The first compound characterized as a sugar conjugate was euxanthic acid, a major component of the dye Indian yellow.
Indian yellow was usually isolated from the urine of cows fed mango leaves. Erdmann observed that euxanthic acid could be hydrolyzed by acid to yield the aglycone.1 In 1855 Schmid reported that euxanthic acid was excreted by camels fed the fruits of Mangostana Mangifer.2 Baeyer characterized the aglycone of euxanthic acid as Euxanthone and the sugar moiety as an oxidized glucose derivative.3

	In the 1870s other sugar containing metabolites were isolated from urine. Von Mering and Musculus isolated urochloralic acid from the urine of humans given chloral hydrate4 while Jaffe, studying nitrotoluene metabolism in the dog found that o-nitrotoluene gave rise to o-nitrobenzyl alcohol excreted as a conjugate.5 Jaffe concluded that the sugar moiety was a carboxylic acid derivative of glucose
Schmiedeberg and Meyer followed up on early studies of camphor metabolism and isolated a camphor conjugate from the urine of dogs dosed with camphor.6 They isolated and characterized the sugar moiety as glucuronic acid.
Schmid had attributed the formation of euxanthic acid as occurring through "the vital process". It wasn't until 1953 that this process was elucidated through the discovery of uridine diphosphoglucuronic acid (UDPGA) by Dutton and Storey and their elaboration of the role of the co-factor in glucuronide formation.7
References: 1O.L.Erdmann, J.Prakt. Chem., 33:190 (1844); 37:385 (184), 2W. Schmid, Ann. Chem. Pharm., 93:83 (1855), 3A.Baeyer, Ann. Chem. Pharm., 155:257 (1870) 4J.Von Mering and F. Musculus, Ber. Dtsch.Chem. Ges., 8:662 (1875), 5M.Jaffe, Z.Physiol.Chem. 2:47 (1878), 6O.Schmiedeberg and H.Meyer, Z.Physiol.Chem. 3:422 (1879), 7Dutton, G.J.and Storey, I.D.E., Biochem J.53:37 (1953) Picture: J.Koch-Weser and P.J.Schechter, Life Sciences 22:1361-1372 (1978)