SummaryPurified human liver arylsulfatase A on polyacrylamide gel electrophoresis at pH 4.0 is separated into two protein forms with enzymatic activity and two distinct inactive subunits. All of these components were immunologically distinguishable using different antisera preparations. In late infantile metachromatic leukodystrophy, only one of the two inactive subunits was immunologically detected, whereas in the juvenile form of metachromatic leukodystrophy, both inactive subunits were antigenically present.SpeculationArylsulfatase A is a dimeric enzyme composed of two distinct subunits. In late infantile metachromatic leukodystrophy, the molecular defect results in the absence of one of the subunits and therefore no active dimer is formed. In contrast, in the juvenile form of the disease, the mutation does not cause the absence of either of the subunits, as they are immunologically detectable, but does cause the formation of an unstable dimer.