How do urea and mercaptoethanol effect ribonuclease?

Mercaptoethanol is the chemical compound with the formula HOCH2CH2SH. ME, as it is commonly abbreviated, is used to reduce disulfide bonds and can act as a biological antioxidant by scavenging hydroxyl radicals (amongst others). It is widely used because the hydroxyl group confers solubility in water and lowers the volatility. Due to its diminished vapor pressure, its odor, while unpleasant, is less objectionable than related thiols.