ORIGIN OF MITOCHONDRIAL ENZYMES .3. DISTRIBUTION AND SYNTHESIS OF CYTOCHROME C IN RAT LIVER TISSUE

Fractionation studies of rat liver have shown that the mitochondria thereof contain more than 90% of the total cytochrome c of the tissue upon their isolation. Reproducible small complements of the remainder of the cytochrome c were found in each of other cell fractions. The intracellular site of synthesis of cytochrome c was determined by use of 3H-δ-aminolevulinic acid (3H-ALA) as a precursor of hematoheme. In short intervals of labeling following intraperitoneal injection of 3H-ALA into mature rats, the specific activity of the hematoheme of microsomal cytochrome c greatly exceeded that of mitochondrial cytochrome c; after 2 hr of incorporation, the two specific activities approached a common value. When short intervals were employed for incorporation of 3H-ALA, followed by an additional interval in the presence of a molar excess of ALA it, was possible to show a precursor-product relationship between the respective microsomal and mitochondrial complements of cytochrome c. The findings indicate that the endoplasmic reticulum catalyzes the complete assembly of cytochrome c whereupon the completed protein is then transferred to the mitochondria. © 1969.