A deficiency of acid maltase but not neutral maltase was observed in cultured fibroblasts obtained from six adults with glycogen-storage myopathy associated with acid maltase deficiency (AMD). Abnormal sequestration of glycogen in membrane-bound sacs, without admixture of cytoplasmic degradation products, was found in cells of three of the six AMD cultures, but the glycogen content was normal in all six cultures. Neither a partial enzyme deficiency nor a selective absence of the enzyme from affected tissues can explain the age-dependent variants of AMD. The diagnosis of AMD in adults can be made on the basis of skin as well as muscle-biopsy specimens. Prenatal diagnosis of the adult form also should be feasible.
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