Base substitution

Protein-Based Substitutes. Several plant and animal-based proteins have been used in processed meat products to increase yields, reduce reformulation costs, enhance specific functional properties, and decrease fat content. Examples of these protein additives are wheat flour, wheat gluten, soy flour, soy protein concentrate, soy protein isolate, textured soy protein, cottonseed flour, oat flour, com germ meal, nonfat dry milk, caseinates, whey proteins, surimi, blood plasma, and egg proteins. Most of these protein ingredients can be included in cooked sausages with a maximum level allowed up to 3.5% of the formulation, except soy protein isolate and caseinates are restricted to 2% (44). 

Aluminum hydroxide and aluminum chloride do not ionize appreciably in solution but behave in some respects as covalent compounds. The aluminum ion has a coordination number of six and in solution binds six molecules of water existing as [Al(H20)g]. On addition of a base, substitution of the hydroxyl ion for the water molecule proceeds until the normal hydroxide results and precipitation is observed. Dehydration is essentially complete at pH 7. 

Given a large population of individuals, a considerable number of sequence variants can be found for a protein. These variants are a consequence of mutations in a gene (base substitutions in DNA) that have arisen naturally within the population. Gene mutations lead to mutant forms of the protein in which the amino acid sequence is altered at one or more positions. Many of these mutant forms are neutral in that the functional properties of the protein are unaffected by the amino acid substitution. Others may be nonfunctional (if loss of function is not lethal to the individual), and still others may display a range of aberrations between these two extremes. The severity of the effects on function depends on the nature of the amino acid substitution and its role in the protein. These conclusions are exemplified by the more than 300 human... 

Lesch-Nyhan syndrome, an overproduction hyperuricemia characterized by frequent episodes of uric acid hthiasis and a bizarre syndrome of self-mutilation, reflects a defect in hypoxanthme-guanine phosphoribo-syl transferase, an enzyme of purine salvage (Figure 34—4). The accompanying rise in intracellular PRPP results in purine overproduction. Mutations that decrease or abohsh hypoxanthine-guanine phosphoribosyltrans-ferase activity include deletions, frameshift mutations, base substitutions, and aberrant mRNA splicing. 

Tvrdik T et al Molecular characterization of two deletion events involving Alu-sequences, one novel base substitution and two tentative hotspot mutations in the hypoxanthine phosphori-bosyltransferase gene in five patients with Lesch-Nyhan-syndrome. Hum Genet 1998 103 3H. 

There is a normal variation of DNA sequence just as is true of more obvious aspects of human structute. Variations of DNA sequence, polymorphisms, occur approximately once in evety 500 nucleotides, or about 10 times per genome. There are without doubt deletions and insertions of DNA as well as single-base substitutions. In healthy people, these alterations obviously occur in noncoding regions of DNA or at sites that cause no change in function of the encoded protein. This heritable polymorphism of DNA structure can be associated with certain diseases within a large kindred and can be used to search for the specific gene involved, as is illustrated below. It can also be used in a variety of applications in forensic medicine. 

M14. Matsuura, S., Igarashi, M Tanizawa, Y., Yamada, M., Kishi, F Kajii, T Fujii, H., Miwa, S Sakurai, M., and Nakazawa, A., Human adenylate kinase deficiency associated with hemolytic anemia. A single base substitution affecting solubility and catalytic activity of cytosolic adenylate kinase. J. Biol. Chem. 264, 10148-10155 (1989). 

V12. Vives Corrons, J. L. 1., Rovira, A., Pujades, A., Vulliamy, T., and Luzzatto, L., Molecular heterogeneity of glucose-6-phosphate dehydrogenase (G6PD) in Spain and identification of two new base substitutions in the G6PD gene, Blood 84 (Suppl. 1), 551a (1994). 

Because these processes require reduction and reoxidation of the silicon, they require large amounts of energy per unit of product. This makes them inherently unattractive and makes a search for replacements for them worthwhile. This naturally leads to a consideration of the silicate-based substitution approach to these compounds. 

Krahmer, M. T. Johnson, Y. A. Walters, J. J. Fox, K. F. Fox, A. Nagpal, M. Electrospray quadrupole mass spectrometry analysis of model oligonucleotides and polymerase chain reaction products determination of base substitutions, nucleotide additions/deletions, and chemical modifications. Anal. Chem. 1999, 71, 2893-2900. 

Myers, R.M., Fischer, S.G., Lerman, L.S. and Maniatis, T. (1985a) Nearly all base substitutions in DNA fragments joined to a GC-clamp can be detected by denaturing gradient gel electrophoresis. Nucleic Acids Research 13, 3131-3145. 

Sheffield, V.C., Beck, J.S., Kwitek, A.E., Sandstrom, D.W. and Stone, E.M. (1993) The sensitivity of single strand conformation polymorphism analysis for the detection of single base substitutions. Genomics 16, 325-332. 

Florian, J. Goodman, M.F. Warshel, A., Free energy perturbation calculations of DNA destabilization by base substitutions the effect of neutral guanine-thymine, adenine-cytosine and adenine-difluorotoluene mismatches, J. Phys. Chem. B 2000,104, 10092-10099. 

Single-base substitutions, which affect the amino acid sequence of proteins and lead to altered protein function, are the most frequent type of polymorphisms associated with many disease phenotypes as well as with variation in drag response... 

The base substitutions are primarily transversions at G C base pairs and the available evidence suggests that these mutations are induced by apurinic sites which are generated as secondary consequences of the initial alkylation event. The significance of these results in the context of carcinogenesis is briefly considered. 

On the other hand, the observation that 95% of the UV induced base substitution mutations arose at the very sites (pyrimidine-pyrimidine sequences) where the major fraction of UV damage is deposited suggested that at least the UV induced mutations were targeted (24). 

We have not directly determined the relative frequencies of frameshift mutations and other mutational events in comparison to the base substitution mutations. However, based on the high frequency of nonsense mutations (11%) among all lacl mutants induced by BPDE and because nonsense mutations are monitorable at less than one-fifth of the lacl codons and, even then, only via certain base pair substitutions, we believe that base substitutions account for a major fraction of mutations induced by BPDE. 

Table I. Summary of Base Substitution Events Generated by BPDE...

Base Substitutions Site No. Independent Occurrences Site No. Independent Occurrences ... 

Luminescent bacteria also allow detection of the carcinogenic effect of ge-notoxics. A dark mutant of a Photobacterium or Vibrio strain that can revert back to luminescence at an increased rate in the presence of base-substitutes or frame-shifts agents, DNA-damaging agents, DNA synthesis inhibitors, and DNA intercalating agents can be employed [171, 172],... 

Point mutations can occur when one base is substituted for another (base substitution). Substitution of another purine for a purine base or of another pyrimidine for pyrimidine is called a transition, while substitutions of purine for pyrimidine or pyrimidine for purine are called transversions. Both types of base substitution have been identified within mutated genes. These changes lead to a codon change which can cause the wrong amino acid to be inserted into the relevant polypeptide and are known as mis-sense mutations. Such polypeptides may have dramatically altered properties if the new amino acid is close to the active center of an enzyme or affects the three-dimensional makeup of an enzyme or a structural protein. These changes, in turn, can lead to change or reduction in function, which can be detected as a change in phenotype of the affected cells. 

A base substitution can also result in the formation of a new inappropriate terminator (or non-sense) codon, and are thus known as non-sense mutations. The polypeptide formed from such mutated genes will be shorter than normal and is most likely to be inactive. Owing to the redundancy of the genetic code, about a quarter of all possible base substitutions will not result in an amino acid replacement and will be silent mutations. 

Base substitutions and frameshift changes occur spontaneously and can be induced by radiation and chemical mutagens. It is apparent that the molecular mechanisms resulting in these changes are different in each case, but the potential hazards associated with mutagens capable of inducing the different types of mutation are equivalent. 

---