THE BIOCHEMICAL EXPLANATION FOR LACK OF PHOTOSENSIT IVITY IN Pb INTOXICATION

Abstract

This study explored the lack of photosensitivity in childhood Pb intoxication (CPI), despite RBCs protoporphyrin (PP) levels often higher than in erythropoietic protoporphyria (EPP). PP was bound to Hgb in both CPI and EPP. Absorption and emission of EPP Hgb solutions were identical to those of protein bound PP (405 and 620 nm); CPI Hgb solutions instead peaked at 424 and 590 nm. Acetone easily detached PP from Hgb in EPP, but not in CPI. The persistence of PP “in vivo” was measured in RBCs separated according to age on discontinuous gradients of arabino-galactan. In CPI, PP decreased only slightly during the RBCs life span and all cells faintly fluoresced. In EPP, PP decreased rapidly “in vivo”, at a rate parallel to reticulocytes; all young RBCs had intense fluorescence, which disappeared at similar rate. Plasma PP levels were normal in CPI, but elevated in EPP. These data indicate that in CPI the relative excess of PP occupies some of the heme sites, available because of defective heme synthesis. In EPP, heme synthesis is normal, and excess PP binds to the surface of the Hgb molecule. Thus, in CPI, the PP remains within the RBC through its life span and cannot (as in EPP) diffuse through the plasma into the skin to induce photosensitivity.