The Duarte allele (D) is a missense mutation (N314D) that produces a characteristic isoform and partial impairment of galactose-1-phosphate uridyltransferase (GALT) in human erythrocytes, fibroblasts, and transformed lymphoblasts. The position of this amino acid is distant, however, from presumptive catalytic site(s) as deduced from a three-dimensional model of crystallized Escherichia coli galT protein. To evaluate the mechanism(s) involved in the partial impairment of enzymatic activity, we compared the activity, abundance, biological stability, and mRNA of GALT in human lymphoblastoid cell lines cultured from individuals homozygous for wild-type (WT/WT) and Duarte alleles (N314D/N314D). No other nucleotide differences were present in their GALT genes. The apparent Vmax was reduced in N314D/N314D cells to 31 +/- 3.6 compared to WT/WT of 54 +/- 6.5 nmole UDP-galactose formed/g cell protein/hour. Both genotypes had similar apparent KMs for UDP-glucose of 0.142 +/- 0.057 mM and 0.133 +/- 0.056 mM. This reduced Vmax was associated with a reduced abundance of the 86kD GALT dimer as determined by Western blots and densitometry. Using RNase protection assays, this reduced GALT protein in the N314D/N314D cell lines was not associated with reduced abundance of GALT mRNA. Using cycloheximide (3-[2-(3,5-Dimethyl-2-oxocyclohexyl)-2-hydroxyethyl]glutarimide) inhibition of de novo protein synthesis, GALT enzyme activity, and its dimeric protein had a biological T1/2 of approximately 24 hours in N314D/N314D cell lines as compared to 50 hours for WT/WT lymphoblasts. Upon exposure to 50 degrees C for 15 minutes, N314D/ N314D lymphoblasts retained 45% of GALT activity, whereas controls retained 77% activity. Reduced activity and thermal sensitivity caused by the N314D mutation reverted to control values when a lysine was substituted for a glutamic acid at amino acid 203 in cis (E203K). In summary, N314D/N314D lymphoblasts have reduced GALT enzyme capacity, dimeric protein abundance, biological, and thermal stability. We conclude that the substitution of aspartate for asparagine at amino acid 314 in the human GALT protein reduces the biostability of the active enzyme in human lymphoblasts.