Labeling of porphobilinogen deaminase by radioactive 5-aminolevulinic acid in isolated developing pea chloroplasts.

A protein had been previously described, which was labeled by radioactive 5-aminolevulinic acid in isolated developing chloroplasts. In the present study we have shown that this protein (Mr approximately equal to 43,000) probably exists as a monomer in the chloroplast stroma. The labeling is blocked if known inhibitors of 5-aminolevulinic acid dehydratase are added to the incubation mixture, and is markedly decreased in intensity if nonradioactive 5-aminolevulinate or porphobilinogen are added to the incubation mixture; other intermediates in the porphyrin biosynthetic pathway, uroporphyrinogen III, uroporphyrin III, and protoporphyrin IX, do not decrease the labeling of the 43-kDa protein appreciably. Nondenaturing gels of the proteins isolated from the incubation with radioactive 5-aminolevulinic acid were stained for porphobilinogen deaminase activity. A series of red fluorescent bands was obtained which coincided with the radioactive bands visualized by autoradiography. It is concluded that the soluble chloroplast protein that is labeled in organello by radioactive 5-aminolevulinic acid is porphobilinogen deaminase.