Degradation of apolipoprotein B-100 in human chylomicrons

Abstract

The purpose of this study was to investigate the molecular forms of apolipoprotein B (ApoB) in human chylomicrons under well-preserved conditions. To this end, plasma and serum were collected from the same normal subjects after ingestion of a fatty meal. The samples were divided into three or four aliquots before the addition of various preservative mixtures, including antibiotics, antioxidants and proteinase inhibitors. The chylomicrons were isolated immediately, and all steps were carried out at or below 4 °C. Changes in the molecular weight of ApoB in chylomicrons were followed by a time study using 3.3% polyacrylamide gel electrophoresis containing SDS. ApoB from chylomicrons analyzed within 5 h of blood collection showed a single band with mobility identical to that of ApoB (ApoB-100) in low-density lipoproteins. When analyzed after 1–2 days, satellite bands smaller than ApoB-100 were observed, and a very faint band with M r , 200000 appeared, which comigrated with intestinal ApoB (ApoB-48). upon storage, the molecular weight of ApoB was smaller in chylomicrons subjected to a higher number of reflotations than those in chylomicrons washed less frequently, suggesting that purified chylomicrons degrade faster. A longer storage time at 4°C (i.e., 7 or 14 days) revealed a stepwise degradation of ApoB, yielding M r 200000 band as the prominent form. The degradation of ApoB-100 was slower when both proteinase inhibitors, leupeptin and ϵ-amino caproic acid, were employed, and the appearance of M r 200000 band was quicker when the chylomicrons were processed at higher temperature (15–25 °C) in the absence of a proteinase inhibitor. Immunoblotting shows that the segment removed from ApoB-100 was the carboxyl-terminal portion. These results suggest the possible presence of a proteinase(s), which copurified with chylomicrons, and which converts ApoB-100 from a large to a smaller molecular form. Although the stop codon has been discovered recently in intestinal ApoB mRNA, which explains the mechanism for direct synthesis of ApoB-48, apparently ApoB-100 is also synthesized in the intestine of all eight subjects studied here, and the ApoB-100 degrades to a form which is ApoB-48-like.