The pH dependency of protoporphyrin IX formation in cells incubated with 5-aminolevulinic acid.

Chinese hamster cells (V79) and human adenocarcinoma cells (WiDr) were incubated with 5-aminolevulinic acid (ALA) at different pH values and the rate of production of protoporphyrin IX (PpIX) was measured. The rate of production increased with pH in the range 6.0-7.5. Above pH 7.5 the rate decreased, possibly due to a reduced metabolic activity of the cells. The observations may be explained by the known pH-dependency of the activity of porphobilinogen deaminase (PBGD) and are in agreement with the assumption that PBGD constitutes a rate-limiting step in the synthesis of PpIX from ALA.