Putrescine oxidase ([PO]; E.C. 1.4.3.4), which catalyzes the oxidative deamination of putrescine into gamma-aminobutyraldehyde, has been partially purified from Candida guilliermondii. Among the substrates tested, putrescine has the highest reaction rate, followed by spermidine and cadaverine. The K-m values for putrescine, spermidine, and cadaverine were 20, 200, and 1.1 mM, respectively. The optimum pH and the temperature for PO were 8.0 and 37 degrees C, respectively. Growth of Candida species on putrescine as the sole nitrogen source induced the synthesis of PO that converts putrescine into Delta(1)-pyrroline and gamma-aminobutyric acid. These two products were detected and identified from the culture medium. The enzyme was not activated by divalent cations. Among the species of Candida tested, the highest enzyme activity was found in cell-free extracts of C. guilliermondii. The pathway of putrescine degradation was identified by substrate analysis to be along the nonacetylated pathway in C. guilliermondii.