Membranes of animal cells. V. Biosynthesis of the surface membrane during the cell cycle

KB cells grown in suspension culture were synchronized by using a double thymidine block. At various times throughout the life cycle aliquots of cells were pulsed with 14 C‐L‐leucine, 14 C‐D‐glucosamine and 14 C‐choline for one hour periods. Surface membranes, cell particulates and soluble proteins were isolated and their 14 C specific activities were determined. It was found that there was a marked increase in the rate of incorporation into surface membrane just after division. The pattern of incorporation was the same for all three isotopic precursors. The rate of incorporation of isotopic precursors into soluble proteins was constant throughout the cycle. Some increase in rate of incorporation of isotope into the particulate fraction was observed during division.