Radiation inactivation of the glycoprotein, invertase

The radiation target size for invertase activity has been determined for the Saccharomyces cerevisiae glycoprotein which contains 50% carbohydrate. Identical inactivation curves were observed for the native enzyme as well as samples depleted of carbohydrate by incubation with Endo-beta-N-acetylglucosaminidase H. The functional unit of 120,000 daltons was unaltered by the per cent of oligosaccharide cleaved by the enzyme, or by the presence or absence of the released sugars. The irradiated samples showed no change in hexose content even after radiation exposures which grossly destroyed enzymatic activity. Reducing sugars appeared in the irradiated samples, indicating radiation damage to the oligosaccharides. These results unequivocally identify the enzymatically functional portion of the invertase molecule as the polypeptide homodimer, independent of the covalently-bound carbohydrates, and indicate that transfer of radiation energy from protein to oligosaccharide or vice versa is inefficient.