Is delta-aminolevulinic acid dehydratase rate limiting in heme biosynthesis following exposure of cells to delta-aminolevulinic acid?

Understanding the regulation and control of heme/porphyrin biosynthesis is critical for the optimization of the delta -aminolevulinic-acid (ALA)-media ted photodynamic therapy of cancer, in which endogenously produced protopor phyrin IX (PPIX) is the photosensitizer, The human breast cancer cell line MCF-7, the rat mammary adenocarcinoma cell line R3230AC, the mouse mammary tumor cell Line EMT-6 and the human mesothelioma cell line H-MESO-1 were us ed to study ALA-induced PPM levels and their relationship to delta -aminole vulinic acid dehydratase (ALA-D) activity in vitro. Incubation of these cel l lines,with 0.5 mM ALA for 3 h resulted in a significant increase in PPIX accumulation, compared with control cells, but there was no significant cha nge in ALA-D activity. Exposure of cells incubated with ALA to 30 mJ/ cm(2) of fluorescent light, a dose that would cause a 50% reduction in cell prol iferation, did not significantly alter the activity of ALA-D. Increasing th e activity of porphobilinogen deaminase (PBGD), the enzyme immediately subs equent to ALB-D, by four- to seven-fold via transfection of cells with PBGD complementary DNA did not alter the activity of ALA-D. How-ever, incubatio n of cells with various concentrations of succinyl acetone, a potent inhibi tor of ALA-D, caused a concomitant decline in both PPIX accumulation and AL A-D activity. These data imply that when tells are exposed to exogenous ALA , ALA-D is an important early-control step in heme/porphyrin biosynthesis a nd that regulation of PPIX synthesis by this dehydratase may impact the eff ectiveness of ALE-mediated photosensitization.