SEQUENCE DIFFERENCES BETWEEN HUMAN MUSCLE AND LIVER CDNAS FOR UDPGLUCOSE PYROPHOSPHORYLASE AND KINETIC-PROPERTIES OF THE RECOMBINANT ENZYMES EXPRESSED IN ESCHERICHIA-COLI

UDP-Glc pyrophosphorylase (EC 2.7.7.9) catalyses the interconversion o f MgUTP plus Glc1P and UDP-Glc plus MgPPi. Complementation of an Esche richia coli strain lacking this activity has allowed isolation of cDNA encoding this enzyme from a human muscle library. Two forms were iden tified and the nucleotide sequence of each was determined; they were f ound to differ only in the 5' region and we suggest that these arise f rom the use of a different first exon in the two transcripts. These nu cleotide sequences are different from that of the cDNA which was isola ted previously from a human liver library [Peng, H.-L. & Chang, H.-Y. (1993) FEES Lett. 329, 153-158] and it is proposed that these liver an d muscle farms are derived from different genes. The cDNA for muscle f orm I, muscle form II, the liver form, and the liver form fused to par t of the lacZ gene were expressed in Escherichia coli and the kinetic properties of each enzyme were characterised, Muscle form I and the La cZ/liver fusion enzyme exhibit Michaelis-Menten kinetics towards all s ubstrates while muscle form II has a sigmoidal dependence of rate upon the concentration of MgPPi. The liver form shows Michaelis-Menten kin etics towards MgUTP. For the remaining three substrates, complex kinet ics were observed involving a combination of sigmoidicity at low subst rate concentration and partial inhibition at high substrate concentrat ion.