| Research Abstract |
The metabolic turnover of HDL-apolipoproteins has been studied in human, and such studies have led to a better understanding of the mechanism of low and high plasma HDL levels. We attempted here to determine the kinetic parameters (turnover) of HDL apo AI in V, WHHL rabbits, normal Japanese white (control) rabbits, with or without cholesterol, probucol feeding. ^<125>I-labeled HDL was injected intravenously and blood samples were taken periodically for 6 days. Kinetic parameters were calculated from the apo AI specific radioactivity decay curves. We found that 1) the deficiency of plasma HDL-apo AI in the WHHL rabbits resulted from an increase in FCR and a decrease in apo AI synthesis, 2) apo AI FCR of the rabbits fed cholesterol was significantly increased but there were no change in synthetic rate, and 3) the decreased HDL or apo AI seen with probucol was apparently the result of an increase in FCR and a decrease in synthesis of HDL-apo AI, compared to findings in normal chow fed rab
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bits. A decreased synthesis of apo AI remained evident even 1 month after discontinuing probucol. The action of probucol on the intracellular synthetic process of apo AI was revealed by the reduced synthesis of proapo AI.
Total RNA was isolated from rabbit intestine under various conditions as stated above, ^<32>P-labeled human apo AI cDNA probe was hybridized to RNA and specific hybridization was visualized by autoradiography. Compared to findings in normal rabbits, the WHHL had 2-3 fold lower levels of intestinal apo AI mRNA, but probucol treated rabbits showed no changes in mRNA level of apo AI. Probucol may affect the extracellular, post-translational process of apo AI, thus decreasing the levels of apo AI.
In vivo conversion of recombinant human proapoplipoprotein AI (rh-Met-proapo AI) from E. coli to apolipoprotein (apo) AI was also investigated in rabbits in vivo. It was found that the radioactivity of rh-Met-proapo AI migrated to more acidic isoproteins, the conversion was complete within 24 hours. Thus, 1) the proteolytic cleavage of proapo AI is an extracellular event, 2) the converting enzyme from rabbits is functional for human proapo AI processing, and 3) the injection of rh-Met-proapo AI into rabbits facilitates understanding of the early events of HDL biogenesis in rabbits. Less
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