Proteolysis in Stored Serum and its Possible Significance in Cell Culture

Abstract

ALMOST all the mammalian cell lines grown in continuous culture require the presence of a small amount of serum protein, usually supplied as whole or dialysed serum. The role of that protein, in an otherwise chemically defined medium¹, is not yet known. It is used to an insignificant degree for the synthesis of cell protein² ; and although it is reported to be necessary for the adhesion and flattening of the cells on the glass surface^3,4, it is no less essential in suspension cultures. It is here shown that serum protein, presumably because of associated enzymes⁵, undergoes proteolysis on storage at 5° C. or during incubation to yield free amino-acids at a rate which may be significant in certain types of experiments. The amino-acids released are themselves not growth-stimulating since they are already supplied in the medium or are formed from glucose and glutamine. However, other breakdown products of the serum protein, or compounds bound to it and released by its degradation, may be the essential growth factors supplied by serum.