Proteins with Growth-Promoting Action on Tissue Cells in vitro.

Abstract

Among the many substances which contribute to the effectiveness of embryonic tissue extracts as growth-promoting media for cells in vitro, some have been found to be associated with the proteins of the extracts. The ease of inactivation of the extracts by heat and by proteolytic enzymes were early indications that proteins played some role in the effects which result in increasing the area of explanted tissue relative to the area of sister control-cultures.

The lability of the high molecular substances in the embryonic extracts makes it necessary to use only very mild methods in attempts to extract or purify active fractions. It has been found by Fischer and Astrup 1 that a rather high activity was associated with a fraction separated by the method used by Hammarsten 2 for preparation of nucleoproteins. Experiments have now confirmed that such fractions contain the relevant activity. It is, however, not yert certain that the activity necessarily resides in a protein, as distince from a protein-complex containing nonprotein material, for one of the results of using mild preparative procedures is that labile, loose linkage may not be broken.

Examination of some crude nucleoprotein fractions has revealed not correlation between activity and the proportions of the 2 types of nucleic acid, i.e. those containing ribose and desoxyribose. Activity has been found in fraction containing only a tract of desoxyribonucleic acid, so it is concluded that, if a nucleic acid is essential for activity, it must be one of the ribose and not of the desoxyribose type. Ribonucleic acid alone, in the depolymerised form in which it is isolated, does not appear to possess the activity. No method has so far been found for dissociating the ribonucleic acid from the protein, with which it is in firm combination, without denaturing the protein, so it cannot be stated whether the protein moiety alone shows any effect.