Purification and Properties of Guanosine Diphosphate Hexose Pyrophosphorylase from Mammalian Tissues

The relative levels of guanosine diphosphate hexose and uridine diphosphate glucose pyrophosphorylases have been measured in extracts of various tissues of the rat and in the lactating glands from several mammals. Calf liver and rat mammary gland are good sources of both pyrophosphorylases. GDP-hexose pyrophosphorylase has been purified about 500-fold from mammalian tissues. The preparation purified 500-fold is not specific for either the nucleoside or the hexose component of the nucleoside diphosphate hexose. In decreasing order of activity, guanosine, inosine, and adenosine diphosphate hexoses are substrates with either glucose or mannose as the sugar. GDP-mannose and mannose 1-phosphate inhibit the formation and cleavage of GDP-glucose, but the rate of pyrophosphorolysis of GDP-glucose alone is faster than that of GDP-mannose alone. Thus, the enzyme has a higher affinity for GDP-mannose, but apparently a higher turnover for GDP-glucose. With GDP-glucose as substrate, the equilibrium constant, optimum pH, metal requirement, and K m have been determined for the purified GDP-hexose pyrophosphorylase.