Metabolic fate of administered [13C]galactose in tissues of galactose-1-phosphate uridyl transferase deficient mice determined by nuclear magnetic resonance

Pathophysiology and targets for treatment in hereditary galactosemia: A systematic review of animal and cellular models

Since the first description of galactosemia in 1908 and despite decades of research, the pathophysiology is complex and not yet fully elucidated. Galactosemia is an inborn error of carbohydrate metabolism caused by deficient activity of any of the galactose metabolising enzymes. The current standard of care, a galactose-restricted diet, fails to prevent long-term complications. Studies in cellular and animal models in the past decades have led to an enormous progress and advancement of knowledge. Summarising current evidence in the pathophysiology underlying hereditary galactosemia may contribute to the identification of treatment targets for alternative therapies that may successfully prevent long-term complications. A systematic review of cellular and animal studies reporting on disease complications (clinical signs and/or biochemical findings) and/or treatment targets in hereditary galactosemia was performed. PubMed/MEDLINE, EMBASE, and Web of Science were searched, 46 original articles were included. Results revealed that Gal-1-P is not the sole pathophysiological agent responsible for the phenotype observed in galactosemia. Other currently described contributing factors include accumulation of galactose metabolites, uridine diphosphate (UDP)-hexose alterations and subsequent impaired glycosylation, endoplasmic reticulum (ER) stress, altered signalling pathways, and oxidative stress. galactokinase (GALK) inhibitors, UDP-glucose pyrophosphorylase (UGP) up-regulation, uridine supplementation, ER stress reducers, antioxidants and pharmacological chaperones have been studied, showing rescue of biochemical and/or clinical symptoms in galactosemia. Promising co-adjuvant therapies include antioxidant therapy and UGP up-regulation. This systematic review provides an overview of the scattered information resulting from animal and cellular studies performed in the past decades, summarising the complex pathophysiological mechanisms underlying hereditary galactosemia and providing insights on potential treatment targets.

The structural and molecular biology of type I galactosemia: Enzymology of galactose 1-phosphate uridylyltransferase

Reduced galactose 1-phosphate uridylyltransferase (GALT) activity is associated with the genetic disease type I galactosemia. This results in an increase in the cellular concentration of galactose 1-phosphate. The accumulation of this toxic metabolite, combined with aberrant glycoprotein and glycolipid biosynthesis, is likely to be the major factor in molecular pathology. The mechanism of GALT was established through classical enzymological methods to be a substituted enzyme in which the reaction with UDP-glucose results in the formation of a covalent, UMP-histidine adduct in the active site. The uridylated enzyme can then react with galactose 1-phosphate to form UDP-galactose. The structure of the enzyme from Escherichia coli reveals a homodimer containing one zinc (II) and one iron (II) ion per subunit. This enzymological and structural knowledge provides the basis for understanding the biochemistry of this critical step in the Leloir pathway. However, a high-resolution crystal structure of human GALT is required to assist greater understanding of the effects of disease-associated mutations.

Effects of galactose on direct and indirect pathway estimates of hepatic glycogen synthesis

Hepatic glycogen is formed by direct and indirect pathways whose activities reflect altered nutrition or disease. Direct/indirect pathway measurements often involve test meals where ∼ 10% of carbohydrate is galactose, but its effects on direct/indirect pathway estimates are unknown. Therefore, direct/indirect pathway contributions in 24-h fasted rats given 2g/kg 100% glucose (GLU, n=6) or 90% glucose-10% galactose (GLU+GAL, n=6) were measured by [U-(13)C]glucose dilution and by position-5/position-2 glycogen enrichment (H5/H2) from 2H(2)O. For GLU+GAL, galactose glycogenesis was independently measured with [1-(13)C]galactose. Glycogenesis was equivalent in both groups but for GLU+GAL, 23 ± 4% of glycogen was derived from galactose. [U-(13)C]glucose reported a 30 ± 3% direct pathway contribution to glycogenesis for GLU but only 20 ± 3% for GLU+GAL (p=0.012 vs. GLU). H5/H2 yielded identical direct pathway estimates (32 ± 3% GLU, 29 ± 6% GLU+GAL). Thus, galactose glycogenesis was undetected by H5/H2 while [U-(13)C]glucose reported a reduced direct/indirect pathway ratio. With [1-(13)C]galactose also present, correct glycogenic source contributions were obtained.

Evidence for function of UDP galactose pyrophosphorylase in mice with absent galactose-1-phosphate uridyltransferase

Mice with deletion of the galactose-1-phosphate uridyltransferase (GALT) gene were examined for their ability to form (13)C labeled hepatic UDP glucose from administered 1-(13)C galactose. NMR analysis of urinary acetaminophen glucuronide, which is derived from hepatic UDP glucose showed (13)C enrichment after concomitant administration of (13)C galactose and acetaminophen. The finding is consistent with the function of UDP galactose pyrophosphorylase as an alternate pathway of galactose metabolism.