PROTEIN TURNOVER IN ATTACHED WHEAT AND TOBACCO LEAVES

Abstract

Attached primary and secondary wheat leaves were supplied continuously with C¹⁴O₂ during daily periods of photosynthesis for 3 days. Samples were analyzed for amounts and total activities of respired carbon, soluble sugars and amino acids, protein amino acids, and protein nitrogen. By labelling all possible protein precursors to the same extent it was possible to eliminate doubts about the specific activity of carbon entering protein. Hence turnover rates could be accurately established. Because tobacco leaves last for a long time, it was possible to label their proteins, wait until soluble compounds were at a low specific activity, and then measure turnover of proteins as radioactivity in them decreased.Protein amino acid turnover rates of 0.4–0.5% per hour were obtained in rapidly growing secondary wheat leaves and 0.2–0.3% per hour in non-growing primary wheat leaves. Turnover rates of 0.15–0.2% per hour were found in expanding tobacco leaves, but little or no turnover was found in fully expanded tobacco leaves.It is suggested that protein turnover is a facet of the biochemical differentiation that accompanies development, enlargement, or change in function of an organ without concomitant net protein synthesis.