Studies of mutant human adenylosuccinate lyase

We have studied the residual adenylosuccinate lyase activity in cultured lymphoblasts from a pair of siblings with infantile autism who have been previously shown to have a deficiency of the enzyme. The rates and distribution of de novo purine synthesis assessed by the utilization of radiolabeled formate by intact cells was nearly normal. We compared the steady-state kinetics and thermal stability of adenylosuccinate lyase in lysates from those cells and normal lymphoblasts. There is no evidence of inhibitory activity in the lysates of the mutant cells. The optimal pH was approximately 7.8 and was indistinguishable from that in control cells. The apparent Km in the two mutant cells lines (2.6 +/- 0.5 microM) is not significantly different from normal (3.3 +/- 0.8 microM), but the mutants displayed markedly decreased maximum steady-state velocities (6.7 +/- 1.1 compared to 13.8 +/- 0.9 nmol.mg-1.min-1). Residual activities in mutant cells show decreased thermal stability (t1/2 = 0.21 minutes at 60 degrees C as compared to 2.2 minutes), suggesting that there is a structural mutation of the adenylosuccinate lyase in the mutant cells.