Studies on the metabolism of the lipoprotein Lp (a) in man.

Lp(a) was isolated and labeled by reductive alkylation. Radioactivity only entered the protein moiety of the lipoprotein. No change in the immunological or physicochemical properties of Lp(a) was noted after the radiomethylation. After incubation of labeled Lp(a) with whole serum for 24 h in vitro, more than 99% of the radioactivity of the incubated sample was found in Lp(a). In 4 subjects Lp(a) was injected intravenously. A linear decline of the specific activity of Lp(a) in the serum was found when it was plotted semilogarithmically against time. Half-lives of Lp(a) in the serum were 35, 38, 53 and 58 h. In one subject, the "soluble" and the "insoluble" apoproteins of Lp(a) showed the same half-life as the whole Lp(a) molecule. This suggests that no exchange of Lp(a) apoproteins with lipoproteins of other density classes took place. At different times after the injection of Lp(a), 3--8% of the radioactivity of the serum was found in Lp B, and less than 2% of the radioactivity was detectable in VLDL and the fraction having a density of greater than 1.110 g/ml.