CHANGES OF AMINO ACID COMPOSITION OFCHLORELLACELLS DURING THEIR LIFE CYCLE

摘要

The cells ofChlorella ellipsoideawere grown synchronously, and at different stages of their life cycle, the cells were analysed for their contents in amino acids existing in free forms as well as in the fractions of bulk protein and peptides. Throughout the algal life cycle, the content of bulk protein (per unit dry weight of cells) remained relatively constant, being about 20 to 40 times those of peptides and free amino acids. The amino acid composition of the protein fraction also remained fairly constant, the predominant amino acids being alanine, glutamic acid, glycine and leucine. The contents in the bulk peptides increased appreciably during the periods of growth and“ripening”(light period), and decreased markedly during the periods of“post-ripening”and cellular division (dark period). Similar modes of change in content were also observed in most of the individual amino acids contained in the peptide fraction. The most abundant component in the peptide fraction was arginine followed by glutamic acid, glycine and cyst(e)ine. Rather irregular was the mode of change of the levels of individual free amino acids, although, as a whole, their behavior was similar to that of bulk peptides, increasing during the light period and decreasing during the dark period. The most predominant free amino acids were glutamic acid and alanine followed by proline.Experimental evidence showed that the processes of formation of free amino acids and peptides are for the most part light dependent, while the synthesis of protein, which is thought to be effected using as building blocks mostly free amino acids—formed directly or indirectly from early photosynthates or derived from pre-formed peptides—is essentially a light-independent process. Peptides, as a whole, seem to have significance as reservoirs of building blocks for the syntheses in the dark of protein and other nitrogenous cellular substances. The synthesis of protein in the dark takes place not only by consuming the free amino acids and peptides that have been accumulated during the light period, but also by assimilating the exogenous nitrogen source (nitrate). The distribution of individual amino acids in the three main fractions mentioned above as it changed during the course of algal cell cycle was followed in detail, and the results obtained were discussed in relation to various relevant data reported by ot