16 MECHANISM OF ATP CATABOLISM INDUCED BY DEOXYADENO SINE AND OTHER NUCLEOSIDES IN ADENOSINE DEAMINASE-INHIBITED HUMAN ERYTHROCYTES

Abstract

In adenosine deaminase (ADA) deficiency and during treatment with deoxycoformycin (dCF), depletion of ATP accompanies accumulation of dATP in erythrocytes and lymphocytes (Bagnara & Hershfield, PNAS, 79: 2673–77, 1982 and refs therein). Stimulation of AMP deaminase and cytosolic 5′-nucleotidase by dATP was proposed to account for this. However, both enzymes are not more stimulated by dATP than by ATP. In human erythrocytes in which ADA was inhibited with 1 μM dCF, a 3- to 30-fold stimulation of adenine nucleotide (AN) catabolism was recorded upon addition of dAdo, but also of other substrates of adenosine kinase (AK) such as Ado, Ara-A, tuberdicin and 6-MMPR. Concomitantly, there was a reproducible increase, from 10 μM up to 100 μM, of AMP and of IMP, indicating an increased activity of AMP deaminase. The effects of Ado were suppressed upon prior inhibition of AK by 10 μM idotubercidin (ITu). Strikingly, they were also suppressed if ITu was added 2 h after dAdo, although dATP was maintained under this condition. ITu also abolished ATP catabolism induced by the other nucleosides. With all nucleosides, AMP increased during their phosphorylation and decreased after inhibition of AK. Studies with erythrocytic AMP deaminase, at low substrate and physiological concentrations of effectors, showed that, because of sigmoidal kinetics, the enzyme is very sensitive to variations of AMP between 5 and 100 μM. We conclude that elevation of AMP, secundary to phosphorylation of the nucleosides, is the main mechanism whereby they stimulate AN catabolism.