Comparison of Enzymatic Reduction of Diphosphopyridine Nucleotide with Ammonia and Urea

In investigating the properties of the dehydrogenating system from ammonia to nitrate, it has been observed that DPN is reduced with urea also in the presence of an enzyme from fowl liver. The properties of the enzyme are very closely similar to those of ammonium dehydrogenase. However, some fundamental differences have been demonstrated by comparing these two enzymes, especially concerning reduction of TPN, substrate inhibition, formation of some nitrogenous metabolites and reoxidation of DPNH in the reaction mixture. The reaction was postulated to be a dehydrogenation of urea by an enzyme. This activity has also been demonstrated in silkworm and yeast.