In vivo metabolism of human apoprotein A-I-phospholipid complexes. Comparison with human high density lipoprotein-apoprotein A-I metabolism.

The metabolism of apolipoprotein A-I was investigated in healthy young adults. The apoprotein was injected intravenously in one of the four following forms: radiolabelled high density lipoproteins, HDL incubated with labelled apo A-I, labelled A-I-dipalmitoyl- or dimyristoylphosphatidylcholine complexes or free apoprotein A-I. The radioactivity-time curves of high density lipoproteins were followed for 15 days. The kinetic studies demonstrate similar fractional catabolic rates, half-lifes and rates of synthesis. Fractional catabolic rate calculated from plasma data was in reasonable agreement with the value derived from the urine/plasma activity ratio. When free apo A-I was injected, the unusual form of the U/P ratio curve is interpreted as the result of different fates of the injected molecules. This study indicates that the half-life of apo A-I seems not to be affected by the form in which the apoprotein is injected and that kinetics of apo A-I-phospholipid complexes may be used as a valid index of apoprotein A-I/HDL metabolism.