Dual Role of Hexose‐1‐phosphate Uridylyltransferase in Galactosamine Metabolism

• Published in: European journal of biochemistry Vol. 128; no. 1; pp. 163 - 168
• Main Authors: WECKBECKER, Gisbert, KEPPLER, Dietrich O. R.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.11.1982
• Subjects: Animals; Cattle; Chemical Phenomena; Chemistry; Chromatography, High Pressure Liquid; Enzyme Activation; Galactosamine - metabolism; In Vitro Techniques; kinetics; liver; Liver - enzymology; Nucleotidyltransferases - physiology; Rats; Substrate Specificity; UDPglucose-hexose-1-phosphate uridylyltransferase; UDPglucose-Hexose-1-Phosphate Uridylyltransferase - physiology; Uridine Diphosphate - metabolism
• ISSN: 0014-2956; 1432-1033
• DOI: 10.1111/j.1432-1033.1982.tb06947.x

The metabolism of 2‐amino‐2‐deoxy‐d‐galactose (galactosamine) in liver involves the formation of UDP‐galactosamine, UDP‐glucosamine, UDP‐N‐acetylglucosamine and UDP‐N‐acetylgalactosamine; however, the conversion of the UDP‐hexosamines to the UDP‐N‐acetylhexosamines requires explanation. We have therefore performed kinetic studies on hexose‐1‐phosphate uridylyltransferase from liver at pH 7.4 and 37°C in order to analyse whether this enzyme catalyzes the synthesis of glucosamine 1‐phosphate as an obligatory intermediate in the formation of UDP‐N‐acetylglucosamine. Glucosamine 1‐phosphate production was demonstrated using enzymatically synthesized UDP‐glucosamine that substituted for UDP‐glucose in the presence of galactose 1‐phosphate or galactosamine 1‐phosphate. The separation of UDP‐glucosamine and UDP‐galactosamine, a prerequisite in these studies, was achieved by anion‐exchange high‐performance liquid chromatography based on the preferential borate complex formation of UDP‐galactosamine. At fixed UDP‐glucosamine concentrations (0.4 and 0.5 mmol/l) the apparent Km values for galactosamine 1‐phosphate and galactose 1‐phosphate were 0.53 and 0.02 mmol/l, respectively. Both Km values increased about 20‐fold when UDP‐glucose served as the constant cosubstrate. Apparent Km values for UDP‐glucosamine at fixed levels of galactosamine 1‐phosphate (2 mmol/l) and galactose 1‐phosphate (0.08 mmol/l) were 0.25 and 0.31 mmol/l, respectively. These Km values are in the range of concentrations determined in vivo earlier in rat liver after a hepatotoxic dose of galactosamine. The estimated velocity of hexose‐1‐phosphate uridylyltransferase in liver in the presence of the galactosamine‐derived substrates can account for the observed increment of UDP‐N‐acetylhexosamines. Uridylyltransferase fulfills a dual role in galactosamine metabolism by the activation of galactosamine 1‐phosphate to UDP‐galactosamine and, after epimerization to UDP‐glucosamine, by the formation of glucosamine 1‐phosphate as the precursor of UDP‐N‐acetylhexosamines.