The Biochemical Function of Cells 



405 



In 1955 we found [30] in our laboratory that, in isolated chloroplasts of the 

 young plant, there are at least two different enzymic systems taking part in the 

 synthesis of the peptide bond. One system brings about the synthesis of peptide 

 bonds by the transpeptidase reaction, while the other catalyses the formation of 

 peptide bonds directly from free amino acids. It has been shown by other workers 

 [31], in experiments in vivo, that methionine containing labelled sulphur is 

 incorporated most quickly into the protein of the fraction which is composed 

 of chloroplasts and mitochondria with a predominance of chloroplasts. In experi- 

 ments with isolated structures from the cells of green plants we [32] fovmd another 

 regularly occurring phenomenon. 



It seemed that the rate of incorporation of [i^Cjglycine into the mitochondria 

 of plant cells was about 10 times greater than that of its incorporation into 

 chloroplasts. Thus if, as Webster's data indicate [21], the incorporation of 

 [i^CJglycine into mitochondria taken from the shoots of broad beans amoimts 

 to 0-03 /<mole of [i4C]glycine/g. protein/hr, then the amount in the chloroplasts 

 will be 0-002-O-005 /^mole. We found a similar picture in our investigation of 

 various fractions of a homogenate of tobacco leaves. 



Table i 



The incorporation of {^'^C\glycine into protein and alterations in their 

 amount in various fractions of a homogenate of tobacco leaves 



As the Table shows, the labelled glycine is incorporated into the protein of all 

 the fractions of a homogenate of tobacco leaves. The greatest incorporation is 

 found in the fraction which comes down at 40,000 g. This fraction has a com- 

 paratively low chlorophyll content and evidently consists of mitochondria con- 

 taminated by fragments of the granules of the chloroplasts and also, of course, 

 of microsomes. In the fraction with a high chlorophyll content, which is thrown 

 down at 3000 g the power to incorporate labelled glycine is weaker. Furthermore, 

 this fraction, which has the highest chlorophyll content, is the only one in which 

 there appears to be any significant power to increase the protein nitrogen at the 

 expense of added amino acids. 



These data suggest that the transformation of proteins, which may be assessed 

 to some extent by the rate of incorporation of labelled glycine, occurs most 



