J. Biochem, 1988, Vol. 103, No. 6 973-978
© 1988 Japanese Biochemical Society
research-article |
Mechanisms Involved in the Cellular Uptake of Hematoporphyrin by Rat Hepatoma Cells1
* Third Department of Internal Medicine, Kansai Medical University Moriguchi, Osaka 570
** Department of Hygiene, Kansai Medical University Moriguchi, Osaka 570
2To whom correspondence should be addressed.
To clarify the mechanisms involved in the specific uptake of hematoporphyrin by cancer cells, we investigated the interaction of the heme- and/or hematoporphyrin-hemopexin complexes with rat hepatoma dRLh-84 cells. Hemopexin bound to the cells in a saturable, time- and temperature-dependent manner. The cells exhibited 0.55 nmol of binding sites/ mg of protein for the heme-hemopexin complex and 0.38 nmol for the hematoporphyrin-hemopexin complex. The dissociation constants (Kd, ) for the heme-hemopexin and hematoporphyrin-hemopexin complexes were 0.57 and 0.54 uM, respectively. Specific binding of the labeled hemopexin was inhibited by the unlabeled heme- and hematoporphyrin-hemopexin complexes but was unaffected by albumin or neoglycoprotein. Hematoporphyrin bound to hemopexin was incorporated into the cells at 37°C, but not at 4°C. These results indicate that hematoporphyrin bound hemopexin was taken up by dRLh-84 cells, via the hemopexin receptors. When the hematoporphyrin-albumin complex was incubated with the cells, the hematoporphyrin- [125I] albumin complex bound to the cells in a time and temperature-dependent manner. Here the binding was not saturated up to 100 µg/ml of albumin. The binding of hematoporphyrin-[125I]albumin was partially inhibited by unlabeled albumin and hemopexin. Hematoporphyrin bound to albumin was taken up by the cells at 37°C. Thus, the albumin-dependent uptake of hematoporphyrin by rat hepatoma dRL-84 cells could be differentiated from the hemopexin-mediated uptake of hematoporphyrin.
1This work was supported by a Grant-in-Aid for Scientific Research (No. 61570357) from the Ministry of Education, Science and Culture of Japan and by grants from the Fugaku Trust for Medicinal Research.