The present invention relates to new substituted xanthines of general formula ##STR00001## wherein R.sup.1, R.sup.2 and X are defined as in the claims, the tautomers, the enantiomers, the diastereomers, the mixtures thereof and the salts thereof, which have valuable pharmacological properties, particularly an inhibiting effect on the activity of the enzyme dipeptidylpeptidase-IV (DPP-IV).