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Catalase activity in blood, liver, and kidney of a mutant strain Cs b has been found to be decreased as compared to the level in normal mice. However, the extent of the reduction largely depends on the conditions used for activity determination, in particular, temperature and duration of the incubation period. In liver, this effect is most pronounced, the observed activity in mutants varying between 21 and 85% of the normal level This dependence on the assay conditions is mainly due to the unusual heat lability of the variant enzyme, which undergoes rapid inactivation when incubated at 37 C.

The immunological properties of the erythrocyte catalase of mice-normal (wild type) strain, one lacking catalase (acatalasemic), and four with only slight catalase activity (hypocatalasemic strains)-have been investigated. Agardiffusion tests and antigen titration of red-cell lysates against rabbit antiserum to catalase from normal mouse blood showed that immunologically identical catalase protein was present in large amounts in the acatalasemic as well as in the hypocatalasemic mutant strains. Despite lack of catalatic activity, the erythrocytes lacking catalase as well as those with only a little catalase contain catalase protein that has been modified at the site of enzyme activity, although the antigenic determinants are identical with those of normal catalase protein. This mutation is purely structural, being characterized by modification of the enzyme active site but not of the antigenic site.

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