Keratins in cultivated human keratinocytes are stable

Cultures of human keratinocytes were established according to the technique of Rheinwald and Green. When cultures were exposed to [14C] leucine, the uptake of leucine and increase in the specific activity of 6 urea-extractable polypeptides was prompt-each keratin achieved 50% of its peak specific activity in 3-6 hours. Cultures were exposed to [14C] leucine for 6 hours and then permitted to grow in unlabeled medium for 10 days. These confluent cultures shed cells into the medium; the amount or protein shed daily was 22.2 microgram or roughly 0.9% of the protein of the attached cells. Thus, protein shed into the medium over a 10-day period of the pulse-chase experiment was 9% of the total extractable protein. The specific activity of individual polypeptides extracted by urea fell an average of 25% during the 10-day chase. Polypeptides extracted by buffer A showed a fall in specific activity of 55% over this period. The relative amounts of individual urea-extractable polypeptides and individual buffer A-extractable polypeptides remained constant over a 10-day period. The rapid labelling of all urea-extractable polypeptides and the relative stability in the specific activity of these polypeptides is evidence that one keratin is not modified to form another keratin and that, once synthesized, the molecules are stable.