Biosynthesis of high density lipoprotein by chicken liver: intracellular transport and proteolytic processing of nascent apolipoprotein A-1.
To study the in vivo processing and secretion of Apolipoprotein A-I (Apo A-I), young chickens were administered individual L-[3H]amino acids intravenously and the time of intracellular transport of nascent Apo A-I from rough endoplasmic reticulum (RER) to the Golgi apparatus was measured. Within 3 to 9 min there was maximal incorporation of radioactivity into Apo A-I in both the RER and the Golgi cell fractions. By contrast, the majority of radioactive albumin was also present in the RER by 3 to 9 min, but did not reach peak amounts in the Golgi fraction until 9 to 25 min. Both radioactive Apo A-I and albumin appeared in the blood at about the same time (between 20 and 30 min). NH2-terminal amino acid sequence analysis of nascent intracellular Apo A-I showed that it contains a pro-hexapeptide extension identical to that of human Apo A-I. After 30 min of administration of radioactive amino acids radioactive Apo A-I was isolated by immunoprecipitation from the liver and serum. NH2-terminal sequence analysis of 20 amino acids indicated that chicken liver contained an equal mixture of nascent pro-Apo A-I and fully processed Apo A-I, whereas the serum only contained processed Apo A-I. Further studies showed that the RER only contained pro-Apo A-I, whereas a mixture of pro-Apo A-I and processed Apo A-I was found in the Golgi complex. These results indicate that, in chicken hepatocytes, there is a more rapid transport of Apo A-I than of albumin from the RER to the Golgi cell fractions, and that Apo A-I remains in the Golgi apparatus for a longer period of time before it is secreted into the blood. In addition these studies show that the in vivo proteolytic processing of chicken pro-Apo A-I to Apo A-I occurs in the Golgi cell fractions.
D000418:Albumins / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000378:metabolism*; D001692:Biological Transport; D002645:Chickens; D004721:Endoplasmic Reticulum / Q000032:analysis; D006056:Golgi Apparatus / Q000032:analysis; D008075:Lipoproteins, HDL / Q000096:biosynthesis*; D008099:Liver / Q000378:metabolism*; D011499:Protein Processing, Post-Translational
D000418:Albumins; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D008075:Lipoproteins, HDL