Studies on the metabolic origin of a glucose-containing tetrasaccharide in human urine

Urinary excretion of a glucose-containing tetrasaccharide, Glc alpha 1-6l alpha 1-4Glc alpha 1-4Glc, designated (Glc)4, is elevated in patients with glycogenoses and other conditions in which glycogen is stored in abnormally large amounts. We have studied the time course of appearance of (Glc)4 during digestion of glycogen by salivary alpha-amylase over a wide range of enzyme/substrate ratios. Aliquots taken from digests at various times of incubation were chromatographed on Bio-Gel P-2 and (Glc)4 was determined in column fractions by radioimmunoassay (RIA) or gas chromatography. (Glc)4 is released by treatment of glycogen with salivary alpha-amylase even at very low enzyme/substrate ratios (0.017 units/25 mg glycogen). At high enzyme/substrate ratios (55 units/25 mg glycogen) release of (Glc)4 is much more rapid but remains incomplete even after 82 h. The rate of release of (Glc)4 from glycogen is about 3 times faster for whole human serum than for an equivalent amount of purified human salivary alpha-amylase. Higher alpha-limit dextrins resistant to digestion by purified human salivary and pancreatic alpha-amylases are converted to (Glc)4 without accumulation of malto-oligosaccharides. These results suggest that both alpha-amylase and neutral alpha-1,4 glucosidase in human serum act upon glycogen and its branched amylolytic cleavage products to release (Glc)4. The amount of (Glc)4 normally excreted in urine per 24 h (0.1-2.5 mg) can be accounted for by intravascular degradation of approximately 30 mg of glycogen, an amount equivalent to 0.01% of total body glycogen stores.