S -Adenosylhomocysteine toxicity in normal and adenosine kinase-deficient lymphoblasts of human origin

Publisher Website
Google Scholar
Abstract
The human lymphoblast line WI-L2 is subject to growth inhibition by a combination of the adenosine deaminase (ADA; adenosine aminohydrolase, EC 3.5.4.4.) inhibitor erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) and adenosine. Although adenosine-induced pyrimidine starvation apparently contributes to this effect, uridine only partially reverses adenosine toxicity in WI-L2 and not at all in strain 107, an adenosine kinase-(ATP:adenosine 5''-phosphotransferase, EC 2.7.1.20) deficient derivative of WI-L2. Treatment of both cell lines with EHNA and adenosine leads to striking elevations in intracellular S-adenosyl-L-homocysteine (AdoHcy), a potent inhibitor of S-adenosyl-L-methionine (AdoMet)-dependent methylation reactions. The methylation in vivo of DNA and RNA is inhibited by concentrations of EHNA and adenosine that elevate intracellular AdoHcy. Addition of 100 .mu.M L-homocysteine thiolactone to cells treated with EHNA and adenosine enhances adenosine toxicity and further elevates AdoHcy to levels approximately 60-fold higher than those obtained in the absence of this amino acid, presumably by combining with adenosine to form AdoHcy in a reaction catalyzed by S-adenosylhomocysteine hydrolase (EC 3.3.1.1). In the adenosine kinase-deficient strain 107, a combination of ADA inhibition and L-homocysteine thiolactone markedly increases intracellular AdoHcy and inhibits growth even in the absence of exogenous adenosine. A form of toxicity from endogenously produced adenosine was demonstrated and the view that AdoHcy, by inhibiting methylation, is a mediator of uridine-resistant adenosine toxicity in these human lymphoblast lines was supported. AdoHcy may play a role in the pathogenesis of the severe combined immunodeficiency disease in most children with heritable ADA deficiency.