Fundamentals of Biochemistry - A. A. Anisimov 1986

Enzymes
Enzyme localization and their comparative activity in individual organs and tissues

All organisms, as well as individual Organs and Tissues of Multicellular Organisms, differ from one another not only in their morphological and Anatomical Features and chemical composition, but also in The Nature of their metabolic processes, which are determined by the specific "set" of Enzymes and their activity. For instance, the enzymes involved in green pigment METABOLISM are characteristic only of photosynthetic organisms, whereas Thrombin and plasmin are found exclusively in humans and animals.

Within The Cell, individual enzymes are generally contained and function within strictly defined Organelles or compartments. Intracellular enzyme localization is directly related to the specific function performed by a given region of the cell. It is now well established that almost all Glycolysis enzymes are found in the Cytoplasm, the Enzymes of the TCA cycle and fatty acid ß-Oxidation are located in the mitochondrial matrix, and Oxidative Phosphorylation enzymes reside in The inner mitochondrial membrane. The enzymes of the Ornithine cycle are present in both the Mitochondria and the cytoplasm; some of them (ornithine carbamoyltransferase, the bulk of carbamate kinase) are localized in the mitochondria, while others (argininosuccinate synthetase, argininosuccinate lyase) are found in the cytoplasm.

The Cell Nucleus contains a small number of enzymes, primarily those involved in nucleic acid synthesis, typical representatives of which are a- and ß-DNA polymerases and RNA polymerase. Lysosomes are extremely rich in enzymatic variety, housing a wide range of hydrolytic enzymes. Lysosomes participate in the "Digestion" of the contents of phagocytic vesicles, which fuse with them in the process. These combined structures remain separated from the rest of the intracellular components by a membrane, which protects the latter from Hydrolysis. At the same time, during tissue autolysis and necrosis, lysosomal enzymes directly take part in The breakdown of the cell's own components. In animals and humans, lysosomes are present in various Cells and are particularly abundant in leukocytes.

Peroxisomes concentrate enzymes related to glycolic acid metabolism and hydrogen peroxide utilization. Microsomal membrane structures are rich in cytochrome P-450, which catalyzes a series of biological hydroxylation reactions and other oxidative processes. The stroma of Chloroplasts contains ribulose-1,5-bisphosphate carboxylase and Other Enzymes involved in carbohydrate synthesis from CO₂. Embedded in the chloroplast membranes are ATPases and electron carriers that function during Photosynthesis. The precise spatial localization of enzymes within the cell ensures their coordinated activity.

Due to their strict arrangement within the cell, many biological catalysts serve as markers for various intracellular structures. For example, studies on rat Liver have shown that acid phosphatase is localized in lysosomes, catalase and D-Amino Acid Oxidase in peroxisomes, adenylate cyclase in The Plasma Membrane, glucose-6-phosphatase and NADPH-cytochrome c reductase in the ER, Lactate dehydrogenase in the cytoplasm, and galactosyltransferases in the Golgi apparatus. However, the Intracellular Distribution of certain enzymes may vary among different species. Thus, phosphoenolpyruvate carboxylase in rat liver is found in the cytoplasm, in rabbits—in the mitochondria, and in guinea pigs—in both the mitochondria and the cytoplasm.

Individual tissues and organs of animals and plants differ not only in their set of enzymes, but also in enzymatic activity. For instance, almost all mammalian tissues contain the enzymes of glycolysis and the TCA cycle, yet their activities vary significantly across cells with different specializations. The highest activity of glycolytic enzymes and creatine kinase is characteristic of skeletal Muscles, ornithine cycle enzymes—of the liver, RNase—of the Pancreas, glutaminase—of the Brain, and ß-glucuronidase—of the Spleen. This tissue Specificity is widely used in clinical practice for diagnosing certain diseases. Specifically, elevated serum creatine kinase activity is a key indicator of Muscle tissue damage.

There are also age-related changes in tissue enzyme activity and composition, which become most pronounced during embryonic development and tissue differentiation. For example, unfertilized chicken eggs show only traces of lactate dehydrogenase activity; this activity increases substantially once embryonic development begins.