The use of 13C-labelling to prove that creatinine is oxidized by mammals into creatol and 5-hydroxy-1-methylhydantoin

Creatinine 1 is not an end-metabolite but a precursor for three heterocycles, 2-amino-5-hydroxy-1-methyl-4(5H)-imidazolone (creatol)2,1-methylimidazolidine-2,4-dione 4 and its 5-hydroxy derivative 5; this is now proven directly by administration of [Me-13C]creatinine 1, to a uraemic rat and subsequent isolation of the corresponding labelled metabolites 2, 4 and 5 from its urine.