var. has high affinity active transport systems for the amino acids -leucine and -tyrosine. Leucine at 0.2 μM was concentrated at least 222-fold and tyrosine 920-fold. Neither uptake process was inhibited by high concentrations of serine, threonine or phenylalanine. The concentration giving half-maximum rate of uptake was 2.5 μM for leucine with a smaller value for tyrosine. The maximum uptake rate for leucine was 10 amol cell min. Uptake of leucine and tyrosine was not inhibited by 0.2 mM-2,4-dinitrophenol, which completely inhibited the incorporation of radioactive leucine and tyrosine into protein. Intracellular radioactive leucine did not exchange with an excess of unlabelled leucine added to the medium, except during nitrogen starvation, and only partial exchange occurred with tyrosine. The apparent overall pool size for these amino acids was dependent upon the growth state of the algal cells. The kinetics of incorporation of radioactive amino acids from intracellular pools into protein suggested that both leucine and tyrosine occupied at least two distinct pools within the cells. Tyrosine and methionine were present in smallest amounts (38 and 37 amol cell, respectively). Leucine was present at 90 amol cell. At least 11 amino acids had higher pool concentrations than leucine, whilst only alanine and glycine were more abundant than leucine in protein.


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