INHIBITION OF INOSINE 5′-PHOSPHATE DEHYDROGENASE FROM EHRLICH ASCITES-TUMOUR CELLS BY 6-THIOINOSINE 5′-PHOSPHATE

Abstract

6-Thioninosine 5[image] -phosphate was freed of its disulphide and of inosine 5[image]-phosphateby anion -exchange chromatography in the presence of mercaptoethanol. The disulphide was prepared from 6-thioinosine 5[image]-phosphate by oxidation with iodine. Inosine 5[image]-phosphate-nicotinamide-adenine dinucleotide oxidoreductase was prepared from Ehrlich ascites cells in a sufficiently pure state for kinetic studied of xanthosine 5[image]-phosphate formation. The validity of a spectrophotometric assay of the dehydrogenase was confirmed by chromatographic methods. At pH 8-0 and 25[degree] in the presence of 40 m[image]-potassium chloride and 0.6 m[image]-nicotinamide-adenine dinucleotide Km for inosine 5unspecified-phosphate was 1-4 x 10-5 [image]. 6-Thioinosine 5[image]-phosphate was shown to be a competitive inhibitor of the dehydrogenase. Ki under these conditions was 3.6x10-6 [image]. The significance of this inhibition in the biological action of 6-mercaptopurlne is discussed.