Stability of the Insoluble Form of Uridine Kinase Coupled to Zn2⊕or Pb2⊕Ions

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Abstract
Partially purified calf brain uridine kinase [EC 2.7.1.48] precipitated by bivalent metal cations was compared with the soluble enzyme fraction regarding its stability in the presence of inactivating factors. The freeze-dried preparations of uridine kinase precipitated by Pb2+ or Zn2+, although enzymatically highly active, are insoluble in aqueous solutions. Th activity of metal-insolubilized enzymes disappears during their preincubation in acidic media or in the presence of Ag2+. Trypsin, chymotrypsin and cathepsin B1 caused decreases in enzyme activity. Fractions which were precipitated by metal ions and freeze-dried were stable at high temperatures; the activity of soluble uridine kinase is completely lost. Both unheated metal-ion precipitated uridine kinase preparations and those heated at 100.degree. C are equally sensitive to the feedback inhibition by CTP.