Uridine Diphosphate Glucose Pyrophosphorylase

The crystalline enzyme uridine diphosphate glucose pyrophosphorylase from calf liver has been shown to be polydisperse. The more rapidly sedimenting components that are present have been shown to be multimers of the basic pyrophosphorylase molecule. Evidence for multimers has come from (a) ultracentrifugal analysis, (b) protein and activity patterns by polyacrylamide gel electrophoresis and sucrose density gradient ultracentrifugal analysis, and (c) redistribution of the sucrose-separable material. Hydrodynamic studies of the principal component in the ultracentrifuge have yielded extrapolated values of 14.10 to 14.45 S for the sedimentation coefficient and 2.72 to 2.87 x 10-7 cm2 per sec for the diffusion coefficient. Combination of these values yielded a molecular weight of about 480,000. Dissociation in 6 m guanidine hydrochloride and 0.1 m mercaptoethanol has yielded subunits with a sedimentation coefficient of 2.14 S and a diffusion coefficient of 2.90 x 10-7 cm2 per sec. A combination of these values produced a molecular weight of 69,100. Estimates obtained by low speed sedimentation equilibrium were 67,500 for both the weight and z-average molecular weights. Electron micrographs have indicated the presence of tetramers. The results are discussed in relationship to an octomer model.