A B S T R A C T Acidic solvenits extract the same porphyrin-protoporphyrin-from the erythrocytes of patients with either erythropoietic protoporphvria or lead intoxication. However, extractable protoporplhyrin disappears rapidly, both in vivo alnd in vitro, frolmi ervthrocytes in erythropoietic protoporphyria but slowly, if at all, in lead intoxicationi. Consistent w-ith these observations, fluorescence spectroscopy revealed that the intracellular state of the erythrocyte protoporphyrin is different in the two diseases. Spectrofluorometric measurements coupled with fractionations and biochemical syntheses showed that in erythropoietic protoporphyria the protoporphyrin is bound as the free base to lhellmoglobin molecules at sites other than the heme bilnding sites. In lead intoxication the fluorescent porphyrin is also bound to hemoglobin but is present as zinlc protoporphyrin. The data suggest that the zilnc protoporphyrin is bound at heme binding sites. Acidic extraction solvents remove the chelated zinc, but zinlc protoporphiyrinl may be extracted intact from erythrocytes w-ith acetone, ethaniol, or the detergent Ammnonyx-LO.

Fluorescence spectra of protoporphyrin bound to its most affinitive site on human serum albumin, bound to human haemopexin and dissolved in human plasma reveal that, when present in plasma, at least 90% of this porphyrin is bound to albumin. Human serum albumin binds protoporphyrin with an affinity KA = 3 X 10(9)M-1 in phosphate-buffered saline. The affinity of haemopexin for protoporphyrin is 4 times smaller. From these data it is concluded that less than 1% of plasma protoporphyrin is bound to haemopexin. Implications of the data for protoporphyrin transport and clearance are discussed.