Current literature and clinical issue

Galactosemia: The uridine diphosphate glactose deficiency-uridine treatment controversy

Classic Galactosemia is an autosomal recessive disorder caused by the deficiency of galactose-1-phosphate uridylyltransferase (GALT), one of the key enzymes in the Leloir pathway of galactose metabolism. While the neonatal morbidity and mortality of the disease are now mostly prevented by newborn screening and galactose restriction, long-term outcome for older children and adults with this disorder remains unsatisfactory. The pathophysiology of Classic Galactosemia is complex, but there is convincing evidence that galactose-1-phosphate (gal-1P) accumulation is a major, if not the sole pathogenic factor. Galactokinase (GALK) inhibition will eliminate the accumulation of gal-1P from both dietary sources and endogenous production, and efforts toward identification of therapeutic small molecule GALK inhibitors are reviewed in detail. Experimental and computational high-throughput screenings of compound libraries to identify GALK inhibitors have been conducted, and subsequent studies aimed to characterize, prioritize, as well as to optimize the identified positives have been implemented to improve the potency of promising compounds. Although none of the identified GALK inhibitors inhibits glucokinase and hexokinase, some of them cross-inhibit other related enzymes in the GHMP small molecule kinase superfamily. While this finding may render the on-going hit-to-lead process more challenging, there is growing evidence that such cross-inhibition could also lead to advances in antimicrobial and anti-cancer therapies.

Galactose is an energy-providing nutrient and also a necessary basic substrate for the biosynthesis of many macromolecules in the body. Metabolic pathways for galactose are important not only for the provision of these pathways but also for the prevention of galactose and galactose metabolite accumulation. Problems with galactose metabolism can cause a variety of clinical manifestations in animals and humans. It has been found that the mammalian ovary is particularly susceptible to damage from the accumulation of galactose and galactose metabolites. The galactose metabolites Gal-1-P, galactitol, and UDPgal are all considered to be important in this toxicity and proposed mechanisms include interference with ovarian apoptosis and gonadotrophin signaling. This review addresses the most recent scientific findings regarding the possible mechanisms of galactose-induced ovarian toxicity and also the possible protective role of hormonal and antioxidant therapy. In addition, the available epidemiologic and scientific evidence linking galactose intake with risk of ovarian cancer is discussed.

A high performance liquid chromatography (HPLC) method has been developed for the measurement of uridine diphosphate galactose (UDPGal) and uridine diphosphate glucose (UDPGlc) in erythrocytes and cultured skin fibroblasts of normal controls and galactosemia patients. The method incorporates an internal standard, UDPxylose, and alkaline phosphatase for the removal of nucleoside phosphates in the regions of UDPGal and UDPGlc in the chromatographic system. UDPGal and UDPGlc were separated on dual Dionex Carbo Pac PA-1 anion exchange columns. Samples derived from galactosemia patients without any detectable erythrocyte galactose-l-phosphate uridyl transferase (GALT) activity (GG), had in comparison to the controls significantly lower levels of UDPGal in both erythrocytes and cultured skin fibroblasts (P = 0.0001), while galactosemia patients with detectable GALT activity (GV) had normal levels of UDPGal. GG patients on oral uridine therapy had normal levels of UDPGal. These findings are consistent with our previous reports using an enzymatic method. In terms of absolute values, the values for both UDPGal and UDPGlc in erythrocytes by HPLC analysis were lower than those reported with the enzymatic method; however, in cultured skin fibroblasts, the values were similar with the two methods.

By limiting galactosylation mechanisms, a cellular deficiency of the uridine sugar nucleotide, UDPgalactose, has been implicated as a cause of the long-term complications seen in patients with classic galactosemia despite dietary treatment. As a result, great interest has been generated in the accurate assessment of UDPgalactose, as well as UDPglucose, from which UDPgalactose may be derived by the function of a ubiquitous, active UDPgalactose-4-epimerase. Since several series of values for the concentration of these compounds in red blood cells (RBCs) of galactosemics have been flawed by the use of methods subsequently shown to be unsuitable, we have quantified erythrocyte UDPgalactose and UDPglucose levels by an accurate high-performance liquid chromatography (HPLC) assay in 116 normals, 76 galactosemics, and 39 patients with other metabolic disorders. These large groups have permitted the evaluation of age, diet, and genotype as influential factors in the steady-state RBC levels of the sugar nucleotides. The data show that age is an important determinant of RBC levels, with children younger than 10 years having higher values than individuals older than 10 years. Mean UDPgalactose levels in galactosemic children younger than 10 years and those older than 10 years were 30% and 18% lower, respectively, than levels in comparable normals. Although the mean differences were highly significant, there was considerable overlap of individual values. There was no difference in UDPglucose levels between galactosemics and normals. Diet plays a role in that erythrocytes of children on special metabolic diets low in protein and therefore also low in lactose show significantly lower levels of both UDPgalactose and UDPglucose than normal children, with the average UDPgalactose level similar to that in galactosemics. An analysis of UDPgalactose content with respect to genotype in galactosemics did not show any correlation with homozygosity of the Q188R allele. The average ratio of UDPglucose to UDPgalactose in RBCs of galactosemics under 10 years of age was 62% higher than the ratio in cells of comparable normals, whereas in older galactosemics there was a 29% increase. In 55% of galactosemics, the ratio was more than 2 SD above the mean of the normals, indicating that an increase in the erythrocyte UDPglucose to UDPgalactose ratio is a characteristic of the majority of galactosemic patients. The difference in the ratio, which deviates from the expectation for the equilibrium normally established by UDPgalactose-4-epimerase, implies a perturbation in epimerase function in galactosemic RBCs, the nature of which remains to be determined by examining the functional cellular pools and flux of hexoses not only in erythrocytes but in other model cell systems.