit Pg activation in vitro and does not bind to CNBr-digested fibrinogen.
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