The Composition of Soluble Nucleotides in the Developing Wheat Grain

A method is described for the extraction and measurement of soluble nucleotides from wheat grain. Nucleotides were separated (80-90% recovery) by paper chromatography followed by electrophoresis. The nucleotides extracted were ADP-glucose, ATP, ADP, AMP, and NAD; UDP-glucose, UTP, UDP, and UMP with smaller quantities of cytidine nucleotides.In grain sampled at 20 days after anthesis, 70% of the UDP-glucose was present in the endosperm and the remainder in the testa and pericarp; 90% of the ADP-glucose was found in the endosperm. Of the four uridine nucleotides UDP-glucose was the most plentiful and the level rose from about 330 mmumoles per g fresh weight on the third day after flowering to 430 mmumoles/g on day 12 and then fell steadily to about 140 mmumoles/g just before complete ripening. Levels of 250 mmumoles and 200 mmumoles per g fresh weight were recorded for UTP and UDP on day 12. Thereafter the content of UTP fell relatively more rapidly than either UDP or UDP-glucose.ATP was the most abundant adenine nucleotide and from 7 days after anthesis to day 40 the quantity per g fresh weight fell from about 350 mmumoles to 100 mmumoles. The level of ADP-glucose rose to a maximum of 140 mmumoles/g between days 15 and 21 and then fell slightly towards maturity while ADP varied between 50 and 80 mmumoles/g. On day 20, coinciding with the maximum rate of starch synthesis in the endosperm, the concentration of ADP-glucose in this tissue was about 0.3 mm, and that of UDP-glucose 0.7 mm.The relationship of these results to the mechanism of transfer of hexose units from sucrose to starch is discussed.