Summary As an initial approach to optimize 8-aminolaevulinic acid (6-ALA)-induced photosensitization of tumours, we examined the response of three enzymes of the haem biosynthetic pathway: 8-ALA dehydratase, porphobilinogen deaminase (PBGD) and ferrochelatase. Only PBGD activity displayed a time-and dose-related increase in tumours after intravenous administration of 300 mg kg-1 6-ALA. The time course for porphyrin fluorescence changes, reflecting increased production of the penultimate porphyrin, protoporphyrin IX (PPIX), showed a similar pattern to PBGD. This apparent correlation between PBGD activity and porphyrin fluorescence was also observed in four cultured tumour cell lines exposed to 0.1-2.0 mm 6-ALA in vitro. The increase in PBGD activity and PPIX fluorescence was prevented by the protein synthesis inhibitor cycloheximide. As the apparent Km for PBGD was similar before and after 6-ALA, the increase in PBGD activity was attributed to induction of enzyme de novo. These observations of an associated response of PBGD and PPIX imply that PBGD may be a ratelimiting determinant for the efficacy of 6-ALA-induced photosensitization when used in photodynamic therapy.Keywords: 6-aminolaevulinic acid: photosensitization; porphobilinogen deaminase: haem biosynthesis; porphyrin fluorescence Haem is an essential prosthetic group in many critical cellular proteins such as haemoglobin, cytochrome P450 and cyclooxygenase (Abraham, 1991). Eight enzymes are involved in the biosynthesis of haem, a process that occurs in two subcellular compartments: the mitochondria and the cytosol. The first enzyme in the haem pathway, mitochondrial 8-aminolaevulinic acid synthase (8-ALA-S), forms 6-ALA from glycine and succinyl CoA and is a target for the regulation of haem biosynthesis. Feedback inhibition of 8-ALA-S occurs when intracellular haem is present in excess (Ade, 1990).The last step in the haem biosynthetic pathway, the insertion of iron into PPIX to form haem, is catalysed by the mitochondrial enzyme ferrochelatase (FC). In unperturbed systems, FC is regulated by the availability of iron and/or PPIX. Two of the metabolic events that occur between 8-ALA-S and FC are catalysed by the cytosolic enzymes 8-ALA dehydratase (8-ALA-D) and porphobilinogen deaminase (PBGD). The dehydratase enzyme catalyses the condensation of two 8-ALA molecules to form porphobilinogen (PBG). It is the first enzyme that will metabolize the administered 8-ALA. The next enzyme in haem biosynthesis, PBGD, forms the tetrapyrole ring from four porphobilinogen molecules (Abraham, 1991).By providing the 8-ALA-S product, 8-ALA, the initial feedback step has been circumvented and this approach has been exploited for use in photodynamic therapy (PDT) of cancer (Kennedy and Pottier, 1992;Grant et al, 1993; Cairnduff et al, 1994;Regula et al, 1995). Traditional PDT regimens consist of the systemic adminis- Accepted 13 June 1997 Correspondence to: R Hilf tration of a photosensitizer followed by an appropriate interval to allow for its maximal accumulation in ma...