Haem precursor effects on [3H]-PK 11195 binding to platelets.

The haem precursors delta-aminolaevulinic acid (ALA) and porphobilinogen (PBG), over-produced in acute intermittent porphyria (AIP), may mediate porphyric neuropathy. Porphyrins bind to peripheral benzodiazepine receptors, putative mitochondrial porphyrin translocation sites. In AIP, ALA and PBG may interfere with porphyrin transport causing a deficit in essential haemoproteins. The effects of ALA, PBG and protoporphyrin 1X on the binding of [3H]-PK 11195 to platelets were examined. Ligand binding was also examined in platelets from porphyric patients. The haem precursor protoporphyrin 1X (10 microM) caused an increase in Kd values (p < 0.05) whereas Bmax values remained unaltered. Neither ALA (10 microM) nor PBG (10 microM) altered ligand binding. Patient platelets showed no change in ligand binding values. ALA and PBG are unlikely to compete with porphyrins for peripheral benzodiazepine sites.