The subcellular distribution and isoelectric focusing profile of neutral maltase were investigated in human skeletal muscle from controls and patients with acid maltase deficiency. After subcellular fractionation of normal muscle by differential centrifugation, 75% of the neutral maltase activity was soluble and 13% sedimented with a "microsomal" fraction; the relative specific activity was highest in this latter fraction. After isoelectric focusing (pH gradient 3.5 to 10) of a soluble fraction from control muscle, three peaks of activity were observed: peak 1 had exclusively neutral maltase activity; peak 2 had predominantly neutral maltase activity; and peak 3 had acid maltase activity predominating. The soluble fraction of muscle from a patient with infantile acid maltase deficiency showed no detectable activity at acid pH in any of the peaks and the neutral maltase peaks were unaltered. In muscle from a patient with late-onset acid maltase deficiency the focusing pattern for neutral maltase was similar to controls; the small amount of residual activity at acid pH was found in peak 3.
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