Suppression of the adipose conversion of 3T3 cells by acidified serum

The adipose conversion of cultured 3T3-F442A cells in strongly inhibited if the fetal bovine serum of the culture medium is briefly acidified before it is used. The inhibitory factor is a polypeptide with an apparent molecular weight of 24,000, and is inactivated by pronase or trypsin. Cells grown to confluence in the presence of this factor do not become spherical or accumulate triglyceride; they also do not increase the activity of their triglyceride-synthesizing enzymes. The factor suppresses adipose conversion even in the presence of untreated serum. Once adipose conversion has begun in the absence of the inhibitory factor, subsequent addition of the factor does not arrest the conversion.