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Preferential heme transport through endoplasmic reticulum associated with mitochondria in rat liver
1Department of Biochemistry, Kyushu University School of Medicine, Fukuoka, Japan.
Abstract:
The transport of de novo synthesized protoheme into the conventional microsomal fraction and endoplasmic reticulum associated with mitochondria (MAER) was studied by injecting amino[14C]levulinic acid (ALA) into phenobarbital-treated rats to evaluate the role of MAER in the trafficking of heme between mitochondria and endoplasmic reticulum. In mitochondria, the specific radioactivity of the radiolabeled heme reached a maximum level at 4 min after the injection of 14C-ALA. The specific radioactivity in cytosol was about 2-fold lower than that in microsomes, suggesting that the cytosolic pathway of the heme transport from mitochondria to endoplasmic reticulum is not predominant, because the specific radioactivity of heme in cytosol should be higher than that in microsomes if heme is transported mainly through cytosol. MAER showed higher specific radioactivity than the conventional microsomal fraction up to 4 min and thereafter the specific radioactivities in MAER and the conventional microsomal fraction became nearly the same. The extents of decrease in cytochrome P-450 and the radioactivity in microsomes by the treatment with allylisopropylacetamide which destroyed cytochrome P-450 but not cytochrome b5, were essentially the same, suggesting that most of the radiolabeled heme in microsomes was incorporated into cytochrome P-450. These results suggest that MAER is a preferential site for the protoheme transport from mitochondria to endoplasmic reticulum.
Insights
Mitochondria-associated endoplasmic reticulum (MAER) is a key site for transporting newly synthesized protoheme from mitochondria to the endoplasmic reticulum. This pathway is more significant than the cytosolic route for heme trafficking.
Area of Science:
- Biochemistry
- Cell Biology
- Heme Metabolism
Background:
- Protoheme is essential for various cellular functions, including respiration and drug metabolism.
- The endoplasmic reticulum and mitochondria are critical organelles involved in heme synthesis and utilization.
- Understanding heme trafficking between these organelles is crucial for cellular homeostasis.
Purpose of the Study:
- To investigate the role of mitochondria-associated endoplasmic reticulum (MAER) in protoheme transport.
- To compare the MAER pathway with the conventional cytosolic pathway for heme trafficking.
- To determine the primary destination of newly synthesized heme within the endoplasmic reticulum.
Main Methods:
- Rats were injected with amino[14C]levulinic acid (ALA) after phenobarbital treatment.
- Radioactivity of labeled heme was measured in mitochondria, cytosol, microsomes, and MAER fractions over time.
- Cytochrome P-450 and cytochrome b5 levels were assessed after treatment with allylisopropylacetamide.
Main Results:
- Peak radiolabeled heme radioactivity in mitochondria occurred at 4 minutes post-ALA injection.
- Heme radioactivity in cytosol was lower than in microsomes, suggesting a non-predominant cytosolic transport pathway.
- MAER exhibited higher specific radioactivity than conventional microsomes initially, indicating its role in early heme transfer.
- Allylisopropylacetamide treatment similarly reduced radioactivity and cytochrome P-450 in microsomes, implying heme incorporation into cytochrome P-450.
Conclusions:
- Mitochondria-associated endoplasmic reticulum (MAER) serves as a preferential site for protoheme transport from mitochondria.
- The MAER pathway is a significant route for heme trafficking to the endoplasmic reticulum.
- Newly synthesized heme is primarily incorporated into cytochrome P-450 within the endoplasmic reticulum.