Photodynamic Therapy of Actinic Keratosis. Porphobilinogen synthase substrate that produces endogenous porphyrins useful for sensitizing cells in photodynamic therapy. Intermediate in heme and chlorophyll biosynthesis. Estimation of 5-aminolevulinic acid dehydratase. PBG formed in the assay is determined spectrophotometrically with Ehrlich's reagent. Used in Porphyrin test for differentiation of Haemophilus species. Enzymes that convert ALA to porphyrins in the biosynthesis of hemin (X factor) are not produced by H. influenzae, H.aegyptius and H. canis. These enzymes are produced by H. parainfluenzae, H. parahaemolyticus, H. gallinarum, H. parasuis, H. parahaemolyticus and H. aphrophilus. A positive reaction is indicated by an orange-red fluorescence.