Influence of N source on ammonia assimilation enzymes

Estimates of the contribution of ammonia versus preformed amino acids to protein synthesis by the mixed ruminal population have been highly variable. 15N studies using 15N-ammonia or urea (which rapidly releases ammonia) infused into the rumen or added as a single dose indicated values of microbial N derived from ammonia that ranged from 18 to 100% (summarized by Salter et a I., 1979). Dietary factors responsible for these differences are the availability of readily fermentable energy source (Ben-Ghedalia et a I., 1978) and the presence of amino acids and peptides (Argyle and Baldwin, 1989). However, little progress has been made on understanding the biochemical basis of this variation in the proportion of microbial N derived from ammonia in the rumen.

The influence of nitrogen source on activities of ammonia assimilating enzyme systems has been studied in a number of ruminal bacteria. The activity of GS has been reported to depend on the N source and concentration in the cells of Preuotella bryantii B:4 (Kirk et al., 2000). It appeared that the bacteria became N-limited when N concentration in the growth medium was 0.5 mM for ammonia or peptides and 2.5 mM for casein. In the N-limited treatments, the highest GS activity was detected from the cells of the bacterium grown on pepticase, followed by ammonia and casein. No activity was detectable in the non-limited N treatments, however. The study also indicated that P. bryantii proteinase activity increased when N availability decreased in the medium, indicating that GS activity could be an indicator of N availability and thus could be involved in the regulation of protease activity in P. bryantii.

Wen and Morrison (1996) found that addition of trypticase (peptides) to the cultures of P. bryantii, P. ruminicola and P. brevis already growing on ammonia resulted in substantial reductions in NADP-linked GDH activity, although they differed in the time required for the decrease in the activity. However, the NAD-linked GDH specific activity in P. ruminicola and P. brevis appeared to increase after the peptides had been added to the growth medium. The authors later confirmed that P. brevis produces a second, NAD-linked GDH in response to growth on peptides (Wen and Morrison, 1997). Likewise, provision of peptides (15 g T1) to P. bryantii decreased NADP-linked GDH activity by tenfold as compared to the cells grown in the presence of 1 mM ammonia (Wen and Morrison, 1996). Our studies with predominant species of non-cellulolytic ruminal bacteria have demonstrated that provision of peptides at differing concentrations decreased the activity of the NADP-linked GDH activity in P. bryantii, Selenomonas ruminantium and Streptococcus bovis, and NAD-linked GDH activity in P. bryantii in a concentration-dependent manner (Table 15.2). A similar pattern was found in 15NH3-uptake experiments with the same species in that the proportion of bacterial cell-N and amino acid-N derived from ammonia fell as the concentration of peptides and amino acids increased in the growth medium (Atasoglu et al., 1998). Thus, in view of the published reports and the results of our studies, it is certain that the form and concentration of N sources present in the microbial environment are involved in the regulation of enzymes of ammonia assimilation in ruminal bacteria, thus are partly responsible for the variation in the utilization of ammonia for microbial protein synthesis.

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