TY - JOUR T1 - Adenosine Triphosphate—Guanosine 5'-Phosphate Phosphotransferase JF - Molecular Pharmacology JO - Mol Pharmacol SP - 128 LP - 138 VL - 8 IS - 2 AU - K. C. AGARWAL AU - R. E. PARKS, JR. Y1 - 1972/03/01 UR - http://molpharm.aspetjournals.org/content/8/2/128.abstract N2 - Four isozymes of guanylate kinase (ATP:GMP phosphotransferase, EC 2.7.4.8) with isoelectric points (pI) of 4.9, 5.1, 5.4, and 5.8 in human erythrocytes and two isozymes with isoelectric points of 5.2 and 5.5 in Sarcoma 180 ascites cells have been identified and separated by two electrophoretic techniques. The molecular weights of the isozymes range from 18,500 to 24,000. GMP, dGMP, and 8-azaguanosine 5'-phosphate serve as substrates for each of the guanylate kinase isozymes. Three of the erythrocytic guanylate kinase isozymes (pI 4.9, 5.1, and 5.4) react with IMP with a velocity of about 0.4%, while with the fourth isozyme (pI 5.8) the velocity is about 1% of that seen with GMP. The analogue nucleotide, 6-thioguanosine 5'-phosphate, behaves as a potent competitive inhibitor of all six guanylate kinase isozymes. The inhibition constant (Ki) values range from 7.5 x 10-5 to 2.1 x 10-4 M. ACKNOWLEDGMENTS The authors wish to thank Dr. Park Gerald and his colleagues at the Boston Children's Hospital for assistance in developing the agarose gel electrophoresis overlay technique, and the New England Enzyme Center at Tufts University for preparing partially purified guanylate kinase from pooled human blood. ER -