Intralysosomal formation and metabolic fate of N-acetylglucosamine 6-sulfate from keratan sulfate

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Abstract
The physiological relevance of the ability of β‐N‐acetylhexosaminidase A to liberate N‐acetylglucosamine 6‐sulfate from polymeric keratan sulfate was investigated. Upon intravenous injection into rats of[35S]sulfate‐labeled proteokeratan sulfate up to 25% of the radioactivity excreted with the urine were identified as N‐acetyl‐glucosamine 6‐sulfate. Within 24 h, however, excretion of inorganic sulfate rose at the expence of the sulfated monosaccharide. Upon incubation in vitro of liver lysosomes from rats treated with proteokeratan sulfate, inorganic sulfate and minor amounts of sulfated monosaccharide were found in the incubation fluid. Cultured rat peritoneal macrophages ingested proteokeratan sulfate with a clearance rate of 6–9 μg × h−1× mg cell protein−1 and degraded it rapidly. Inorganic sulfate but not N‐acetylglucosamine 6‐sulfate was delivered to the culture medium. During a chase period the amount of intracellular N‐acetylglucosamine 6‐sulfate fell, and a corresponding amount of sulfate could be found extracellularly. Significant amount of N‐acetylglucosamine 6‐sulfate were only found in the culture medium when the cells were challenged with zymosan. These results suggest that N‐acetylglucosamine 6‐sulfate is a physiological intermediate during the degradation of keratan sulfate, but is usually hydrolyzed intralysosomally by N‐acetylglucosamine‐6‐sulfate sulfatase. Genetic deficiency of the sulfatase in humans therefore results in excessive excretion of the sulfated amino sugar but not of keratan sulfate.