CYTOSOLIC BIOSYNTHESIS OF GTP AND ATP IN NORMAL RAT PANCREATIC-ISLETS

GTP and ATP are necessary for glucose-induced insulin secretion; howev er, the biosynthetic pathways of purine nucleotides have not been stud ied in pancreatic islets. The present work examines the cytosolic path ways of purine nucleotide synthesis using intact rat islets cultured o vernight in RPMI 1640 medium containing either [C-14]glycine (to label the de novo pathway) or [H-3]hypoxanthine (to mark the salvage pathwa y), with or without mycophenolic acid or L-alanosine (selective inhibi tors of cytosolic GTP and ATP synthesis, respectively). Addition of my cophenolic acid decreased total GTP content (mass) by 73-81%; although the incorporation of labeled hypoxanthine into GTP also fell by 87%, the incorporation of glycine did not change. Similarly, L-alanosine de creased ATP mass by 26-33% in the presence of either label; whereas th e incorporation of hypoxanthine into ATP fell 59%, the incorporation o f glycine was again not significantly decreased. Thus, both the de nov o and salvage purine nucleotide biosynthetic pathways are present in r at islets; however, the salvage pathway appears to be quantitatively t he more important source of nucleotides. This conclusion was supported by additional studies of the effects on nucleotide content and insuli n secretion of various site-specific inhibitors of purine synthesis. T hese findings have potential relevance to the processes of mitogenesis , cell proliferation and differentiation of islet cells, as well as fo r the control of insulin secretion.