The presence in animal tissues of an enzyme which splits off purines from purine nucleosides has been recognized for many years (1). This enzyme, purine nucleoside phosphorylase (PNP), has been studied by Kalckar (2-4) and has been shown to be involved in the following reaction: ribose-l-purine + phosphate = ribose-1-phosphate + purine Purine nucleoside phosphorylase from rat liver also possesses a certain specificity with regard to the nitrogenous bases. Inosine and guanosine are the only ribosides which undergo phosphorolysis in the presence of the enzyme. Adenosine and xanthosine are inert in the system, as are pyrimidine ribosides. Hypoxanthine and guanine are the only nitrogenous bases which are incorporated into ribosides by the enzyme. Nucleoside phosphorylases fractionated by various means catalyze the phosphorolysis of purine desoxyribosides as well as of the ribosides. Cleavage of the purine base from the pentose does not occur in the ab- (Summer, 1954).It is the purpose of this paper (a) to demonstrate the presence of PNP activity in erythrocytes; (b) to indicate the comparative activity of PNP in the red blood cells of man, dog, rabbit, pig, and chicken; and (c) to report experimentally induced changes in the PNP activity of dog erythrocytes. METHODSRed cell enzyme preparations were made as follows. Blood obtained by venipuncture was anticoagulated with heparin and centrifuged for 30 minutes at 3,000 r.p.m. The plasma and buffy coat were drawn off. The cells were hemolyzed with distilled water. They were not washed prior to hemolysis since this procedure did not modify the activity of the preparations.

1. Erythrocyte hemins, extracted with acid-acetone, exhibited multiple components on two chromatographic systems. However, this heterogeneity can be explained by various physicochemical properties of hemin in solution and does not imply that heme, when attached to globin in the native hemoglobin molecule, is heterogeneous. 2. The presence of a non-polar fraction may be accounted for by esterification of protohemin as a result of reaction with small amounts of methanol contaminating the acetone and/or the presence of some un-ionized protohemin molecule in solutions of low pH. 3. The presence of poorly soluble fractions is probably the result of polymerization of the hydroxyhemin molecule into so-called β- and γ-hematins. 4. The formation of mesohemin is considered unlikely, but the formation of deuterohemin or hematohemin cannot be excluded. 5. No changes in hemin chromatographic patterns were noted in disease. 6. It is necessary to evaluate reports concerning chromatographic hemin heterogeneity in the light of the artifacts described above before attributing physiologic or pathogenetic significance to the findings.