Protein Chemistry - Part 2 - Selected Chapters of Special Protein Chemistry - Ashmarin I. P. 1968

Histones
General Characteristics of Histones

Histones are Nuclear Proteins characterized by a high content of Lysine and Arginine, which imparts basic properties to them. Histones are typically low in aromatic (Tryptophan and Tyrosine) and sulfur-containing (Cysteine, cystine, Methionine) Amino Acids, exhibit high isoelectric point values (pH 10–11.5), are soluble in acids, and show a high affinity for DNA. The exact values of their molecular weights remain unknown, as some histone fractions tend to aggregate, and their N-terminal groups are masked (acetylated). Molecular weight values calculated from N-terminal Amino Acids and sedimentation rates often do not coincide, ranging from 10,000 to 50,000 for individual histone fractions. Due to the high content of lysine and arginine, the nitrogen content in histones is higher than in most proteins (~18%), and the conversion factor from nitrogen to protein amount is 5.5.

Histones are nearly universally distributed in nature. They have been found in The Cell nuclei of animals and plants: in the Thymus, Liver, Kidneys, Brain, Pancreas, and Placenta of mammals; in the erythrocytes of various fish, amphibians, and birds; in the spermatozoa of certain fish, amphibians, and Mollusks; as well as in grains, seedlings, roots, and perianths. In addition, histones are present in the Tissues of certain insects, Chlorella, amoebae, and some microorganisms (B. subtilis, Staphylococcus aureus, Micrococcus lysodeikticus). However, the presence of histones in Bacteria remains questionable, since the basic proteins isolated from their DNA complex differ from true histones by having different N-terminal groups or distinct ratios of basic to acidic amino acids. In most cases, detecting histones in single-celled organisms is altogether unsuccessful, and it is assumed that low-molecular-weight Polyamines may serve as their bacterial analogues.

The histone content in mammalian tissues varies and depends on both The amount of nuclear material and the DNA content within The Nucleus (Table 7). In most cases, the weight of histones accounts for 70–80% of the DNA weight, and their yield ranges from 18–28% of the nuclear weight for normal tissues and 4–28% for tumor tissues.

Class="center">Table 7 Histone content in various mammalian tissues (Johns, 1962)

Regarding the Intracellular Localization of histones, it should be noted that the bulk of them is located in Chromatin as a complex with DNA, and only a negligible amount is found in the nucleoli and the nuclear ribonucleoprotein network. Interestingly, histone-like proteins have also been detected in the extranuclear substance of frog Cells, as well as in liver microsomes and the Ribosomes of rabbit reticulocytes and HeLa cells. These proteins are similar in Amino Acid Composition to nuclear histones (10–28 mol % of basic acids) and are capable of forming aggregates with them. However, the basic proteins of microsomes and ribosomes are apparently not identical to histones, as they differ significantly from the latter in having a lower Alanine content, distinct characteristics of N-terminal groups, and different patterns upon cation-exchange Chromatography and starch or Polyacrylamide gel Electrophoresis.