Pyrimidine nucleoside phosphorylation in developing seeds and germinating seedlings of wheat

Uridine- and thymidine-phosphorylating enzymes were measured in developing and germinating seeds of Triticum aestivum v. Arthur and T. aestivum v. Lemhi.

Because crude extracts were to be used in the developmental study, characteristics of unpurified nucleoside phosphotransferase (NPTase) were examined. Crude NPTase was found to be relatively heat and cold stable with a broad pH optimum of between 7.5 and 8.0. It did not require sulfhydryl protection and could not be separated from an associated AMP phosphohydrolase activity by ammonium sulfate fractionation.

In the developmental study with two varieties of wheat, NPTase activity was found to be very low in all of the true seed tissues during seed maturation. Uridine-phosphorylating activity was due primarily to uridine kinase. Thymidine phosphorylation was very low in all tissues throughout seed maturation, with a brief appearance by thymidine kinase in the developing embryo. In germinating seeds, uridine-phosphorylating activity was present from earliest stages of germination but showed a decrease in activity followed by a recovery after 48 hours imbibition. Experiments using [α-²P]ATP and [γ-²P]ATP indicated that uridine kinase was present during early germination but had disappeared by 96 hours. Uridine phosphorylation at later stages of germination was accomplished by NTPase. Thymidine iii IV phosphorylation did not begin until after 36 hours of germination and was the result of NPTase activity.