Residual nitrogen and its components
An integrative indicator that allows assessing the severity of disorders in the final stages of protein catabolism is the level of non-protein (residual) nitrogen in blood serum. Normally, this indicator is strictly regulated by the body and ranges from 14.3 to 28.5 mmol/L.
The blood non-protein nitrogen pool includes the following main components:
- Urea
- Free amino acids
- Uric acid
- Creatinine
- Creatine
- Ammonia
Among all the listed substances, ammonia has the most pronounced pathogenic (cytotoxic) effect. The mechanism of its damaging effect on cells is due to the ability of ammonia to easily and unhinderedly penetrate the lipid bilayers of cell membranes. Once inside the cell, it directly damages vital enzymes, various cytosolic components, and the membrane structures themselves.
The role of urea in pathogenesis
Urea itself is a substance that does not possess a direct toxic effect on the body's tissues. Under physiological conditions, its synthesis occurs predominantly in the liver via the ornithine cycle (urea cycle), and only to a minor extent in other tissues. The excretion of generated urea is carried out mainly through the kidneys, and partially through sweat glands.
However, pathogenesis changes dramatically with the development of renal failure:
- Due to a sharp decrease in the filtration function of the kidneys, a large amount of urea begins to be eliminated via a compensatory pathway — through the intestines.
- In the intestinal lumen, urea undergoes active catabolism under the influence of local bacterial flora enzymes.
- As a result of this bacterial cleavage, highly toxic ammonia is formed.
Thus, it is this secondary ammonia formed in the intestines that acts as a crucially significant link in the pathogenesis of renal failure and the development of uremia.
Creatine, creatinine, and hyperazotemia
Creatine and creatinine also serve as important markers of protein metabolism status. The causes of changes in their blood and urine concentrations are quite diverse. The main factors include:
- Renal impairment
- Muscle tissue hypotrophy
- Inflammatory and autoimmune muscle lesions (myositis and myasthenia)
- Prolonged starvation states
- Endocrine pathologies (specifically, diabetes mellitus)
The accumulation of nitrogenous waste products in the blood leads to a condition called hyperazotemia. It is the main pathogenetic link of such life-threatening conditions as uremic and hepatic coma. The main clinical effect of hyperazotemia lies in its significant toxic impact on the nervous system, which causes severe neurological symptoms.
Dysproteinoses
Dysproteinoses are considered separately within standard forms of protein metabolism pathology. Their main characteristic is a pathological change in the physicochemical properties of proteins. Consequently, a profound disorder of the normal functions of protein molecules occurs: enzymatic, structural, receptor, and informational.
According to their localization in the body, dysproteinoses are classified into two large groups:
- Cellular dysproteinoses
- Extracellular dysproteinoses
From the standpoint of clinical significance, the greatest attention in medical practice is paid to extracellular dysproteinoses. The most severe forms of this pathology include amyloidosis and hyalinosis, which lead to pronounced structural changes in organs.
Briefly: answers to topical questions
Which enzymes are involved in the ornithine cycle of urea formation?
Mitochondrial and cytosolic enzymes participate in the ornithine cycle of urea formation.
Depending on intracellular compartmentalization, the following enzymes are distinguished:
- Carbamoyl phosphate synthetase I — the initiating mitochondrial enzyme responsible for the synthesis of carbamoyl phosphate.
- Ornithine transcarbamylase — a mitochondrial enzyme catalyzing the synthesis of citrulline.
- Argininosuccinate synthetase — a cytosolic enzyme synthesizing argininosuccinate.
- Argininosuccinate lyase — a cytosolic enzyme catalyzing the cleavage of argininosuccinate into arginine and fumarate.
- Arginase — a cytosolic enzyme carrying out the hydrolysis of arginine to form the end product — urea, as well as ornithine, which completes the cycle.
To remember the components of residual nitrogen, use the phrase: «We Actively Can Control Nitrogen Amounts» (Urea, Amino acids, Uric acid, Creatinine, Creatine, Ammonia).
Class and exam questions
Which substance from the residual nitrogen composition is the most toxic?
Ammonia has the most pronounced cytotoxic effect. It easily penetrates cell membranes, damaging intracellular enzymes and cytosolic structures.
Why does urea become dangerous in renal failure?
Urea itself is non-toxic, but in renal failure, it begins to be excreted through the intestines. There, bacterial flora breaks it down to form highly toxic secondary ammonia.
Which organ system is most affected by hyperazotemia?
The main target of hyperazotemia is the nervous system. The toxic effect of nitrogenous waste on nerve tissues underlies the development of hepatic and uremic coma.
Which types of dysproteinoses have the greatest clinical significance?
Extracellular dysproteinoses are the most significant in clinical practice. These include such severe pathologies as amyloidosis and hyalinosis.
What else to study on this topic
- Mechanisms of cell membrane and enzyme damage by ammonia
- Ornithine cycle (urea cycle) in the liver
- Pathogenesis of uremic and hepatic coma development
- Etiology and mechanisms of extracellular dysproteinosis development (amyloidosis, hyalinosis)
- Alteration of receptor and informational functions of proteins in dysproteinoses
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