Deoxyadenosine metabolism in the erythrocytes of children with sever, combined immunodeficiency

Abstract

Deoxyadenosine metabolism was compared in intact erythrocytes from 2 children with severe combined immunodeficiency: one had normal adenosine deaminase (ADA; Ec 3.5.4.4) levels and the other, a homozygote for ADA deficiency, had (following a bone-marrow graft) 15% of normal activity in lysed erythrocytes. Deamination was the principal route of deoxyadenosine metabolism, under all incubation conditions, in intact erythrocytes from both patients and control subjects. When the same studies were performed with the ADA inhibitor EHNA (erythro-9[2-hydroxy-3-nonyl]adenine), further differences between the metabolism of deoxyadenosine and adenosine became apparent. Deamination of deoxyadenosine was not completely inhibited by 5 .mu.M EHNA even at the lowest substrate concentrations. Significant deoxynucleotide formation occurred only at unphysiological phosphate levels (18 mM Pi) and most of the substrate remained unmetabolized. In these in vitro studies at high phosphate concentrations deoxyadenosine was incorporated into the deoxynucleotides dAMP:dADP:dATP in the same ratio (10:1.0:0.1) as was found in the ADA-deficient erythrocytes prior to bone-marrow graft. Under physiological conditions deoxyadenosine - unlike adenosine - is deaminated, not phosphorylated, and is dependent on intact ADA activity for its normal metabolism. The fact that even low levels of ADA (as assessed by lysate activity) are adequate in intact cells to perform the important detoxifying function of this enzyme may explain the existence of immunocompetent children deficient in erythrocyte ADA. Intact cells may give a better indication of prognosis in suspected cases of ADA deficiency.