Biological Chemistry - Berezov T. T., Korovkin B. F. 1998

Kidneys and Urine
Certain features of metabolism in renal tissue in health and disease

Complex physiological processes within the renal tissue involve the constant consumption of large amounts of energy released during metabolic reactions. At least 8–10% of total oxygen uptake in a resting human is utilized for oxidative processes in the Kidneys. Energy consumption per unit mass is higher in the kidneys than in any other organ.

The renal cortex exhibits a predominantly aerobic METABOLISM, whereas anaerobic processes prevail in the medulla. The Kidney is among the Organs richest in Enzymes. Most of these enzymes are also found in other organs; for instance, LDH, AST, ALT, and Glutamate dehydrogenase are widely distributed in both the kidneys and other Tissues. At the same time, certain enzymes are largely specific to renal tissue. Glycine amidinotransferase (transamidinase) is foremost among such enzymes. This enzyme is present in the Tissues of the kidneys and Pancreas and is virtually absent in other tissues. Glycine amidinotransferase transfers an amidine group from L-Arginine to glycine, yielding L-Ornithine and glycocyamine:

Class="center">L-arginine + Glycine —> L-ornithine + Glycocyamine.

This reaction represents the initial stage of creatine synthesis (see Chapter 20). Glycine amidinotransferase was discovered as early as 1941, but it was not until 1965 that W. Horner et al., followed by S.R. Mardashev and A.A. Karelin (1967), first noted the diagnostic value of measuring this serum enzyme in kidney disease. The appearance of this enzyme in the Blood may be associated either with renal damage or with the onset or progression of pancreatic necrosis.

The highest serum activity of glycine amidinotransferase is observed in Chronic Pyelonephritis during the phase of impaired nitrogen-excretory renal function, followed in decreasing order by chronic nephritis with hypertensive and edema-hypertensive syndromes and moderate impairment of nitrogen excretion, chronic nephritis with an isolated urinary syndrome and preserved nitrogen-excretory function, and residual effects of acute diffuse Glomerulonephritis.

Renal tissue belongs to the category of tissues with high activity of the LDH₁ and LDH₂ Isoenzymes. Examination of tissue homogenates from various renal layers reveals a distinct differentiation of LDH isoenzyme spectra. The activity of LDH₁ and LDH₂ predominates in the cortex, whereas LDH₅ and LDH₄ prevail in the medulla. In ACUTE RENAL FAILURE, the activity of anodic LDH isoenzymes—that is, isoenzymes with high electrophoretic mobility (LDH₁ and LDH₂)—increases in the blood serum.

The Study of Alanine aminopeptidase (AAP) isoenzymes is also of considerable interest. Five AAP isoenzymes are known. Unlike LDH isoenzymes, AAP isoenzymes in various organs are not represented by a full spectrum (all 5 isoenzymes), but typically appear as a single isoenzyme. Thus, the AAP₁ isoenzyme is primarily localized in Liver tissue, AAP₂ in the pancreas, AAP₃ in the kidneys, and AAP₄ and AAP₅ in various sections of the intestinal wall. In renal tissue injury, the AAP₃ isoenzyme is detected in the blood and urine, serving as a specific marker of kidney damage.

The investigation of urinary enzyme activity is equally crucial for the Diagnosis of renal diseases. Acute inflammatory processes in the kidneys are primarily characterized by increased permeability of the glomerular membranes, which leads to The excretion of Proteins, including enzymes, into the urine. Overall, metabolic shifts in renal tissue can be triggered by the blockade of glomerular blood flow, impaired filtration and reabsorption, obstructed urine outflow, damage to the juxtaglomerular apparatus, impaired secretion, and other factors.