RESPIRATORY ENERGY-REQUIREMENTS AND RATE OF PROTEIN-TURNOVER IN-VIVO DETERMINED BY THE USE OF AN INHIBITOR OF PROTEIN-SYNTHESIS AND A PROBETO ASSESS ITS EFFECT

Protein turnover is generally regarded as a major maintenance process, but experimental evidence to support this contention is scarce. Here we quantify the component of dark respiration rate associated with ove rall protein turnover of tissues in vivo. The effect of an inhibitor o f cytosolic protein synthesis (cycloheximide, CHM) on dark respiration was tested on a cell suspension from potato (Solanum tuberosum L.) an d quantified on leaf discs of expanding and full-grown primary leaves of bean (Phaseolus vulgaris L.). The in vivo effect of CHM on protein biosynthesis was assessed by monitoring the inhibition of the inductio n of the ethylene-forming enzyme (EFE) activity. The present method yi elds the energy costs of turnover of the total pool of proteins irresp ective of their individual turnover rates. Average turnover rates were derived from the respiratory costs and the specific costs for turnove r. Inhibition of respiration by CHM was readily detectable in growing- cell suspensions and discs of expanding leaves. The derived respirator y costs of protein turnover in expanding leaves were maximally 17-37% of total respiration. Turnover costs in full-grown primary leaves of b ean amounted to 17-21% of total dark respiration. The maximum degradat ion constants (i.e. K-d-values) derived for growing and full-grown lea ves were up to 2.42 x 10(-6) and 1.12 x 10(-6) s(-1), respectively.