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.