1. Adenosine kinase was measured in dialysed extracts from Ehrlich ascites-tumour cells by a chromatographic procedure. 2. In the absence of added Mg2+ the Km values for ATP and adenosine were 0·22mm and 2·8μm respectively. 3. The maximum velocity of adenosine kinase with free ATP was about three times that with the Mg2+–ATP complex. Free Mg2+ was a non-competitive inhibitor of the reaction. A small amount of added Mg2+, Mn2+ or Ca2+ was required for maximum adenosine kinase activity after cation bound to the enzyme had been released by treatment with p-chloromercuribenzoate and then removed by dialysis. 4. GTP, ITP, deoxy-ATP, deoxy-GTP, CTP, xanthosine triphosphate, UTP and thymidine triphosphate could partially or completely replace ATP as a phosphate donor. 5. The reaction of ATP with adenosine kinase was competitively inhibited by AMP, GMP, IMP, ADP, deoxy-ADP and IDP (Ki 0·2, 1·1, 5·9, 1·2, 0·5 and 0·78mm respectively). Enzymic activity was markedly affected by the relative concentrations of AMP, ADP and ATP in assay mixtures. 6. The results are discussed in terms of possible mechanisms regulating the rate of adenosine kinase in vivo.
Initiation of Protein Synthesis in Ehrlich Ascites Tumour Cells. Evidence for Physiological Variation in the Association of Methionyl-tRNAf with Native 40-S Ribosomal Subunits in vivo
