Analysis of UDP-galactose 4'-epimerase mutations associated with the intermediate form of type III galactosaemia

Type III galactosaemia is a hereditary disease caused by reduced activity in the Leloir pathway enzyme, UDP-galactose 4'-epimerase (GALE). Traditionally, the condition has been divided into two forms-a mild, or peripheral, form and a severe, or generalized, form. Recently it has become apparent that there are disease states which are intermediate between these two extremes. Three mutations associated with this intermediate form (S81R, T150M and P293L) were analysed for their kinetic and structural properties in vitro and their effects on galactose-sensitivity of Saccharomyces cerevisiae cells that were deleted for the yeast GALE homologue Gal10p. All three mutations result in impairment of the kinetic parameters (principally the turnover number, k (cat)) compared with the wild-type enzyme. However, the degree of impairment was mild compared with that seen with the mutation (V94M) associated with the generalized form of epimerase deficiency galactosaemia. None of the three mutations tested affected the ability of the protein to dimerize in solution or its susceptibility to limited proteolysis in vitro. Finally, in the yeast model, each of the mutated patient alleles was able to complement the galactose-sensitivity of gal10Delta cells as fully as was the wild-type human allele. Furthermore, there was no difference from control in metabolite profile following galactose exposure for any of these strains. Thus we conclude that the subtle biochemical and metabolic abnormalities detected in patients expressing these GALE alleles likely reflect, at least in part, the reduced enzymatic activity of the encoded GALE proteins.