Hepatic clearance of rat liver aspartate aminotransferase isozymes: Evidence for endocytotic uptake via different binding sites on sinusoidal liver cells
✍ Scribed by Seikoh Horiuchi; Yukio Kamimoto; Yoshimasa Morino
- Publisher
- John Wiley and Sons
- Year
- 1985
- Tongue
- English
- Weight
- 882 KB
- Volume
- 5
- Category
- Article
- ISSN
- 0270-9139
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✦ Synopsis
Rat liver aspartate aminotransferase (AAT) (EC 2.6.1.1) exists in two isozymic forms, cytosolic (c-AAT) and mitochondria1 (m-AAT). The previous study (Kamimoto, Y. et al., Hepatology an accompanying paper in this issue) demonstrated that these isozymes were cleared from blood at different half-lives via adsorptive endocytosis by sinusoidal liver cells. To understand the cellular mechanism for the differential uptake of the isozymes, we have further studied in vivo uptake of '261-labeled AAT isozymes by sinusoidal cells. The results indicated that the uptake of the isozymes occurred via a typical endocytotic pathway: the initial binding, internalization and subsequent degradation in the lysosomes. Quantitation of the isozymes bound to the cell surface prior to internalization either by binding at 4OC or by a combined use of anti-AAT antibody and '251-protein A at 37°C revealed that the differential plasma clearance of AAT isozymes could be attributable to the isozymic difference in capacity of surface membranes to bind the isozymes. The uptake of lZ5I-c-AAT was inhibited by unlabeled c-AAT more significantly than by m-AAT, but not by other ligands known to be taken up by sinusoidal cells via receptor-mediated pathways. Similarly, the uptake of '261-m-AAT was inhibited predominantly by unlabeled m-AAT. Similar ligand specificity was also demonstrated in the binding study at 4°C. The binding of 1251-m-AAT and c-AAT followed saturation kinetics with an apparent Kd of 1.3 x 1O-'M and 1.7 X lo-' M, respectively.
These findings indicate that differential uptake of AAT isozymes results from the presence on the surface membrane of two binding sites with a definite specificity for each isozyme.
Since the discovery by Ashwell and Morrel (1) of the specific receptor for asialoglycoproteins on the hepatocyte plasma membranes, many studies have been focused on the mechanisms by which plasma proteins are cleared from the blood. It has been shown that the degree of adsorption of circulating proteins on the surface of endocytosing cells can be the main factor governing the rate of plasma clearance and catabolism of these proteins. The first step of their plasma clearance is the endocytotic uptake by a specific organ(s), followed by subsequent intercellular degradation by a lysosomal protease system. Among the organs involved in elimination of the circulating proteins, liver has been known to take