The temperature dependence of protoporphyrin IX production in cells and tissues

The formation of protoporphyrin DL (PpIX) in human skin during topical appl ication of 5-aminolevulinic acid (ALA) was found to be strongly temperature dependent, with an activation energy of about 17 kcal/mol. This temperatur e dependence is mainly related to porphyrin production and not to ALA penet ration into the skin. The penetration of AL,A into mouse and human skin was almost temperature independent. The activation energy of PpIX production i n mouse skin was practically identical with that in human skin. The activat ion energy of ALA uptake by cells irt vitro was about 10 kcal/mol and that for PpIX production was about 13 kcal/mol, The latter activation energy was within the error limits similar to that for the activity of the enzyme por phobilinogen deaminase, suggesting that this enzyme might represent a rate- limiting step for PpIX production in living tissue.