Purine biosynthesis in Chinese hamster cell mutants and human fibroblasts partially deficient in adenylosuccinate lyase

Neurologic aspects of adenylosuccinate lyase deficiency

Adenylosuccinate lyase deficiency is an autosomal-recessive disorder of the purine de novo synthesis pathway, diagnosed up to now in approximately 40 patients. The clinical presentation is characterized by severe neurologic involvement including seizures, developmental delay, hypotonia, and autistic features. Neonatal seizures and a severe infantile epileptic encephalopathy are often the first manifestations of this disorder. The existence of genetic heterogeneity for the adenylosuccinate lyase defect could account for variability of the clinical presentation. Deficiency of purine nucleotides, impairment of energy metabolism, and toxic effects are potential mechanisms of cerebral damage. Laboratory investigations show the presence in urine and cerebrospinal fluid of succinylpurines, which are normally undetectable. Currently, no effective treatment is available for adenylosuccinate lyase deficiency. A search for this disorder should be included in the screening program of children with unexplained neonatal seizures or severe infantile epileptic encephalopathy.

Inborn errors of the purine nucleotide cycle: adenylosuccinase deficiency

Adenylosuccinase catalyses two reactions in purine metabolism: the conversion of succinylaminoimidazole carboxamide ribotide (SAICAR) into aminoimidazole carboxamide ribotide (AICAR) along the de novo synthesis of purine nucleotides, and the conversion of adenylosuccinate (S-AMP) into AMP in the conversion of IMP into AMP. The hallmarks of adenylosuccinase deficiency are the presence of succinylaminoimidazole carboxamide riboside (SAICAriboside) and succinyladenosine (S-Ado) in body fluids. These normally undetectable succinylpurines are the products of the dephosphorylation, by cytosolic 5'-nucleotidase, of the two substrates of adenylosuccinase. The clinical picture of the enzyme deficiency is markedly heterogeneous with, as a rule, a profound, but nevertheless variable degree of psychomotor delay, often convulsions and/or autistic features, sometimes growth retardation and muscular dystrophy. The diagnostic tests that can be used for diagnosis, the enzyme and gene defects that have been identified, and the hypotheses that have been put forward to explain the pathophysiology of the disorder are reviewed.

Residual adenylosuccinase activities in fibroblasts of adenylosuccinase-deficient children: parallel deficiency with adenylosuccinate and succinyl-AICAR in profoundly retarded patients and non-parallel deficiency in a mildly retarded girl

Adenylosuccinase (ASase) catalyses both the conversion of succinyl-aminoimidazole carboxamide ribotide (succinyl-AICAR) into AICAR and that of adenylosuccinate into AMP in the synthesis of purine nucleotides. Its deficiency results in the accumulation in body fluids of the nucleosides corresponding to both substrates, succinyl-AICAriboside and succinyladenosine. Two main subtypes of the defect are type I with severe mental retardation and succinyladenosine/succinyl-AICAriboside ratios around 1, and type II with slight mental delay and succinyladenosine/succinyl-AICAriboside ratios around 4. We report that in fibroblasts of type I patients, the activity of ASase with both adenylosuccinate and succinyl-AICAR is about 30% of normal. In contrast, in type II fibroblasts, the activity with adenylosuccinate is only 3% of normal, whereas that with succinyl-AICAR is also 30% of normal. If also present in other tissues, this non-parallel deficiency provides an explanation for the higher concentration of succinyladenosine in type II. In type I fibroblasts, ASase is further characterized mainly by a 3-fold to 4-fold increase in Km for succinyl-AICAR, and by retarded elution from an anion exchanger. In type II fibroblasts, ASase is characterized by a similar increase in Km for succinyl-AICAR but by a potent inhibition by KCl and nucleoside triphosphates, and by a normal elution profile. These results suggest a modification of the surface charge of ASase in type I, and the addition of one or more positively charged residues in the active site in type II.

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.

The purine nucleotide cycle and its molecular defects

Three enzymes of purine metabolism, adenylosuccinate synthetase, adenylosuccinate lyase and AMP deaminase, have been proposed to form a functional unit, termed the purine nucleotide cycle. This cycle converts AMP into IMP and reconverts IMP into AMP via adenylosuccinate, thereby producing NH3 and forming fumarate from aspartate. In muscle, the purine nucleotide cycle has been shown to function during intense exercise; the metabolic flux through the cycle has been proposed to play a role in the regeneration of ATP by pulling the adenylate kinase reaction in the direction of formation of ATP, and by providing Krebs cycle intermediates. In kidney, the purine nucleotide cycle was shown to account for the release of NH3 under the normal acid-base status, but not under acidotic conditions. In brain, the purine nucleotide cycle might function under conditions that induce a loss of ATP, and thereby contribute to its recovery. There is no evidence that the purine nucleotide cycle operates in liver. Deficiency of muscle AMP deaminase is an apparently frequent disorder, which might affect approximately 2% of the general population. The observation that it can be found in clinically asymptomatic individuals suggests, paradoxically, that the ATP-regenerating function which has been attributed to the purine nucleotide cycle is not essential for muscle function. Further work should be aimed at identifying the conditions under which AMP deaminase deficiency becomes symptomatic. Adenylosuccinate lyase deficiency provokes psychomotor retardation, often accompanied by autistic features. Its clinical heterogeneity justifies systematic screening in patients with unexplained mental deficiency. Additional studies are required to determine the mechanisms whereby this enzyme defect results in psychomotor retardation.