In the present study, irreversible thermoinactivation of holo-and apo-yeast alcohol dehydrogenase (YADH, EC 1.1.1.1) and protection by sugars (mannitol, sorbitol, sucrose and trehalose) were investigated at 50 degreesC and pH 7.8. The apo-protein was obtained by removing the structural zinc with the catalytic zinc remaining on the enzyme. Thiol group oxidation, aggregation and deamidation were examined. Hypochlorous acid and, cupric chloride were used in relation to thiol group oxidation. Zn2+ mobilization was measured spectrophotometrically using the metallochromic indicator 4-(2-pyridylazo)resorcinol (PAR). The presence of sugars provided significant protection against all the three deleterious processes. It is concluded that use of sugars may provide an effective approach for stabilization of holo- and apo-YADH via alteration, of protein microenvironment.