Biogenesis of embryonic chick liver δ-aminolevulinate synthase: Regulation of the level of mRNA by hemin

The effects of hemin on the concentration of the mRNA for δ-aminolevulinate synthase (ALA synthase) and on the association of the messenger with polysomes were investigated in primary cultures of embryonic chick hepatocytes incubated with allylisopropylacetamide (AIA). A synthetic 24-mer DNA complementary to ALA synthase mRNA was used to determine by solution hybridization the effects of AIA and of AIA plus hemin on the ALA synthase-specific RNA sequences in the cells. The results indicated that ALA synthase mRNA concentrations increased significantly in hepatocytes incubated for 5 h with AIA (0.075 mg/ml), and that hemin in the medium (2 or 10 μ m) blocked the increase in the messenger. When δ-aminolevulinic acid (ALA) and FeCl 3 were added into the culture medium (1 m m and 5 μ m, respectively), the increase in ALA synthase mRNA brought on by AIA was also inhibited. Neither ALA nor FeCl 3, when individually added to the cultures, was as effective as the combination of the two. The results with ALA + FeCl 3 suggested that stimulation of intracellular production of heme was also effective in blocking the increase in ALA synthase mRNA caused by AIA. Finally, the distributions of ALA synthase mRNA were compared in polysomes isolated from hepatocytes which had been incubated with AIA for 5 h in the presence and absence of 10 μ m hemin in the medium. Although a drop was detected in the concentration of ALA synthase mRNA in polysomes from hepatocytes incubated with hemin for 30 min, the decrease was explained by the effect of hemin on the mRNA concentration in the cells.