Assessment of fractional rates of protein synthesis in cardiac muscle cultures after equilibrium labeling.

Protein synthesis, accumulation, and breakdown were examined in growing cultures of contractile embryonic chick heart cells, and the fractional synthetic rates of several individual proteins were compared. Fractional rates of protein synthesis were evaluated with a procedure that combined equilibrium and pulse labeling for determination of specific radioactivities of the precursor in the medium and in proteins isolated by electrophoresis. Kinetic analysis during equilibration of the precursor specific radioactivity with that in labeled proteins indicated that the specific radioactivity of leucine in the culture medium closely approximated that of the immediate reaction precursor for protein synthesis. Protein accumulation and breakdown were evaluated by standard growth and decay kinetics. Fractional protein synthesis rates determined during either short pulse labeling or during continuous labeling to equilibrium were found to be in agreement with independent measurements of protein accumulation and breakdown. We also determined fractional synthetic rates of proteins isolated from cultured heart cells on single and two-dimensional electrophoresis and found the following order of fractional synthesis rates: fibronectin greater than or equal to alpha-actinin greater than myosin heavy chain = tropomyosin = myosin light chains greater than 55,000-dalton proteins (desmin and tubulin) greater than or equal to actin. The half-lives of these proteins ranged from T 1/2 = 1.1 days for fibronectin and 2.0 days for myosin heavy chain to 4.7 days for actin.