Effect of bound carbohydrate on the action of trypsin on lupine seed glycoproteins

Proteolysis by trypsin of the storage proteins of lupin seed is less extensive than that of bovine serum albumin or of casein, and the decrease is related to the amount of carbohydrate bound to the protein. Purified conglutin γ and vicilins 4 and 6, the three seed globulins having the highest sugar content, were first incubated with jack bean exoglycosidases, which removed 75, 81, and 80% of the carbohydrate, respectively. This increased the subsequent action of trypsin in liberating small trichloroacetic acid (TCA) soluble peptides and/or in forming large TCA-insoluble fragments evidenced only by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The increase was related to the amount of sugar detached. The effects observed may depend on the removal of a steric hindrance due to the bound carbohydrate and on other additional mechanisms that involve the relationship between the amount of bound sugar and extent of hydrophobic areas on the protein surface.