Deficiency of erythrocytic and lymphocytic adenosine deaminase (ADA) occurs in some patients with severe combined immunodeficiency disease (SCID). SCID with ADA deficiency is inherited as an autosomal recessive trait. ADA is markedly reduced or undetectable in affected patients (homozygotes), and approximately one half normal levels are found in individuals heterozygous for ADA deficiency. The metabolism of purine nucleosides was studied in erythrocytes from normal individuals, four ADA deficient patients, and two heterozygous individuals. ADA deficiency in intact erythrocytes was confirmed by a very sensitive ammonia liberation technique. Erythrocytic ADA activity in three heterozygous individuals (0.07, 0.08, and 0.14 μmolar units/ml of packed cells) was between that of the four normal controls (0.20-0.37 μmol/ml) and the ADA deficient patients (no activity). In vitro, adenosine was incorporated principally into IMP in the heterozygous and normal individuals but into the adenosine nucleotides in the ADA deficient patients. Coformycin (3 β (D) ribofuranosyl 6,7,8 trihydroimidazo [4,5 d] [1,3] diazepin 8 (R) ol), a potent inhibitor of ADA, made possible incorporation of adenosine into the adenosine nucleotides in normal and heterozygous erythrocytes. Coformycin did not alter the pattern of nucleotide incorporation in the ADA deficient cells. These results indicate that coformycin causes normal and heterozygous cells to behave like ADA deficient cells. Similarly, p nitrobenzylthioguanosine (2 amino 6 ([4 nitrobenzyl] thio) 9 β (D) ribofuranosylpurine) (NBTGR), an inhibitor of nucleoside transport, drastically reduced the incorporation of adenosine into the inosine nucleotides and enhanced the incorporation into adenosine nucleotides in the normal erythrocytes. NBTGR had no effect upon ADA deficient cells. NBTGR inhibited the liberation of ammonia from adenosine in intact normal cells. In two patients previously treated with bone marrow or fetal liver transplantation, incubation of erythrocytes with inosine resulted in accumulation of ITP, whereas in the erythrocytes from the heterozygotes, the normal individuals and the two other ADA deficient patients, IMP accumulated. Reexamination of one of the transplanted patients 10 and 15 mo later revealed his erythrocytic inosine incorporation into ITP to be decreased from that observed at the initial examination. Guanosine was incorporated into guanosine nucleotides in all erythrocytes studied, suggesting that the nucleoside transport system and the enzymes purine nucleoside phosphorylase, hypoxanthine guanine phosphoribosyltransferase, and 5 phosphoribosyl 1 pyrophosphate synthetase are functional in the erythrocytes of ADA deficient patients. Formycin A (7 amino 3 β (D) ribofuranosylpyrazolo [4,3 d] pyrimidine), an adenosine analogue, was converted to formycin A mono, di, and triphosphate nucleotides in ADA deficient patients' erythrocytes to much greater extent than occurred in normal cells. This confirms the ability of formycin A to act as an excellent substrate for both human erythrocytic ADA and adenosine kinase. These results are consistent with the hypothesis that the disposition of adenosine in human erythrocytes is the result of the relative activities and Michaelis constants of the enzymes ADA and adenosine kinase. Various possible biochemical mechanisms by which ADA deficiency causes cytotoxicity are discussed.
ASJC Scopus subject areas