Food Chemistry and Fish in Nutrition ( 2+1)

6.5.2.Dehydrogenases and carriers of reducing equivalents

a) Aerobic dehydrogenases

They catalyze the removal of hydrogen from a substrate and use either oxygen or artificial substances such a methylene blue as hydrogen acceptor. H₂O₂ is formed as a product. They are flavoprotein enzymes having FMN (flavin mononucleotide) or FAD (Flavin adenine dinucleotide) as prosthetic groups. Many of the flavoprotein enzymes contain a metal for which they are known as metal for which they are known as metalloflavoprotein enzymes.

(i) D-amino acid dehydrogenase (D-amino acid oxidase):  It is an FAD-linked enzyme..It is found particularly in liver and kidney.  It catalyzes the oxidative deamination of the unnatural (D-) forms of amino acids.

(ii) L-amino acid dehydrogenase (L-amino acid oxidase):  It is an FMN-linked enzyme. It is found in kidney. It catalyzes the oxidative deamination of naturally occurring L-amino acids.

(iii) Xanthine dehydrogenase (Xanthine oxidase): It occurs in milk and liver. In the liver, it converts purine bases to uric acid.  It contains FAD as the prosthetic group.  It is highly significant in the liver and kidneys of birds which excrete uric acid as the end product of purine metabolism and also of protein and amino acid catabolism. It is a metalloflavoprotein containing nonheme iron and molybdenum.  It also oxidizes all aldehydes.