Work in my laboratory first directly demonstrated the rapid hepatic clearance of apolipoprotein B of lower molecular weight (apo B1) from rat plasma compared to apolipoprotein B of higher molecular weight (apo Bh). The use of SDS-gel filtration column chromatography enabled a direct comparison of apo B1 label in whole serum to liver homogenates, allowing a direct calculation of hepatic apo B1 clearance rates. In rats apo B1 represents a secretory protein of liver and intestine as opposed to apo Bh which is an hepatic secretory protein. Apo Bh containing secretory lipoproteins (Lps) undergo progressive delipidation and eventually apo Bh constitutes the major apo B protein of LDL. Recent experiments described in this proposed provide an explanation. The liver has a binding site for apo B1 enriched Lps which allows discrimination from apo Bh containing Lps. Hepatic recognition requires prior lipase interaction with Lps. Once bound the apo B1 enriched Lps are internalized and degraded. The apo Bh-Lps remain in the plasma in LDL with a slow catabolic rate. I propose studies using isolated liver perfusion to characterize the apo B1 hepatic binding site. These experiments will involve study of characterized Lp particles which vary in apoprotein composition. Using apo B1-Lps the binding site will be characterized in terms of affinity, saturation, turnover, apoprotein specificity and lysosomal interaction. A calculation of receptor number will be made and it will be determined whether dietary or metabolic state alters the number. Using particles of mixed composition, favorable apoprotein composition for hepatic clearance of Lps will be determined with particular interest in the role of surface protein (apo C, apo E and apo A-1) on binding of apo B1-Lps. Considering the evidence that apo E may mediate Lp interaction with liver, a reasonable hypothesis is that apo E increases the hepatic sinusoidal concentration of Lps for action as a substrate for sinusoidal lipases. Apo B1 recognition may occur only following lipase modification and apo B1 hepatic clearance would, therefore, be facilitated by apo E present on the Lp. Particle composition of apo B1-Lps is seen as critical to their eventual hepatic catabolism. These studies assume importance because of the relationship of apo B-Lps, especially IDL and LDL, in development and progression of human atherosclerosis.
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