One of the groups of conjugated proteins, characterized by presence of nonprotein prosthetic group, nucleic acid and attached to one or more molecules of a simple protein, a basic protein histone or protamine is called nucleoprotein. Nucleoproteins are so named because they constitute a large part of nuclear material.
Chromatin is largely composed of nucleoproteins, which indicates that these compounds are involved in cell-division, and transmission of hereditary factors. Nucleoproteins are found in all animal and plant tissues and have been extracted from a variety of tissues. Most easily isolated from yeast cells or from tissues with large nuclei where the cells are densely packed such as thymus gland.
Extracting agents used include water, dilute alkali, NaCl solutions and buffers ranging from pH of 4.0 to 11.0. In each case extraction is followed by precipitation with acid, saturated (NH4)2 SO4 or dil. CaCl2. When the purified nucleoprotein is hydrolysed with acid or by the use of enzymes, various components as shown in the box are obtained.
D-ribose and D-2-deoxyribose are the only sugars so far found in the nucleic acids from which the sugars have been isolated and identified, and they are assumed to be the sugars universally present in nucleic acids. Both sugars are present in nucleic acids as the β-Furanoside ring structures.
Pyrimidine bases found in nucleic acids are mainly three:
• Cytosine is found both in DNA and RNA
• Thymine is found in DNA only
• Uracil is found in RNA only. All the pyrimidine bases can exist in lactam form and Lactim form. If the group is –HN–CO–, it is called the Lactam type (keto), while the same if isomerises to – N = C – OH, it is called Lactim form (enol). At the physiological pH, the lactam (keto) forms are predominant.
Chemically it is 2-deoxy-4-amino pyrimidine, it can exist in both lactam or lactim forms. Cytosine is found in all nucleic acids except DNA of certain viruses.
2. Thymine (5-methyl uracil)
Chemically it is 2, 4-deoxy-5-methyl pyrimidine. Thymine occurs only in DNA, in nucleic acids which contain deoxyribose as Sugar. Minor amounts have recently been found in t-RNA.
Chemically it is 2, 4-dioxy pyrimidine. Uracil is confined to RNA only, not found in DNA. In addition to three major pyrimidine bases, there occurs in small quantities bases like 5–OH-methyl cytosine, methylated derivatives and reduced uracil compounds.
The Purine ring is more complex than the Pyrimidine ring. It can be considered the product of fusion of a pyrimidine ring with an imidazole ring. Adenine and guanine are the two principal purines found in both DNA and RNA.
1. Adenine: Chemically it is 6-aminopurine.
Chemically it is 2-amino-6-oxypurine. Guanine can be present as lactam and lactim forms. In addition to above, small amount of methylated purines have been shown to be present in nucleic acids. Two other purine bases, hypoxanthine and xanthine occur as intermediates in the metabolism of adenine and guanine. In human beings, a completely oxidised form of purine base uric acid occurs, which is of great biomedical importance. Uric acid is the catabolic end product of purines in human beings.
In nature, numerous unusual minor bases can exist in addition to the five major bases discussed above. Some of these unusual substituted bases are present only in the nucleic acids of bacteria and viruses. But many are also found in DNA and transfer RNAs of both prokaryotes and eukaryotes. Examples
• Bacteriophages contain 5-hydroxy methyl cytosine.
• Certain unusual bases are found in m-RNA molecules of mammalian cells like N6 – methyl adenine, N7 – methyl guanine and N6 – N6 – dimethyl adenine.
• Both bacterial and human DNA have been found to contain significant quantities of 5-methyl cytosine.
• A uracil modified at N3 position by attachment of a propyl group has also been detected in bacteria. The functions of such substituted Purine and Pyrimidine bases are not clear.
B. In plants:
Quite a number of purine bases containing methyl substituents occur in plants and plant products and many of them have definite pharmacologic properties. Some of them which occur in foodstuffs having biomedical importance are:
• Theophylline: Chemically 1, 3-dimethyl xanthine is found in tea.
• Theobromine: Occurs in cocoa which is chemically 3, 7-dimethyl xanthine.
• Caffeine: Present in coffee, is chemically 1, 3, 7-trimethyl xanthine.