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

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 μμmoles per mg (dry weight) per minute, and maximal incorporation was 11 to 333 μμmoles 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 14 C 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.