Abstract
Resting lymphocytes are in the G~0~ phase of the cell cycle. Upon activation by PHA, they progress into G~1~ with accompanying increased protein and RNA synthesis, initiate DNA synthesis and divide. We have studied the kinetics of inhibition of macromolecular synthesis during activation in the absence of single amino acids. Three types of kinetics are observed. In the absence of tryptophan or isoleucine, stimulated lymphocytes show a normal increase in protein and RNA synthesis during the first 30 hours of stimulation, initiate DNA synthesis but are subsequently inhibited. In phenylalanine‐deficient medium, no DNA synthesis occurs in spite of a slight increase in protein synthesis. No increase in macromolecular synthesis is observed in medium lacking any one of the other essential amino acids (eg: lysine).
Our results indicate that the kinetics of macromolecular synthesis in tryptophan‐deficient medium is the result of a limited reserve of protein‐bound tryptophan which becomes exhausted after 30 hours. On the other hand, delayed inhibition of synthesis in isoleucine‐deficient medium probably reflects an initially low requirement for this amino acid followed by inhibition of the synthesis of isoleucine‐rich proteins involved in some late event of stimulation. Partial deprivation of lysine results in kinetics of protein synthesis similar to that in tryptophan‐ or isoleucine‐deficient media. The results indicate that the kinetics of macromolecular synthesis during activation of lymphocytes in the absence of an essential amino acid is a function of the quantitative requirement for that amino acid, at a given time during stimulation.
Upon replacement of lysine, lymphocytes inhibited by lysine deficiency begin RNA and protein synthesis immediately and at a rate faster than that of unstimulated cultures to which PHA is added. They also initiate DNA synthesis earlier and therefore, are closer to the S phase than resting lymphocytes. It is concluded that lymphocytes stimulated in the absence of lysine are activated even though no overall increase in macromolecular synthesis is observed. Furthermore, the kinetics of DNA synthesis following reversal of inhibition by phenylalanine suggests that lymphocytes stimulated during phenylalanine deprivation become arrested at most six hours before S. These results indicate that amino acid deficiencies lead to arrest of activated lymphocytes at various stages of stimulation, depending on how stringent these deficiencies are.