Sugars are not only energy sources but also building blocks for cell-surface glycans and storage polysaccharides. To build these structures, monosaccharides must be “activated” before being transferred. Leloir proved that the activation step is accomplished by coupling a sugar to a nucleoside diphosphate (UDP, GDP, etc.), forming a sugar nucleotide. This clarified how glycosyltransferase enzymes receive a sugar from the nucleotide and add it, train-car style, to growing polysaccharide chains. Clinically, the work explained disorders such as galactose metabolism defects and improved diagnostics. It also laid foundations for vaccine design and bio-based materials.

In the late 1940s, Leloir isolated UDP-glucose from liver extracts and identified it as an activated sugar containing a high-energy phosphate bond. He subsequently discovered other sugar nucleotides such as UDP-galactose and GDP-mannose and demonstrated that transferase enzymes use them as substrates. These findings established the stepwise enzymatic model of glycosylation, in which each transferase selects a specific sugar nucleotide and extends carbohydrate chains. The work also explained the pathogenesis of classical galactosaemia caused by galactose-1-phosphate uridylyltransferase deficiency, now mapped as the “Leloir pathway.” Today, sugar nucleotides are indispensable reagents in glycotechnology, carbohydrate-based therapeutics, and advanced mass-spectrometric glycan analysis, underpinning both life science research and the bio-industry.

Leloir employed P-32-labelled ATP in liver soluble fractions to demonstrate the enzymatic formation of UDP-glucose from glucose-1-phosphate and UTP. He quantitatively confirmed, using radiochemical assays, the reversible interconversion of UDP-glucose and UDP-galactose via UDP-glucose 4-epimerase, proposing nucleotide-level regulation of galactose metabolism. His group partially purified multiple glycosyltransferases, analysed donor-substrate specificity and inferred active-site stereochemistry. The work indicated that the shared ΔG of sugar nucleotide phosphoanhydride hydrolysis drives disaccharide phosphate chemistry and that sequential oligosaccharide synthesis depends on ordered enzyme assemblies. Present-day UDP-sugar analogue probes and sugar nucleotide synthase engineering trace their methodological lineage directly to Leloir’s quantitative enzymology and intermediate-trapping strategies.