The cell growth-promoting factor.

It was recently shown that macromolecular serum proteins [1-3] as well as some of their hydrolyzed products, especially peptides of molecular weight around 5000[4] and even much less[5,6], are able to promote the growth of cells. This paper describes how the serum proteins were separated by salt precipitation and polyacrylamide electrophoresis into various albumin and globulin fractions and their growth-promoting activities ascertained. Subsequently, these macromolecules were treated with alkali, acids or proteolytic enzymes, and the activity of the products obtained was determined. We also isolated growth-promoting peptides from the liver by enzymatic hydrolysis, followed by gel filtration, or by ultrafiltration through Diaflo membranes.