Biosynthesis of the Diguanosine Nucleotides. I. Purification and Properties of an Enzyme from Yolk Platelets of Brine Shrimp Embryos

Abstract

An enzyme that catalyzes the synthesis of P 1 P 4 -diguanosine 5′-tetraphosphate (Gp 4 G) has been isolated and purified from yolk platelets of encysted embryos of the brine shrimp, Artemia salina. The enzyme GTP:GTP guanylyltransferase (Gp 4 G synthetase) utilizes GTP as substrate, has a pH optimum of 5.9–6.0, a temperature optimum of 40–42 °C, and requires Mg 2+ and dithiothreitol for optimal activity. The synthesis of Gp 4 G is inhibited markedly by pyrophosphate, whereas orthophosphate has no effect on the reaction. In the presence of GDP the enzyme also catalyzes the synthesis of P 1 ,P 3 -diguanosine 5′-triphosphate (Gp 3 G), but the rate of synthesis is low compared with Gp 4 G synthesis and dependent upon other small molecular weight components of yolk platelets.