AbstractThe stability of immobilized preparations of xanthine oxidase and urate oxidase was studied, and optimized, because of the potential joint use of both enzymes in clinical analysis. Xanthine oxidase was immobilized on cellulose, Sepharose, hornblende, Enzacryl‐TIO, and Porous glass. The half‐lives of these preparations at 30°C ranged from 40 min to 5.0 hr. In this respect immobilized enzyme resembled soluble enzyme in dilute solution (0.11 mg/ml), when the half‐live was about 3.5 hr. More concentrated enzyme solution (1 mg/ml), had a half‐life of 64 hr, and was, therefore, considerably more stable than the untreated immobilized xanthine oxidase preparations. Inclusion of albumen in storage and assay buffer increased the half‐life of bound xanthine oxidase. So also did treatment with glutaraldehyde: in the case of xanthine oxidase bound to Enzacryl‐TIO such treatment increased the half‐life at 30°C from 3 hr to about 100 hr. Immobilized xanthine dehydrogenase was more stable than immobilized xanthine oxidase: the dehydrogenase lost no activity during continuous assay for 5 hr at 30°C. The stability of immobilized urate oxidase depended on the quantity of enzyme used and on the time of stirring during immobilization: thus a preparation was made (by stirring urate oxidase (48 mg/g support) with Enzacryl‐TIO for 24 hr) which lost no activity during 350 hr at 30°C.