Comparison of the characteristics of cholesterol oxidases from different sources was made by a new polarographic method for measurement of the oxygen-consumption rate. 2. A pH optimum of 7.0 was observed for cholesterol oxidases isolated from Nocardia and Brevibacterium, pH 5.0 for the enzyme from Schizophyllum and pH 7.5 for the enzyme from Streptomyces. 3. In the system used in the present study, Ca2+ and Mg2+ had no effect on these enzyme activities. On the other hand, the Schizophyllum enzyme was strongly inhibited by increasing concentrations of Cu2+, whereas the brevibacterium enzyme was slightly activated by them and Nocardia and Streptomyces enzymes were not affected. Hg2+ strongly inhibited the activities of enzymes the Schizophyllum enzyme. 4. Using serum as substrate, the cholesterol oxidases employed, except for the enzyme from Streptomyces, were not active without detergent in the reaction mixture. Effects of various detergents at various concentrations on the enzyme activities were studied. 5. Results of studies on the reaction of cholesterol oxidases on free cholesterol in low-and high-density lipoproteins were also compared.
