Carbon Sources for the In-Vivo Nitrate Reductase Activity in Azospirillum Brasilense Cells
Carbon sources representative of various entry pathways of carbon metabolism viz. fructose, gluconate, glucose and TCA cycle intermediates were used as energy sources to study the invivo nitrate reductase activity in whole cells of A brasilense ATCC 29145. Enzyme activity was linear when using cells grown and immediately assayed using nutrient broth. Organic acids such as malate, succinate, pyruvate, and gluconate were all effective as sources of energy for the reduction of nitrate to nitrite under aerobic conditions. Among the carbohydrates, glucose and sucrose could not support nitrate reduction, while fructose was able to supply energy after a lag of 10–20 minutes. This lag was also observed when using organic acids.
Most interesting was the observation that A brasilense cells were able to metabolize and used α-Ketoglutarate (α-KG) as an external source of energy for nitrate reduction. The use of α -KG as energy source by the A brasilense cells was confirmed by the uptake studies using radioactively labelled C14 α -KG. The cells did not grow in semi-solid nitrogen-free medium containing α -KG as sole carbon source but limited growth in liquid medium was observed with fixed nitrogen sources.
KeywordsNitrate reduction nitrate reductase activity carbon metabolism A. brasilense
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