Contribution of endosomes to intrahepatic distribution of apolipoprotein B and apolipoprotein E

The endosomal contribution of apolipoprotein B and apolipoprotein E to the intrahepatic distribution of these 2 apolipoproteins was studied. The endosomal contribution of the apolipoproteins was estimated by comparing the specific radioactivities of the injected apolipoproteins with that of apolipoproteins in the liver. The endosomal contribution is further validated by using a least-squares linear combination method that has been described previously (Wong and Pino: European Journal of Biochemistry 164:357-367, 1987). The results using these two independent analyses were similar. There was no difference in endosomal contribution of the apolipoproteins between the fasted and fed rats. In both cases, total endosomal contribution in the microsomal fraction of apolipoprotein B was estimated at 7-10% and that for apolipoprotein E was estimated at 62-66%. The molecular weights of the two apolipoproteins in the endosomes closely resembles those of the injected apolipoproteins in the very low density lipoproteins, indicating little or no apolipoprotein loss during receptor-mediated endocytosis. It is concluded that while there was no substantial contribution of apolipoprotein B to the intrahepatic distribution of that protein, the contribution of extrahepatic apolipoprotein E to intrahepatic apolipoprotein E distribution is substantial.