The effect of deuterium oxide on protein turnover in Lemna minor

Lemna minor fronds transferred to a sterile culture medium containing 50% (v/v) deuterium oxide (²H₂O) rapidly undergo a loss of soluble protein with a corresponding increase in free amino acids. The loss of protein is due to two factors: (i) the inhibition of protein synthesis for 4 h followed by a slower rate of synthesis than normal, (ii) a rapid 9–10 fold increase in protein degradation. In plants grown for longer periods (3–6 days) in 50% ²H₂O medium, protein synthesis is inhibited by 20% and the rate constant of degradation is 2–3 times that measured in fronds growing in normal (H₂O containing) complete medium. The initial loss of protein is not due to the breakdown of any specific protein fraction. Investigation of several enzymes indicates that all proteins are catabolised in response to ²H₂O treatment. The implications of these results with regard to the interpretation of density-labelling experiments are discussed.