Isolation and crystallization of ubiquitin from mature erythrocytes

A relatively simple and reproducible procedure for the isolation of the active form of ubiquitin has been developed. The amount of this protein in mature erythrocytes appears to be about 0.15 mg. per gm. of hemoglobin and 60% of the material may be recovered. The protein can be crystallized from ammonium sulfate in the presence of mercaptan.