The possible relationship between heme synthesis and mitochondrial biogenesis

Administration of allylisopropylacetamide (AIA)¹ to rats causes a rapid induction of hepatic δ-aminolevulinic acid (ALA) synthetase. Liver mitochondria obtained from animals after AIA treatment had a 50–60% greater rate of amino acid incorporation in vitro as compared to mitochondria from control animals. Similarly, pulselabeling by (³H) leucine in vivo was stimulated 50% in submitochondrial fractions containing proteins insoluble in acetic acid and a membranous preparation of cytochrome oxidase. Previous studies had indicated that 15–20% of the proteins in these fractions were labeled in vivo by a cycloheximide-insensitive system, presumably that of the mitochondria (Fed. Eur. Biol. Soc. Lett. 9, 232 1970). In time-course studies, it was observed that increased incorporation of (¹⁴C)glycine into heme in vivo followed shortly after ALA synthetase induction. A slight decay occurred before the incorporation of (³H)leucine into cytochrome oxidase or total mitochondrial protein was stimulated. At these latter times a 20–30% increase in the cytochrome content was also observed. Administration of aminotriazole, an inhibitor of ALA dehydratase and hence overall heme synthesis, prevented the stimulation of leucine incorporation in vivo and the increase in cytochrome content observed after AIA treatment. These results suggest that mitochondrial protein synthesis and cytochrome formation may be stimulated when heme biosynthesis is increased due to ALA synthetase induction.