The use of porphyrins as probes to examine the requirement for hemin in the maintenance of protein synthesis in reticulocyte lysates.

Abstract

Hemin was compared with other porphyrins for the ability to sustain protein synthesis in reticulocyte lysates and to inhibit the phosphorylation of the alpha subunit of initiation factor 2 (eIF-2 alpha). Iron-containing porphyrins (hemin, mesohemin IX, deuterohemin IX) were found to maintain protein synthesis while iron-deficient porphyrins (protoporphyrin IX, mesoporphyrin IX) were ineffective. All of the porphyrins and metalloporphyrins tested were observed to inhibit phosphorylation of eIF-2 alpha by highly purified hemin-controlled repressor, although at different concentrations. Phosphorylation of eIF-2 alpha was examined in reticulocyte lysates. In the absence of porphyrin, phosphorylation of eIF-2 was rapid and linear for the first 7 min, during which time protein synthesis was inhibited. Both hemin and protoporphyrin IX inhibited phosphorylation of eIF-2 alpha in the lysate during this period; however, hemin sustained protein synthesis whereas protoporphyrin IX did not. Thus, the requirement for iron-containing porphyrins in the maintenance of protein synthesis is not due strictly to an inhibition of the protein kinase activity of the hemin-controlled repressor but involves an alternative, although yet undeciphered, mode of action.