Anabolism and Vitamin

Vitamin Bn-deficient P. shermanii grows well under strictly anaerobic conditions. However, under microaerophilic conditions compounds containing reduced sulfur methionine, cysteine, thiosulfate, reduced glutathione, or tryptone must be added to the medium (Fig. 5.3) (Vorobjeva and Iordan, 1976). These compounds are necessary to prevent the oxidation of thiol groups. A direct determination showed (Iordan et al., 1974) that the 

Survival rates of vitamin Bi 2-deficient cells irradiated with UV light were up to 10 times lower than of the Bn cells (Vorobjeva and Iordan, 1976). It is known (Samoilova, 1967) that thiol groups can protect UV-irradiated cells. But the protective effect of vitamin B, as we found, was greater than that of cysteine. Accordingly, the survival rate of vitamin Bi 2-deficient cells, irradiated in the presence of AdoCbl, was higher than in the presence of cysteine but without AdoCbl, or in the absence of either. It follows that the protective effect of AdoCbl against UV-irradiation may be distinct fi om the effect of SH-groups, and may be due to the increased levels of nucleotides 

On the other hand, the inhibitory effect of the excessive synthesis of vitamin B on the growth rate may be rationalized in terms of the competition for some common metabolites (Vorobjeva, 1976). A number of observations suggest the existence of a threshold level of vitamin B12 in the cell (1000 Lig/g biomass), above which competition with other anabolic reactions for common intermediates occurs, since some of them, such as ATP, NAD, FAD, are sources of the structural units of the vitamin Bn molecule. Probably, this is the reason why factors that delay growth and decouple the anabolic and catabolic processes lead to an increased vitamin Bi2 synthesis (Konovalova and Vorobjeva, 1970 Ibragimova and Sakharova, 

On the other hand, it has been shown (Kanopkaite and Gibavichyute, 1965) that the addition of ATP to the medium increases vitamin B12 synthesis. Similarly, adding amino acids to minimal medium causes the growth rates of cultures producing excessive quantities of vitamin Bn to approach those of cultures synthesizing optimal amounts (Fig. 5.5) (Iordan et al., 1984). 

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