Immunocytochemical studies on the localization of 5-aminolevulinate synthase in rat liver

The localization of 5-aminolevulinate synthase (ALAS) in hepatocytes of untreated and porphyrinogenic drug-treated rats has been examined by an immunocytochemical approach using a monoclonal antibody and protein A-gold labeling. Gold particles representing antigenic sites for ALAS were observed in the mitochondria and cytoplasm of untreated and drug-treated cells. Quantitative analysis of the labeling density showed that levels of ALAS increased significantly in both of these cellular compartments following drug treatment. Evidence that the detected cytoplasmic form of ALAS represents the precursor of the enzyme was obtained from immunoblotting experiments. The direct detection of cytosolic ALAS in vivo rules out the possibility that enzyme activity previously detected in the cytosol fraction resulted from mitochondrial leakage during cell fractionation. The results indicate that the cytosolic accumulation of ALAS is not a consequence of the inability of mitochondria to accommodate more enzyme. However, the molecular basis for this cytosolic accumulation is not known. The studies also established that the mitochondrial enzyme is predominantly, if not exclusively, associated with the matrix side of the inner mitochondrial membrane.