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

Nerve Tissue
Chemical Composition of the Brain
Proteins

The grey matter of the Brain is composed primarily of neuronal Cell bodies, whereas the White matter consists of axons. Consequently, these two Regions of the brain differ significantly in their chemical composition. These differences are primarily quantitative. The Water content of the brain's grey matter is noticeably higher than that of the white matter (Table 19.1). In grey matter, Proteins account for half of the dry weight, whereas in white matter, they constitute one-third*. Lipids account for more than half of the dry residue in white matter, compared to only about 30% in grey matter.

Class="center">Table 19.1. Chemical composition of human brain grey and white matter (as a percentage of fresh tissue mass)

Proteins account for approximately 40% of the dry mass of the brain. Brain tissue is a challenging subject for protein composition analysis due to its high lipid content and the presence of protein-lipid complexes.

A.Ya. Danilevsky was the first to divide brain tissue proteins into water- and salt-soluble proteins and insoluble proteins. Extensive research in this field was also conducted by A.V. Palladin and co-workers, who divided Nervous Tissue proteins into 4 fractions: those extractable with water, 4.5% KCl solution, 0.1% NaOH solution, and the insoluble residue. It was established that grey matter is richer in water-soluble proteins than white matter, at 30% and 19% respectively. In contrast, white matter contains a much higher proportion of insoluble protein residue (22%) than grey matter (5%).

* When calculated on The basis of fresh tissue mass, proteins are distributed approximately evenly between the grey (8%) and white (9%) matter of the brain.

Subsequently, 5 to 10 fractions of soluble brain proteins, differing in their electrophoretic mobility, were isolated.

Currently, by combining buffer extraction, DEAE-Cellulose Column Chromatography, and polyacrylamide gel Disc Electrophoresis, researchers have isolated approximately 100 different soluble protein fractions from brain tissue.

Nervous tissue contains both simple and conjugated (complex) proteins. Simple proteins include albumins (neuroalbumins), globulins (neuroglobulins), cationic proteins (Histones, etc.), and structural proteins (neuroscleroproteins).

Albumins and globulins differ somewhat in their physicochemical properties from analogous Serum proteins; hence, they are termed neuroalbumins and neuroglobulins. The amount of neuroglobulins in the brain is relatively high—averaging 5% of all soluble proteins. Neuroalbumins constitute the primary protein component of nervous tissue Phosphoproteins, accounting for the bulk of soluble proteins (89–90%). Free neuroalbumins are rarely encountered; in particular, they readily associate with lipids, Nucleic Acids, CARBOHYDRATES, and other non-protein components.

Proteins that migrate toward the cathode during electrophoretic Separation at pH 10.5–12.0 are designated as cationic proteins. The principal representatives of this group in nervous tissue are histones, which are divided into five main fractions depending on the content of Lysine, Arginine, and Glycine residues in their polypeptide chains.

Neuroscleroproteins can be characterized as structural-support proteins. The main representatives of this group include neurocollagens, neuroelastins, neurostromins, etc. They constitute approximately 8–10% of the total simple proteins in nervous tissue and are localized primarily in the brain's white matter and the Peripheral Nervous system.

Conjugated (complex) proteins of nervous tissue include Nucleoproteins, Lipoproteins, proteolipids, phosphoproteins, Glycoproteins, etc. Brain tissue also contains significant quantities of even more complex supramolecular structures, such as liponucleoproteins, lipoglycoproteins, and potentially lipoglyconucleoprotein complexes.

Nucleoproteins are proteins belonging to either deoxyribonucleoproteins or ribonucleoproteins. Some of these proteins are extracted from brain tissue with water, others with salt media, and a third portion with 0.1 M alkali solution.

Lipoproteins constitute a significant portion of the water-soluble proteins in brain tissue. Their lipid component consists predominantly of phosphoglycerides and Cholesterol.

Proteolipids are protein-lipid complexes extracted from brain tissue by organic Solvents. They differ from water-soluble lipoproteins in being insoluble in water but soluble in a chloroform-methanol mixture. Once freed from lipids, the proteins become soluble in water as well as (owing to their high content of hydrophobic Amino Acids) in a chloroform-methanol mixture. The highest concentration of proteolipids is found in myelin, with smaller amounts present in synaptic membranes and synaptic vesicles.

Phosphoproteins are present in the brain in higher quantities than in other Organs and tissues—accounting for about 2% of all complex brain proteins. Phosphoproteins have been detected in the membranes of various morphological structures of nervous tissue.

Glycoproteins represent an extremely heterogeneous group of proteins. Based on their protein and carbohydrate content, they can be divided into two main groups. The first group comprises glycoproteins containing 5% to 40% carbohydrates and their derivatives, with the protein moiety consisting predominantly of albumins and globulins. Glycoproteins of the second group contain 40% to 85% carbohydrates and frequently feature a lipid component; compositionally, they can be classified as glycolipoproteins.

A number of specific proteins have been discovered in nervous tissue, notably the S-100 protein and the 14-3-2 protein. The S-100 protein, also known as Moore's protein, is referred to as an acidic protein because it contains a high proportion of glutamic and aspartic acid residues. This protein is localized primarily in neuroglia (85–90%), with no more than 10–15% of the total brain protein content found in Neurons. It has been established that the concentration of S-100 protein increases during animal learning (training). There is as yet no evidence to suggest that S-100 protein is directly involved in memory formation and storage, though its involvement in these processes may be indirect. The 14-3-2 protein is also classified as an acidic protein. Unlike the S-100 protein, it is localized predominantly in neurons, with only minor amounts found in neuroglial Cells. The Role of the 14-3-2 protein in fulfilling the specific Functions of Nervous Tissue remains unclear.

Enzymes. Brain tissue contains A large number of enzymes that catalyze the METABOLISM of carbohydrates, lipids, and proteins. To date, only a few enzymes have been isolated in crystalline form from the mammalian Central Nervous System, notably acetylcholinesterase and creatine kinase.

A significant number of enzymes in brain tissue exist in Multiple molecular forms (Isoenzymes): LDH, aldolase, creatine kinase, hexokinase, malate dehydrogenase, Glutamate dehydrogenase, cholinesterase, acid phosphatase, monoamine oxidase, etc.