Functional studies in fibroblasts of adenylosuccinase-deficient children

In fibroblasts of severely retarded (type I) adenylosuccinase (ASase)-deficient children, activities with the two substrates of the enzyme, succinylaminoimidazole carboxamide ribotide (succinyl-AICAR) and adenylosuccinate are decreased in parallel, to about 30% of normal. In a markedly less retarded (type II) patient, ASase activity with adenylosuccinate reaches only 3% of normal, whereas activity with succinyl-AICAR is also about 30% of normal. To assess the functional significance of a partial versus a profound deficiency of ASase, precursor incorporation studies were performed in intact fibroblasts. In cells from controls and from type I patients, incorporation of 0.2 mmol/L [14C]formate into adenine and guanine nucleotides was not accompanied by accumulation of either [14C]succinyl-AICAR or [14C]adenylosuccinate. Similarly, incorporation of 20 mumol/L [14C]hypoxanthine was not accompanied by accumulation of [14C]adenylosuccinate. In contrast, in fibroblasts of the type II patient, in accordance with the profound deficiency of ASase with adenylosuccinate, and with the inhibitory effect of Cl- and nucleotides on the activity with succinyl-AICAR, incorporation of [14C]formate resulted in accumulation of [14C]succinyl-AICAR and [14C]adenylosuccinate, and incorporation of [14C]hypoxanthine in a marked build-up of [14C]adenylosuccinate. That both precursors were still incorporated into the adenine nucleotides of the fibroblasts of the type II patient indicates that adenylate synthesis remains possible even with 3% residual ASase activity, as also shown by their grossly normal ATP concentrations. The results suggest that the pathophysiology of ASase deficiency may be mediated at least in part by accumulation of succinyladenosine and succinyl-AICAriboside.