The radiation-induced inactivation of external yeast invertase in dilute aqueous solution

The inactivation of external yeast invertase by irradiation in dilute aqueous solution has been investigated. The contributions of the individual radical species from water radiolysis to inactivation and amino acid degradation were estimated from the results of experiments in which solutions were saturated with nitrogen, nitrous oxide or oxygen, and on addition of hydroxyl radical scavengers. Under conditions where inactivation by hydroxyl radicals predominates, the rate of inactivation increased with increasing dose, indicating that in the initial stages of the radiolysis the mannose-rich oligosaccharide chains of the glycoprotein protect the polypeptide chain from radical attack. Amino acid analysis of the irradiated external invertase showed that there was significant destruction of tyrosine, phenylalanine, methionine and histidine residues. Destruction of methionine and histidine residues may be responsible for the free radical-induced inactivation of this enzyme.