Studies on the turnover of plasma membranes in cultured mammalina cells. I. Rates of synthesis and degradation of plasma membrane proteins and carbohydrates.

The rate of turnover of membrane proteins and membrane-bound carbohydrates in exponentially growing and in confluent contact-inhibited cultures of strain MK-2 cells was investigated. Cells were labelled with 14-C]leucine and 3-H]glucosamine, incubated in isotope-free medium and, at various times thereafter, the cells were harvested and membranes isolated from them. The rate of decay of the protein and carbohydrate components was determined from specific activity dilution of the labeled components in the isolated membranes. Although the rate of membrane synthesis is different in exponential and contact-inhibited cells, the rate of degradation (turnover) of membrane proteins and carbohydrates was found to be the same (25% per generation (42 h) or 0.6%/h).