An increased amount of N-acetylaspartic acid was found in urine and plasma of three patients, from two families, with the diagnosis of cerebral spongy degeneration (Canavan disease). Aspartoacylase was assayed in cultured skin fibroblasts from one patient of each family and a profound deficiency of this enzyme was found. Although the function of N-acetylaspartic acid is not understood, it is known to occur in high concentration in human brain. The finding of a defect in the metabolism of N-acetylaspartic acid causing progressive spongy degeneration of the brain may lead to a better understanding of the function of this amino acid derivative. The aspartoacylase assay affords a new tool for determining the diagnosis of Canavan disease. Since aspartoacylase activity was present in cultured amniotic cells and chorionic villi, it is likely that the assay for this enzyme can be used for the prenatal diagnosis of Canavan disease.

Canavan disease, an autosomal recessive disorder, is characterized biochemically by N-acetylaspartic aciduria and aspartoacylase (N-acyl-L-aspartate amidohydrolase; EC 3.5.1.15) deficiency. However, the role of aspartoacylase and N-acetylaspartic acid in brain metabolism is unknown. Aspartoacylase has been purified to apparent homogeneity with a specific activity of approximately 19,000-20,000 nmol of aspartate released/mg of protein. The native enzyme is a 58-kDa monomer. The purified aspartoacylase activity is enhanced by divalent cations, nonionic detergents, and dithiothreitol. Low levels of dithiothreitol or beta-mercaptoethanol are required for enzyme stability. Aspartoacylase has a Km of 8.5 x 10(-4) M and a Vmax of 43,000 nmol/min/mg of protein. Inhibition of aspartoacylase by glycyl-L-aspartate and amino derivatives of D-aspartic acid suggests that the carbon backbone of the substrate is primarily involved in its interaction with the active site and that a blocked amino group is essential for the catalytic activity of aspartoacylase. Biochemical and immunocytochemical studies revealed that aspartoacylase is localized to white matter, whereas the N-acetylaspartic acid concentration is threefold higher in gray matter than in white matter. Our studies so far indicate that aspartoacylase is conserved across species during evolution and suggest a significant role for aspartoacylase and N-acetylaspartic acid in normal brain biology.

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