Special features of histidine metabolism with different kinds of animals

Natural amino acids being a part of food protein are known to be divided into essential and non-essential; essential ones if they are not present in food cannot be synthesized. There are two amino acids, threonine and histidine, which up to the present time have been a mystery for the biochemical science. However, it is universally recognized that threonine can be referred to as an essential amino acid as its absence from the food of all vertebrates results in the negative nitrogen balance. That is why specific features of threonine transformation with reference to different kinds of animals are not dealt with in the present paper. At the same time it was as far back as the thirties of the XX century that the fact of histidine being an essential amino acid for rats, mice, dogs and chickens but non-essential for men was proved. On the basis of the histidine essentiality thesis, a steady viewpoint that every living thing is not capabale of histidine synthesis was formed. But in this case the question arises: in what way has histidine become a non-essential amino acid for a healthy adult whereas for children and patients suffering from nephritic problems it has remained an essential amino acid? In view of the above different theories were put forward: 1) histidine reserves in haemoglobin and carnisone; the latter is enough with reference to adults but insufficient with reference to children; 2) histidine synthesis is caused by intestinal bacteria. But except mice whose muscles do not contain carnosine, the latter is a source of histidine for rats and dogs, histidine being an essential amino acid. It is only in the XXI century that the discovery of histidinol dehydrogenase enzyme  has allowed to answer the question. Histidine essentiality / non-essentiality as well as any other amino acid should be taken into account when making up rations for people and agricultural animal.