Table. Isotope components of growth media and characteristics of bacterial growth
Media components, % (v/v)
H2O 2H2O MetOH [U -2H]
MetOHLag-phase (h)Yield of
biomass (%) Generation time (h)Production of phenylalanine (%) (a)9802020100.02.2100.0(b)73.524.5023485.92.697.1(c)49.049.0024460.53.298.8(d)24.573.5024947.23.887.6(e)098.0026030.14.937.0
The production of L-phenylalanine was linear with respect to the time up to exponentaly growth cells (see Fig.1). During the fermentation the formation rate of L-phenylalanine was about 5 mmol/day. As shown in Fig. 1, the substitution by deuterium atoms pronons of water and methanol caused the decreasing both the production of L-phenylalanine and the yieald of biomass. Hawever, the decreasing of L-phenylalanine production (up to 0,5 g\L) was observed in those experiments (10) (Fig.1) when using non adapted cells on media with 98 % (v/v) 2H2O. The growth rate and generation time for adapted cells were found to be the same as in control in ordinary water despite of small increasing of lag-phase. In contrast to adapted cells, the growth of non-adapted species on maximal deuterated media was strongly inhibited by deuterium. These data are shown in Fig. 2.
A smart attempt was made to intensificate the growth and biosynthetic parameteres of cells to grow on media containing highly concentration of deuterated substrates. We employed a "step by step" adaptation method, combined with the selection of clones resistent to deuterium using agaric media supplemented with C2H3O2H 2% (v/v) and with increasing 2H2O content starting from pure water up to 98 % (v/v) 2H2O. The degree of cell survive on maximum deuterated medium (10), containing 98 v/v.% 2H2O and 2 v/v.% C2H3O2H was about 40%. Figure 1 shows characteristic growth and biosynthesis curves for adapted to 2H2O (10) and non-adapted (10) cells in conditions compared with the control (1) in H2O. The transfer of fully deuterated cells to ordinary protonated medium results eventually in normal growth.
The results on adaptation testified, that the generation time for adapted bacteria was approximately the same as in the control (1) despite the two-fold increase of the lag-phase (Table, Expt. 10). The yield of microbial biomass and level of phenylalanine production for adapted bacteria on maximally deuterated medium (Table, Expt. 10) were decreased relative to the control (1) by 13 and 5.3% respectively. Figure 1 shows growth (Expts. 1a, 2 a, 3 a) and production of phenylalanine (Expts. 1 b, 2 b, 3 b) for non-adapted (2) and adapted (3) bacteria on maximally deuterated medium under conditions like the control (1) on protonated medium. As shown from Fig. 1, the curves of phenylalanine production were close to a linear extrapolation with respect to the exponential phase of growth dynamics. The level of phenylalanine production of non-adapted bacteria on maximally deuterated medium was 0.39 g/liter after 80 hours of growth (Fig. 1, Expt. 2 b). The level of phenylalanine production for adapted bacteria under those growth conditions was 0.9 g/liter (Fig. 1, Expt. 3 b). Thus, the use of adapted bacteria in growth conditions to be the same as in the control (1), enabled us to improve the level of phenylalanine production on maximally deuterated medium by 2.3 times. However, phenylalanine is not the only product of biosynthesis; other metabolically related amino acids (alanine, valine, and leucine/isoleucine) were also produced and accumulated in the growth medium in amounts of 5-6 mol in addition to phenylalanine. This fact required, for the future prospects of the production of labeling molecules of amino acids with deuterium, an efficient separation of 2H-labeled phenylalanine from other amino acids of growth medium. Recently such separation was done using a reversed-phase HPLC method developed for methyl esters of N-Dns- and Bzl-amino acids with chromatographic purity of 96-98 and yield of 67-89%.
For evaluation of deuterium enrichment methyl esters of N-DNS-amino acids were applied because the peaks of molecular ions (M+) allow to monitor the enrichment of multicomponential mixtures of amino acids being in composition with growth media metabolites, therefore EI MS allows to detect samples with amino acids of 10-9-10-12 mol (Karnaukhova, 1994). N-DNS-amino acids were obtained through the derivatization of lyophilized M9 with DNSCl. To increase the volatality of N-DNS-amino acids, the methylation with DZM was made to prevent the possible isotopic (1H-2H) exchange in molecule of Phe. With DZM treatment it occured the derivatization on NH2 group in the molecule, so that its N-methylated derivative was formed to the addition of methyl ester of N-DNS-Phe.
Mass spectra EI MS of methyl esters of N-DNS-amino acid mixtures, obtained from M9 where used 0; 73.5 and 98% (v/v) of 2H2O (Table, Expts. (a), (d), (e)) are shown in consecutive order in Figs. 1-3. The fragmentation pathways of methyl esters of N-Dns-amino acids by EI MS leads to the formation of the molecular ions (M+) from whom the fragments with smaller m/z ratio further are formed. Since the value of (M+) for Leu is as the same as for Ile, these two amino acids could not be clearly estimated by EI MS. A right region of mass spectra EI MS contains four peaks of molecular ions (M+) of methyl esters of N-DNS-amino acids: Phe with m/z 412; Leu/Ile with m/z 378.5; Val with m/z 364.5; Ala with m/z 336.4 (see Fig. 1 as an example). A high continuous left background region at m/z 80 - 311 is associated with the numerious peaks of concominant metabolites and fragments of further decay of methyl esters of N-DNS-amino acids.
The results, firmly established the labeling of amino acids as heterogenious, juging by the presence of clasters of adduct peaks at their molecular ions (M+); the species of molecules with different numbers of deuterium atoms were visualised. The most aboundant peak (M+) in each claster was registered by mass spectrometer as a peak with average m/z ratio, from whom the enrichment of each individual amino acid was calculated. Thus, in experiment (d) shown in Fig. 2 where used 73.5% (v/v) 2H2O the enrichment of Phe was 4.1, calculated at (M+) with m/z 416.1 (instead of m/z 412 (M+) for non-labeled compound); Leu/Ile - 4.6 (M+) with m/z 383.1 instead of m/z with 378.5 (M+)); Val - 3.5 (M+ with m/z 368 instead of m/z (M+) with 364.5); Ala - 2.5 deuterium atoms ((M+) with m/z 338.9 instead of m/z with 336.4 (M+)).
With increasing of 2H2O content in liquid M9, the levels of amino acid enrichment varried propotionaly. As seen in Fig. 3 in experiment (e) where used 98% (v/v) 2H2O the enrichment of Phe was six ((M+) with m/z 418 instead of m/z 412 (M+)); Leu/Ile - 5.1 ((M+) with m/z 383.6 instead of m/z with 378.5 (M+)); Val - 4.7 ((M+) with m/z 369.2 instead of m/z (M+) with 364.5); Ala - 3.1 deuterium atoms (M+) with m/z 339.5 instead of