Uptake and Utilization of Amino Acids by Resting Cells of Nitrosomonas europaea

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Abstract
Incorporation of selected amino-acids by resting cells was studied with regard to effects of concentration, rate and extent of incorporation, cellular distribution, effect of ammonium ion on uptake, and competitive effects. L-Aspartic acid, L-alanine, L-serine, and glycine presented at trace levels were incorporated at rates ranging from 0.11 to 8.2 [mu][mu]moles per mg (dry weight) per minute, and maximal incorporation was 11 to 333 [mu][mu]moles per mg (dry weight). When glycine and aspartic acid were supplied at substrate level, the rate of incorporation increased 14- and 109-fold, respectively. The presence of ammonium ion further increased both the rate and extent of uptake of glycine and aspartic acid. The distribution of cellular radioactivity arising from Cl4 amino-acids indicated that cell pool radioactivity was concentrated from 1.2-to 24.5-fold over the external medium. Aspartic acid pool radioactivity accounted for 50% or more of the total cellular radioactivity, whereas radioactivity in glycine and serine pools dropped from initially high levels to 20 to 25% during incubation. The decrease in pool radioactivity with both glycine and serine was accompanied by an increase in other fractions, especially in the cell residue. The growth-inhibiting amino-acid L-valine, supplied at substrate level, contributed more carbon per milligram (dry weight) than any other amino-acid studied. L-Leucine, in the presence of L-valine, was observed to decrease valine Incorporation.