A comparison of two methods to investigate the metabolism of human apolipoproteins A - I and and A - I 1

Two methods are compared for measuring the kinetic parameters of apolipoprotein A-I and A-I1 metabolism in human plasma. In the first, high density lipoprotein apoproteins were radioiodinated in situ in the lipoprotein particle (endogenous apoprotein labeling) while in the second, individually labeled apolipoprotein A-I or A-I1 was incorporated into the particle by in vitro incubation (exogenous apoprotein labeling). The catabolic clearance rate of exogenously labeled apolipoprotein A-I was consistently faster than that of endogenous apolipoprotein A-I. Conversely, endogenously and exogenously labeled apolipoprotein A-I1 were catabolized at identical rates. The fractional plasma clearance rates of endogenous apolipoproteins A-I and A-I1 were the same.