The hexoses so obtained are not necessarily identical: thus cane sugar yields d-glucose and d-fructose (invert sugar); milk sugar and melibiose give d-glucose and d-galactose, whilst maltose yields only glucose.
Of especial note is the transformation of maltose by maltase into glucose, and of cane sugar by invertase into a mixture of glucose and fructose (invert sugar); other instances are: lactose by lactase into galactose and glucose; trehalose by trehalase into glucose; melibiose by melibiase into galactose and glucose; and of melizitose by melizitase into touranose and glucose, touranose yielding glucose also when acted upon by the enzyme touranase.
Other forms are: d- and l-gulose, prepared from the lactones of the corresponding gulonic acids, which are obtained from d- and /-glucose by oxidation and inversion; d- and l-idose, obtained by inverting with pyridine d- and l-gulonic acids, and reducing the resulting idionic acids; d- and l-galactose, the first being obtained by hydrolysing milk sugar with dilute sulphuric acid, and the second by fermenting inactive galactose (from the reduction of the lactone of d, l-galactonic acid) with yeast; and d- and l-talose obtained by inverting the galactonic acids by pyridine into d- and l-talonic acids and reduction.
Of the ketoses, we notice d-sorbose, found in the berries of mountain-ash, and d-tagatose, obtained by Lobry de Bruyn and van Ekenstein on treating galactose with dilute alkalis, talose and l-sorbose being formed at the same time.
The remaining aldohexoses discovered by Fischer are derived from d-galactose from milk-sugar.
On reducing the lactone prepared from the inactive acid an inactive galactose is obtained from which l-galactose may be separated by fermentation.
Lastly, when d-galactonic acid is heated with pyridine, it is converted into talonic acid, which is reducible to talose, an isomeride bearing to galactose the same relation that mannose bears to glucose.
It can be shown that d-galactose is CH 2 (OH) + - + - COH, and hence d-talose is CH 2 (OH) + - + + COH.
Lactose is thus found to be glucosido-galactose and melibiose a galactosido-glucose.
Cooh, is obtained by the oxidation of milk, sugar, dulcite, galactose, quercite and most varieties of gum by nitric acid.
Boletus edulis, in the Oriental Trehala and in ergot of rye; melibiose, C12H22011, formed, with fructose, on hydrolysing the trisaccharose melitose (or raffinose), C18H32016.5H20, which occurs in Australian manna and in the molasses of sugar manufacture; touranose, C12H22011, formed with d-glucose and galactose on hydrolysing another trisaccharose, melizitose, C,8H32016 2H20, which occurs in Pinus larix and in Persian manna; and agavose, C12H22011, found in the stalks of Agave americana.