Hemin inhibits protein synthesis and degradation in isolated brown adipose tissue mitochondria

The primary objectives of this work were to evaluate the effects of hemin on protein synthesis and degradation in isolated brown adipose tissue (BAT) mitochondria. Exposure of mice to a cold environment (4 degrees C) caused a significant increase in 5'-aminolevulinate synthase (ALV synthase) activity in BAT coincident with an increase in the tissue mitochondrial protein content. Hemin caused significant inhibition of protein synthesis and of ATP-stimulated proteolysis in isolated BAT mitochondria. These effects were specific, as protoporphyrin IX, a precursor of heme, increased protein synthesis and had no effect on ATP-stimulated degradation of mitochondrial translation products. The end products of degradation of proteins synthesized within isolated mitochondria were amino acids released to the outside of the organelles. Hemin appeared to inhibit an early step of the degradation pathway, as it caused a significant reduction in labelled methionine release from the organelles but no accumulation of peptides. Hemin caused significant inhibition of ATP-stimulated hydrolysis of labelled casein by soluble mitochondrial fractions, while addition of protoporphyrin IX was much less effective. In contrast, protease activity associated with mitochondrial membranes was almost equally sensitive to inhibition by hemin and protoporphyrin IX. These results suggest that heme may play a role in BAT mitochondriogenesis by its action on protein synthesis and degradation within the organelles.