Delta-aminolevulinic transport by mammalian peptide tansporters

Delta-aminolevulinic acid (ALA) is the precursor of porphyrin synthesis and has been recenctly used in vitro and in clinical studies as an endogenous photosensitizer in photodynamic therapy for treatment of various tumors. When administered by the oral route it shows excellent intestinal absorption. ALA is also efficiently reabsorbed in the renal proximal tubule. However, the pathways and mechanisms for its transmembrane transport into epithelial cells of intestine and kidney are unknown. Here we demonstrate that ALA utilizes the intestinal and renal apical peptide transporters for entering into epithelial cells. Characteristics of ALA transport were determined in Xenopus laevis oocytes and Pichia pastoris yeast cells expressing either the cloned intestinal peptide transporter PEPT1 or the renal form PEPT2. By using radiolabeled ALA and electrophysiological techniques in these heterologous expression systems we established that a) PEPT1 and PEPT2 translocate radiolabeled ALA by saturable and pH dependent transport mechanismus, b) that ALA and di-/tripeptides but not GABA or related amino acids compete at the same substrate binding site of the carriers and c) that ALA transport is electrogenic in nature as a consequence of H+/ALA cotransport. RT-PCR demonstrates that particularly the PEPT2 mRNA is expressed in a variety of other tissues including lung, brain and mammary gland which have been shown to accumulate ALA. This suggests that these tissues could take up the prophyrin precursor via expressed peptide transporters providing the endogenous photosensitizes for efficient photodynamic therapy.