Respiratory energy requirements and rate of protein turnover in vivo determined by the use of an inhibitor of protein synthesis and a probe to assess its effect

Protein turnover is generally regarded as a major maintenance process, but experi­ mental evidence to support this contention is scarce. Here we quantify the component of dark respiration rate associated with overall protein turnover of tissues in vivo. The effect of an inhibitor of cytosolic protein synthesis (cycloheximide, CHM) on dark respiration was tested on a cell suspension from potato (Solanum tuberosum L.) and quantified on leaf discs of expanding and full-grown primary leaves of bean (Phaseo­ lus vulgaris L.). The in vivo effect of CHM on protein biosynthesis was assessed by monitoring the inhibition of the induction of the ethylene-forming enzyme (EFE) activity. The present method yields the energy costs of turnover of the total pool of proteins irrespective of their individual turnover rates. Average turnover rates were derived from the respiratory costs and the specific costs for turnover. Inhibition of respiration by CHM was readily detectable in growing-cell suspensions and discs of expanding leaves. The derived respiratory costs of protein turnover in expanding leaves were maximally 17-37o/c of total respiration. Turnover costs in full-grown primary leaves of bean amounted to 17-21% of total dark respiration. The maximum degradation constants (i.e. "Ki-values) derived for growing and full-grown leaves were up to 2.42 x 106 and 1.12 x 10-6 s1 , respectively.