We have prepared a series of oligosaccharides to assess the substrate specificity of exo sulfatase activity in cultured human skin fibroblasts toward N-acetylglucosamine-6-sulfate residues present in keratan sulfate (KS) and heparan sulfate (HS). Non-reducing end alpha-GlcNAc-6-SO4 residues (derived from HS) were desulfated by a specific sulfatase that when deficient leads to the accumulation of HS and the expression of mucopolysaccharidosis type IIID (Sanfilippo D). Under the in vitro conditions studied there are two pathways for the degradation of oligosaccharides containing non-reducing end beta-GlcNAc-6-SO4 residues (derived from KS). In one pathway beta-N-acetylglucosaminidase produces GlcNAc-6-SO4 which is then desulfated. In the other pathway the beta-GlcNAc-6-SO4 residue is desulfated and then cleaved by the action of an beta-N-acetylglucosaminidase activity. There was no detectable beta-N-acetylglucosaminidase activity in fibroblasts from a Tay-Sachs patient to produce GlcNAc-6-SO4 from beta-GlcNAc-6-SO4 residues in KS of oligosaccharides. There was approximately 10% of this normal beta-N-acetylglucosaminidase activity in fibroblasts from a Sandhoff patient, suggesting the A and S forms may be involved in this reaction. Desulfation of GlcNAc-6-SO4 residues in KS, HS and the monosaccharide GlcNAc-6-SO4 was considerably reduced or not detected in fibroblasts from a Sanfilippo D patient. As KS was not detected in the urine of a Sanfilippo D patient we propose that KS degradation in these patients proceeds by the action of a beta-N-acetylglucosaminidase activity to produce GlcNAc-6-SO4 which is not further degraded.(ABSTRACT TRUNCATED AT 250 WORDS)