122 MICROSOMAL PARTICLES 



also showed that, under the influence of the cells, free C 1 l -L-proline was most 

 rapidly incorporated into the proteins of the supernatant (obtained after final 

 centrifugation), and into the "small-granule" fraction of the cytoplasm (sedi- 

 mented at 105,000^) ; these fractions also contained the greatest amount of 

 ribose. Subsequently part of the labeled proline appeared as hydroxyproline in 

 a large-granule fraction (~3000 A particle size) as well as in fractions of larger 

 particle size; for example, after 21 hours of culture growth followed by contact 

 with radioactive proline for 1 hr it was found that for every 100 residues of 

 protein-bound proline 16.9 residues were hydroxylated in this fraction. With 

 longer contact times the radioactivity of the larger-granule fractions increased 

 considerably. 



The significance of these results in relation to the synthesis of collagen pro- 

 tein by the osteoblasts and in the mechanism of the formation of typical collagen 

 fibrils was discussed. 



REFERENCES 



M. R. Stetten, 1949, /. Biol Chew., 181, R. H. Smith and S. Fitton Jackson, 1957, 



31. /. Biophys. Biochem. CytoL, 3, 692. 



S. Fitton Jackson and R. H. Smith, 1957, 

 /. Biophys. Biochem. CytoL, 3, 679. 



