Hepatic clearance of rat liver aspartate aminotransferase isozymes: Evidence for endocytotic uptake via different binding sites on sinusoidal liver cells

Authors

  • Seikoh Horiuchi M. D.,

    Corresponding author
    1. Department of Biochemistry and Department of Surgery, Kumamoto University Medical School, Kumamoto 860, Japan
    • Department of Biochemistry, Kumamoto University Medical School, 2–2–1, Honjo, Kumamoto 860, Japan
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  • Yukio Kamimoto,

    1. Department of Biochemistry and Department of Surgery, Kumamoto University Medical School, Kumamoto 860, Japan
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  • Yoshimasa Morino

    1. Department of Biochemistry and Department of Surgery, Kumamoto University Medical School, Kumamoto 860, Japan
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Abstract

Rat liver aspartate aminotransferase (AAT) (EC 2.6.1.1) exists in two isozymic forms, cytosolic (c-AAT) and mitochondrial (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 125I-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 4°C or by a combined use of anti-AAT antibody and 125I-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 125I-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 125I-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 125I-m-AAT and c-AAT followed saturation kinetics with an apparent Kd of 1.3 × 10−6M and 1.7 × 10−6M, 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.

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