Abstract: The present invention provides a process for the purification or enrichment of biologically active proteins by the addition of a selective precipitation agent, wherein an aqueous solution containing the desired protein is mixed at a pH value below its isoelectric point with such an amount of an arylsulphonic acid azo dyestuff with two sulphonate groups, which are separated from one another by 8 to 10 double bonds standing in resonance relationship, among which there is present at least one N.dbd.N double bond, that at least the greater part of the biological activity of the desired active protein is present in the precipitate, the precipitate is separated off and the active protein is again brought into solution at a pH value above its isoelectric point.

Abstract: Storage-stable aqueous cholesterol oxidase compositions comprising cholestrol oxidase, a buffer and from 2 to 3.5 M sodium or potassium chloride as a stabilizing agent, are significantly superior to conventional compositions with respect to storage stability and fungibility.