Release of nucleotides and their derivatives

As mentioned above, immobilized cells are studied mainly for practical reasons, since they show a number of economic advantages over the use of growing cells or cell suspensions. Production of organic acids is one of the prospective applications of immobilized cells. Another one is related to the release of nitrogenous bases and some nucleosides by immobilized cells. In nitrogen-starved immobilized cells the levels of all metabolites (first of all, nucleotides) are reduced (Leps and Ensign, 1979). It was shown (Ikonnikov et al., 1982) that immobilized cells of propionic acid bacteria, incubated periodically in nitrogen-free medium, released substances of protein and nucleic acid nature, whose quantity decreased with the time of incubation in 

The main source of nucleic acid-derived substances appears to be ribosomal RNA (rRNA), since the rRNA content decreased faster than the total RNA content (Ikonnikov, 1985), and Mg ions, known to have a stabilizing effect on ribosomes, prevented the release of nucleic acid derivatives. The main species released by the immobilized cells were nitrogenous bases adenine, guanine, cytosine, uracil, as well as nucleosides adenosine and guanosine (Fig. 6.10). The total yield of nucleic acid derivatives in the first 48 h reached 45 p-g/ml, which is rather high. Reactivation of the biocatalyst, especially with rich medium, increased the release and led to its stabilization (cf Fig. 6.9). Periodic reactivation allows maintaining 60-70% of the initial release level. Synthetic medium was much less effective. 

As an alternative to immobilized cells, the use of resting cells of P. shermanii, made permeable to nucleotides, has been suggested (Iordan and Pryanishnikova, 1994). The cells were permeabilized by treatment with diethyl ether or Triton X-100. The best effect was obtained by using Triton X-100 in combination with subsequent freezing and thawing of the cells. The biocatalyst prepared in this way was permeable to both the substrate of 

RNR and its reaction product, ADP and 2-deoxy-ADP, respectively. 2-deoxy-ADP accximulated in the incubation mixture. Moreover, inhibition of DNA synthesis in these cells by adding antibiotics, among which bleomycin (0.4-0.5 ig/ml for 2.5 h) was found to be the most effective, increased the RNR activity twofold as compared with the control the yield was 9.8-10.5 nmoles of 2-deoxy-ADP per mg dry cells per h. 

The nucleosides GMP, IMP and XMP are known to enhance food flavor. They display a synergistic action with soditim glutamate (Demain, 1968). Nucleotide derivatives are recommended for the treatment of thromboses. Inosine (under the name Riboxin) is a pharmaceutical agent used in the treatment of various heart and liver diseases in Russia. Ribosides and adenine support high levels of ATP in erythrocytes. Deoxyribonucleotides are used in chemical syntheses of some antitumor and antiviral preparations (Sidermene et al., 1985). In addition, 2-deoxyribonucleotides (-sides) serve as substrates for enzymatic and chemical syntheses of specific oligodeoxyribonucleotides, which at present are widely used as biosensors (Efremenko et al, 1990). 

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