1. A non-diffusible mucoid, showing a single peak in the ultracentrifuge, was isolated from human colloid breast carcinoma by treatment with trypsin and pepsin. The material contained threonine, leucine (isoleucine), valine, proline, glycine and glutamic acid in the approximate molar proportions 5:1:1:2:1:1. Smaller amounts of aspartic acid and serine were also found. For each 5 threonine residues, 6 N-acetylgalactosamine and 3-4 galactose residues were present. 2. The mucoid possessed reducing properties by the Park & Johnson (1949) procedure; these were attributable to the action of mild alkali, as employed in this procedure. Mild alkaline treatment by the Aminoff, Morgan & Watkins (1952) procedure gave rise to a diffusible N-acetylgalactosamine chromophore that gave an enhanced colour with Ehrlich's reagent. That galactosyl-(1-->3)-N-acetylgalactosamine residues were liberated was supported by periodate studies. 3. Alkaline liberation of hexosamine residues was accompanied by a specific destruction of threonine. After 40 min. at 100 degrees in 0.18 n-lithium hydroxide, both moieties had almost completely disappeared from the ninhydrin-positive components formed on subsequent acid hydrolysis. Glycine and alpha-oxobutyric acid were present in the acid hydrolysate, showing that both possible pathways of a beta-elimination reaction were involved. Formation of diffusible peptide on very mild alkaline treatment was attributable to the rupture of the original peptide core, necessitated by the second of these two pathways. 4. Hydroxamate formation on treatment with hydroxylamine showed the presence of carbohydrate linkage to glutamic acid or aspartic acid residues or both. This could account for the single N-acetylgalactosamine residue not linked to threonine. 5. The native mucin contained sialic acid, which was cleaved by the acid environment used in the treatment with pepsin. A statistical model of the mucin would require each prosthetic group to be linked, via N-acetylgalactosamine, to threonine, which would occupy every alternate position among the amino acids in the peptide core.
1. Glycosidic linkage of carbohydrate to the primary hydroxyl groups of threonine and serine has been established in human blood-group A and Lea substances, bovine submaxillary-gland mucin and human pseudomyxomatous mucin. 2. Treatment of these substances in 0 09N-lithium hydroxide at 100°for lhr. led to ,B-elimination at these glycosidic linkages with the resultant formation of a-oxobutyric acid and glycine from threonine linkages, and pyruvic acid from serine linkages. Though most of the threonine was destroyed in every ease, about onethird to one-half of the serine residues resisted alkaline cleavage. Such results, indicative of the presence of unbound serine residues, allow, in submaxillary mucin, for a close correlation between the remaining serine, threonine, glutamic acid and aspartic acid and the available sialyl-(2 -÷6)-N-acetylgalactosamine prosthetic groups. 3. The stoichiometry of the f-eliminations has been demonstrated for pseudomyxomatous mucin. The a-oxo acids were separated and determined as their quinoxalinol derivatives by thin-layer chromatography on silica gel. Reaction at the threonine centres favoured a-oxobutyric acid formation (70%, via the intermediary dehydropeptide) over the alternative pathway to glycine (30%). 4. 100% ofthe hexosamine was destroyed in submaxillary-gland mucin, 85% in pseudomyxomatous mucin and about 60% in the blood-group substances. In the latter cases, the glucosamine/galactosamine ratio was increased from about 4:1 to 8-10:1, suggesting a preferential destruction of galactosamine. Evidence was obtained, however, for a further destruction of hexosamine, in addition to that which could be theoretically attached to peptide at possible known binding sites. 5. The major part of the alkali-resistant hexosamine in the blood-group substances was nondiffusible and was accompanied by the constituent carbohydrates in similar molar proportions to the native materials.
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