Galactose-1-phosphate uridyl transferase in density-fractionated erythrocytes. Studies of normal and mutant enzymes

Genetic basis of galactosemia

Classic galactosemia is an inborn error of galactose metabolism and results from deficiency of the ubiquitously expressed enzyme galactose-1-phosphate uridyltransferase (GALT). Nine missense mutations, three splicing mutations, three GALT protein polymorphisms, and one silent nucleotide substitution have been identified to date. Most of the disease-causing mutations are rare among patients. The most common mutation, Q188R, has a frequency of only one-fourth in the patient population examined. Three classes of disease-causing mutations have been reported: CRM+ missense mutations (the most common class), CRM- missense mutations, and splicing mutations. Thus, galactosemia is heterogeneous at the molecular level, which is noteworthy in light of the well-documented clinical variability observed in this disorder. It has also been shown that eight of nine galactosemia missense mutations occur in evolutionarily well-conserved domains, suggesting that they affect functionally and/or structurally important residues. In contrast, all protein polymorphisms alter variable amino acids which presumably are not important for the enzyme's function.

Molecular analysis of 11 galactosemia patients

Galactosemia is a human inborn error of galactose metabolism due to deficiency of galactose-1-phosphate uridyl transferase. In this paper, I describe the molecular analysis of genomic DNA, mRNA and protein from 11 different galactosemic patients by Southern, Northern and Western blotting. The results of these experiments lead me to conclude that galactosemia is caused mostly by missense mutations. The unusual preponderance of missense mutations in galactosemia led me to investigate its cause. I demonstrate that all 9 patients I investigated have detectable residual enzyme activity (ranging from 0.7-6.9% of normal). This finding is of potential importance in addressing the long-term complications of galactosemia.

Characterization of a novel biochemical abnormality in galactosemia: deficiency of glycolipids containing galactose or N-acetylgalactosamine and accumulation of precursors in brain and lymphocytes

Classic galactosemia, an inborn error of human galactose metabolism, is characterized by a deficiency of the enzyme galactose-1-phosphate uridyltransferase (GALT). The current model for the pathophysiology of this disease ascribes most of its symptoms to the toxicity of intracellular galactose-1-phosphate (Gal-1-P), one of the substrates of GALT which accumulates in the untreated disease state. Recently, a reduction in the intracellular concentration of UDP-Gal (uridine diphosphogalactose), one of the products of GALT, has been described in treated galactosemic patients. We investigated whether galactosemic patients might also have reduced amounts of those macromolecules that depend on UDP-Gal for their biosynthesis. We report a reduction in glycolipids that contain either galactose or its derivative N-acetylgalactosamine and an accumulation of the precursors to these compounds in the brain of a neonate with galactosemia. We also found an imbalance in glycolipids in galactosemic lymphoblasts. This novel biochemical abnormality observed in galactosemic patients is not addressed by dietary galactose-restriction therapy and could explain some of the chronic neurologic and other complications of galactosemia.

Regulation of galactose metabolism: implications for therapy

In view of evidence that dietary therapy of galactose-1-phosphate uridyltransferase deficiency has failed to prevent complications of the disorder, there is a need for new strategies in treatment. The enhancement of residual enzyme activity in tissues of galactosaemic patients should provide such an approach. This possibility is derived from knowledge of the regulation of transferase activity in normal animal tissues. The pertinent observations summarized herein are: (1) that hepatic transferase activity is modulated by various cellular metabolites, uridine nucleotides being of particular significance; (2) that transferase activity in the young rat liver is subject to developmental programming with a several-fold increase after birth; (3) that transferase activity in pregnant rat liver is significantly increased which may be related to hormonal effects of progesterone; and (4) that pharmacological doses of folic acid may increase transferase activity. The basis of such regulation can give insight into sufficient augmentation of the residual activity to increase galactose utilization and thereby better the long-term outcome.