Germany); Urea was obtained from Bio Lab (Jerusalem, Israel); Xanthydrol and
labeled 15N2-urea (98%) were purchased from Aldrich; 4-dimethylaminocinnamaldehyde
(p-DMAC, UN-1 reagent) was purchased from ACRO¯ S organics (NJ, USA);
Hydrochloric acid was obtained from Frutarom (Haifa, Israel).