Biochemistry

CHAPTER 9. NUCLEIC ACID METABOLISM 9.1 NUCLEIC ACID METABOLISM IN THE ORGANISM Nucleic acids are part of cells in the form of complex proteins - nucleoproteins, which enter the body with food. They break down into simple proteins and nucleic acids in the stomach, under the action of hydrochloric acid and pepsin. Hydrochloric acid breaks the bonds between nucleic acid and proteins (histamines and protamines), and the rest, more stable bonds at the pH of gastric juice (1.5-2.5), breaks down mainly pepsin. In the small intestine under the action of trypsin, chymotrypsin and peptidases, proteins are broken down to amino acids, which are absorbed by the mucous membrane and enter the bloodstream. Nucleic acids under the influence of pancreas enzymes and small intestine - nucleases break down to mononucleotides. Mononucleotides under the action of intestinal juice enzymes, lose phosphoric acid and turn into nucleosides. Further, nucleosides under the action of nucleosidases breakdown into components - nitrogenous bases and pentoses. However, small intestine nucleosidases are not active enough and cannot ensure the cleavage of the entire mass of nucleotides. In this regard, mononucleotides and nucleosides, which undergo specific transformations in tissues, are absorbed into the blood mainly. Tissue nucleic acids under the action of nucleases are cleaved to mononucleotides, and then to nitrogenous bases, pentose and phosphoric acid. Phosphoric acid is involved in various phosphorylation processes occurring in the body, as well as in buffer systems, the synthesis of phospholipids, phosphoproteins, ATP and other compounds. It is excreted mainly in the form of acidic sodium salts with urine. Pentoses can be oxidized to CO 2 and H 2 O or used for the synthesis of glucose, nucleotides. Uric acid formation Purine bases in tissues undergo deamination under the influence of deaminases. As a result, uric acid is formed from adenine and guanine through a series of intermediate steps; with deamination, adenine is first converted to hypoxanthine (Fig. 9.1). 179